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Managing Viruses in Potato Collections
Overview
This is a review of viruses and viroids that infect the potato (Solanum tuberosum). This article is targeted at the advanced amateur, particularly anyone considering doing some testing of their collection. Most articles about potato viruses focus on just a few of the most common ones. I have tried to cover the viruses that infect potato here as comprehensively as possible, so that you don’t need to go digging through the research. This should at least be a good place to start for anyone interested in learning more about potato viruses.
- Potato viruses produce a range of symptoms, from asymptomatic infection to major yield loss and unmarketable tubers.
- Most countries restrict the importation of potatoes to reduce the spread of potato viruses.
- The tubers of an infected plant are almost always infected, so infection with a virus is effectively permanent.
- Viruses can be removed from potatoes, but the process is complex, time-consuming, and expensive.
- Growing potatoes from true seed can eliminate some viruses, but not others.
- Tests are available to detect most potato viruses.
- 83 viruses are known to naturally infect potatoes worldwide and 47 of those are known to be present in the USA.
- 3 viroids are known to naturally infect potatoes worldwide and 2 of those are known to be present in the USA.
- Modern technology is making it easier than ever to discover new viruses, so the number of known potato viruses is certain to increase.
Introduction
Viruses and viroids are the smallest organisms. They are incomplete organisms in the sense that they contain genetic material, but lack the ability to reproduce. Instead, to reproduce, they must invade the cell of another organism and use its cellular machinery to make new viruses. A typical virus is a small strand of RNA or DNA encapsulated within a protein shell. The shell protects the genetic material and takes part in delivering the RNA or DNA into the cell of a host organism. There are viruses that infect all forms of cellular life, including plants. A viroid is an even simpler organism, consisting of nothing but a piece of RNA. Viroids are only known to infect plants.
There are many important viral pathogens of plants. When a virus infects a plant, the plant diverts some of its energy to manufacturing viruses instead of the products that the plant cell normally produces. This means that the plant has less energy to devote to its normal activities, so growth and yield tend to be reduced. There may also be more complex interactions with the plant. For example, the virus may cause the plant to produce RNA products or proteins that directly interefere with the normal functions of the plant. Not all viruses are pathogenic, in the sense that they cause symptoms. Some are well tolerated by plants and may actually add value. There are some ornamental plants, for example, that produce prized flower colors or patterns only when infected by a virus. Generally though, viruses are harmful to crop plants, because we are most interested in obtaining a high yield of edible parts and yield will generally be reduced when the plant has to compete for resources with the virus.
Viruses produce a wide range of symptoms. Many viruses are latent, essentially undetectable, although they often reduce yield in a slow and insidious fashion over a period of years. Other viruses have clear and obvious symptoms like mosaic patterns or mottling of the leaves, yellowing, necrotic spots or veins, all the way up to complete defoliation and death of the plant. Some viruses get worse over time, while others can coexist with the plant indefinitely. Plants can be infected with more than one virus at a time and cases of multiple infection usually have worse symptoms.
Viruses can transmit horizontally, plant to plant through direct contact or through pests that feed on the plant, or they can transmit vertically, through the pollen and seeds of a plant. Most plant viruses transmit horizontally. Viruses are a particular problem in potatoes because the plants are reproduced clonally, by replanting tubers. Once a plant is infected with a virus, the tubers are normally all infected. That means the plants of the next generation will all be infected and will all produce infected tubers. Before the advent of tissue culture techniques to remove viruses from potatoes, all varieties eventually “ran out,” due to virus burden. They simply became uneconomical to grow anymore and were abandoned. Because most viruses are transmitted horizontally, true seed of the potato is usually virus free – the pollen and seeds are not infected. This is not always the case and this is covered in more detail below.
Most potato varieties that are not grown commercially are infected with viruses. It is somewhat expensive to remove the viruses from a potato variety and then to keep a clean source for future crops. Because of this, most heirloom varieties have been infected with viruses for decades and they often have poor performance and lose the ability to flower. Commercial potatoes are grown in certification programs, where the plants are sampled for major viruses and only allowed to be sold for seed when the percentage of virus infected plants is low.
Potato Viruses and International Quarantine
I didn’t originally include this section in the article and quickly received some emails informing me that I was being irresponsible. That certainly wasn’t the objective. The whole purpose of this article to to try to help collectors to be more responsible! So, let’s talk about international controls on movement of plants. Your country, almost regardless of which one it is, has regulations in place that limit which plants you can bring into the country and from where. It is virtually certain that potatoes are on that list. The potato is an important crop that is infected by a large number of diseases, as you will see if you keep reading. The USA, which is the focus of this article, since that is where I live, prohibits importing potato tubers from any other country, unless you work through the official quarantine process. In many cases, that may not even be possible for reasons too complex to bother with here. The USA also prohibits the import of true potato seeds with a few exceptions that still require a permit. So, if you bring potatoes or TPS into the country from anyplace else, you are breaking the law and taking a risk.
Most people think that the risk ends when you get through customs, but that is really when it begins. If you bring a potato in from South America, homeland of the potato and, not so coincidentally, homeland of potato diseases (and pests), it is very likely that you will bring disease with it. If you share that potato with others, you are spreading sources of that new disease around the country. Nothing bad may ever happen. On the other hand, a year, a decade, even multiple decades later, that disease may suddenly make the jump to a commercial field. Maybe an ambitious aphid carried it. Maybe it became naturalized in a compatible weed and has slowly infected everything between you and the potato field. Maybe a rodent or a bird carried infected tubers and dropped enough to cause volunteer plants to grow elsewhere. Now, some intrepid field roguer notices unusual symptoms and sends off a sample to a lab and, RED ALERT, we have discovered a new potato disease that escaped quarantine. Will the authorities catch you? Quite possibly. What will happen? Nothing good. Even if they don’t catch you, you may have just ruined a crop or a field for that potato farmer. The point is, you could get in serious trouble or cause serious trouble for others years after you think that you have gotten away with it. If you are growing varieties that you imported into the country on the sly, you should stop growing them, thoroughly destroy the plants, and don’t grow potatoes in the same area for at least several years. You should probably contact your state plant health authorities, but I am trying to be realistic here.
All of that said, if you are reading this, you probably aren’t the problem and may well be part of the solution, inasmuch as there is a solution. The problem with only telling people to follow the rules is that a lot of people don’t know the rules exist, don’t take them seriously because they don’t understand why they exist, or simply don’t care about rules. This is not going to change. People have been breaking the rules for as long as they have existed and will continue to for as long as they do. Social media provides abundant evidence of this. Foreign varieties are going to make their way into private collections and then they are your problem. I’ll be the first to admit that I have not always been aware of, understood, or followed the rules. That has been a progression as I have moved from hobbyist to professional. I have written previously about how I got a surprise visit from the USDA after ordering true potato seeds from Europe. So, this article is partly my penance. This document sets out to help you develop the knowledge necessary to test and respond to viruses if you find them and to hopefully avoid making costly and counter-productive mistakes. If you better understand the scope of the problem, then you can make better decisions and you can help others to be better informed so that they can do the same.
Some viruses are classified as quarantine pests in the USA, with mandatory reporting to the agricultural authorities. This is noted in the descriptions below. The fact that a virus is not designated for quarantine does not mean that it is not potentially serious or that it won’t be added to the list if it appears in the USA and causes problems. If you live in another country, you will have a different list of quarantine viruses.
Potato Viruses in Collections
Potato collectors should have increased awareness of potato viruses, beyond those that are common in the commercial crop. Collectors have unique exposure to viruses for a variety of reasons. Potatoes grown in gardens have much different virus exposure than potatoes grown in monocrop fields. For example, most gardens contain other Solanaceae like tomatoes, peppers, and eggplants, which can be sources of viruses that are not common in potato fields. I have noticed that Tomato Spotted Wilt Virus is fairly common in potatoes from private collections, but I have never seen it from a commercial source. I would guess that it spreads from tomatoes, in which is is a very common virus. Virus carrying weeds that don’t occur in potato fields due to herbicide use may also grow in or near your garden.
Many collectors grow potato varieties that have been maintained as clones for decades without any virus cleanup. There are still plenty of heirloom varieties in the USA that predate plant health regulations. These varieties may carry viruses that have been extirpated in the commercial crop. Viruses like Potato Spindle Tuber Viroid, Potato Aucuba Virus, and Potato Latent Virus have been detected in private collections in recent years even though they have been out of the commercial crop for a long time. Collections also often contain varieties that escaped quarantine. Anyone who has spent much time in social media knows that people routinely carry potatoes across borders. Once they are here and have changed hands, there is no way for you to know if they came in through quarantine or in somebody’s backpack. There must literally be hundreds of such varieties floating around out there. And it isn’t just potatoes; many garden vegetables have some virus overlap with potatoes.
I suspect that there are a lot more exotic potato viruses floating around in collections than anyone realizes. Viruses are usually determined to be extirpated from a region when they no longer show up in the testing results of certification programs. That is a sensible enough approach when you are looking to minimize economic damage, but it means that there could be many exotic and economically unimportant viruses present in the country without any recognition. Collections are rarely tested. Even when they are tested, they are mostly tested for things that are common, because testing is expensive. Exotic viruses probably don’t become a problem because private collections are downstream from commercial cultivation and the commercial crop is grown almost entirely from certified seed, but that doesn’t mean that they won’t become a localized problem for you. So, when you are testing a collection of older varieties or varieties of unknown origin, you should probably cast a wider net.
Eliminating Potato Viruses Through True Seed
It is commonly believed that growing plants from true potato seed (TPS) is a guaranteed way to eliminate viruses. That’s not really true. It is largely true if your exposure is limited to viruses that are common in North America. Of course, growing potatoes from true seed preserves the genetics, but not the phenotype, so this method is no help for preserving particular clones. The really common potato viruses – Potato Leafroll Virus, Potato Mop Top Virus, Potato Virus A, Potato Virus M, Potato Virus S, Potato Virus X, Potato Virus Y, and Tobacco Rattle Virus – are not transmitted through true seed. Exotic viruses may be. The most common example is Potato Spindle Tuber Viroid, which spreads easily through true seed. Some viruses are transmitted from pollen directly to seeds, without infecting the parent plant. This is a really difficult problem, since you could collect seeds from a plant that you have tested and found clean, only to later discover that those seeds were directly infected with a virus. The viruses in the USA that are capable of transmitting via this route are not common in potatoes and are inefficient, but there are viruses elsewhere that both infect potatoes easily and transmit efficiently by the pollen to seed route. Some viruses are able to infect the seed coat and can be difficult to sterilize. This includes Tobacco Mosaic Virus and other Tombamoviruses. Even with standard surface sterilization treatments with bleach or trisodium phosphate, Tobamoviruses can sometimes still transmit at a low rate on seeds.
Many people are under the impression that you can produce virus free true seeds with heat treatment. This advice mostly comes from tomato growers. I don’t know how true this is for tomatoes, but it probably isn’t sufficient for true potato seed. Heat treatment as it is usually described for tomatoes reduces the odds of transmission of bacterial and fungal pathogens on TPS, but does little to eliminate viruses if they are carried in the seed. The incidence of some viruses can be reduced by dry baking seeds, which may be a reasonable precautionary measure if you think you have exposure to viruses that might infect true seed, as long as it is followed up by testing to confirm. In most cases though, you can’t expect to get completely clean seed from this procedure.
Quarantining New Acquisitions
Whenever you acquire new, non-certified varieties from other collectors, there is a pretty high risk that they will carry some kind of virus. Because of this, I quarantine new varieties that I am considering using for breeding or adding to my collection. Viruses spread by direct contact between plants, by insect feeding or potentially even just by insect pollination in some cases, or by contact with soil organisms. To quarantine, I grow plants in pots, on either a concrete or plastic surface with no soil contact. I cover each pot with a frame covered in insect netting and I use a wide spectrum pesticide on the plants. This is the only part of my operation where I use pesticides. The goal is to keep insects off of the aerial plant and to prevent exchange of soil organisms with the environment. I keep the plants like this for the first season, giving plenty of opportunity to observe them for symptoms and to collect foliage for testing. If I encounter something suspicious or can confirm infection by testing, I destroy the plant and sterilize the soil. The soil goes into a plastic barrel with bleach and then sits sealed in the sun for several years. I put the actual plant matter in an incinerator and reduce it to ash. This is probably overkill, but I don’t have a lot of land, so better safe than sorry.
If you have a collection that is substantially free of the common potato diseases, it is worth considering quarantining even certified seed potatoes. Certified seed programs don’t test for uncommon viruses and they have an infection rate as high as 0.5% for some viruses. That translates to 1 in 200 plants. Those are pretty good odds, but not high enough that I would put my collection at risk for something like PLRV or PVY, particularly when quarantining plants doesn’t involve a lot of work or expense.
Remember that viruses typically have multiple host species. If you are protecting a potato collection, you should also quarantine other host plants that are likely to carry potato viruses. I recommend quarantining most clonally propagated plants, particularly other Solanaceae and Andean root and tuber crops. You might also consider any clonally propagated Asteraceae. Unfortunately, the more species that you grow, the more complicated the web of host/virus relationships becomes.
Testing for Potato Viruses
There are several different kinds of tests available for plant viruses. These fall into two categories: serological (immunological) tests and genetic tests. Serological tests are usually virus specific, while genetic tests can be used to test for wider groups of related viruses or even for totally unknown viruses.
The widest selection of tests is available from commercial labs. I have purchased from both Agdia and Nano Diagnostics in the USA with no complaints about either one, but you may be able to get some testing done (and probably at a lower cost) through your state extension or local university.
Serological Tests
Serological tests are based on the use of animal antibodies that were produced by exposure to the plant virus. The antibodies are then exposed to a sample to see if they react. Immunological tests include field tests and ELISA tests. With one notable exception, immunological tests are usually specific to a particular virus, occasionally cross reacting only with other closely related viruses. The exception is the Potyvirus group, which can probably detect every virus in the Potyvirus genus. There are both field and ELISA Potyvirus tests. Because there are a lot of potato Potyviruses, this is a very valuable test and I recommend that everyone screen for Potyviruses.
Field Tests
The simplest, consumer-oriented tests are field test kits. These are usually a dip-stick kind of test, where you grind up some plant material and insert a test strip, which develops a positive or negative indicator. These tests require no skill beyond being able to read and follow instructions. I use a lot of these. They are handy, keep well in the refrigerator, and require no setup, so I am likely to test something suspicious immediately rather than put it off until I have a lab day. Unfortunately, they are only available for the most common viruses and not all of those. These tests cost on the order of $3-$5 per sample, with minimum purchases usually between $50 and $100.
ELISA
ELISA requires some tools and some skill and practice, but if you can follow moderately complex instructions, like making a pastry or wiring a three way switch, you can manage ELISA. You will probably have to spend about $200 on tools. Cost per sample is generally in the range of $1 to $3 in smaller quantities, but the minimum purchase is usually between $150 and $300. There is a much wider selection of ELISA than field tests, but still not everything is covered. If you anticipate doing a lot of testing, it is well worth learning ELISA. If you only want to do the occasional test or doing your own ELISA is too much work, you can have a lab perform the tests for you and the cost is usually pretty reasonable.
Genetic Tests
Genetic tests are based on either amplification of signature regions of RNA or DNA, or on actual gene sequencing. PCR and NAH are based on biochemical amplification. Next generation sequencing (NGS) and high throughput sequencing (HTS) actually examines the gene sequences.
PCR & NAH
The next step is PCR. You will probably not do your own PCR. The tools are more expensive than for ELISA. Even though I have the tools, I don’t find it practical to do my own PCR tests. You will probably outsource these to a lab and the cost per sample for small quantities is usually $50 or more. To warrant this expense, you should have some suspicion that the virus you are interested in is present. Also in this general category is nucleic acid hybridization, which is typically used to test for specific viroids. Potato Spindle Tuber Viroid is a serious concern in potatoes and this is the only reason you would generally be interested in a NAH test. Although these tests are more expensive, you will generally be testing for viruses that aren’t very likely. You can bulk several samples from different plants together. In that case, you will have saved a lot of money if the test comes back negative, but you will need to resubmit individual samples to determine the source if you get a positive.
Sequencing (NGS/HTS)
The final level is next generation / high throughput sequencing, where a lab takes your sample, sequences it, and then basically runs matches against known virus sequences. This has the benefit of being comprehensive, but it is currently outside of the budgets of most collectors. It is currently in the vicinity of $1000/sample. It will have to come down in price by another order of magnitude to be practical for most people. The price is likely to keep coming down though and this will become a very interesting and useful option when it does.
Treating Potatoes to Remove Viruses
Once you know that you have an infected variety, you face the decision of what to do about it. If you have uninfected plants of the same variety or if it is possible to acquire disease free material elsewhere, then the decision is easy: destroy the infected plants. If you are conserving a rare variety and have no source of clean material, then you may want to consider cleaning it. Cleaning potatoes of viruses is a complex procedure that requires some facilities, tools, and skills that you will have to obtain. I will not detail the procedure here, but I will provide an overview.
Removing viruses usually requires either meristem culture or a combination of chemotherapy and thermotherapy. Meristem culture is the process of removing very small sections, less than half a millimeter in size, from the growing tip of the plant and growing them on tissue culture media. The reasons why meristem culture works are complex and not completely understood. Meristems have not yet developed vascular channels, which limits the potential for virus spread only to cell to cell contact and the rapid rate of meristem growth can outpace viral replication. In some cases, the virus primarily infects tissues that have not yet formed in the meristem. RNA silencing may also defend against viruses entering the meristem. A technician with excellent technique, who can obtain the smallest possible tips, can have a very high rate of disease free plants, but some viruses are able to infect at least the lower part of the meristem. In practice, is it usually necessary to follow meristem culture with chemo- and thermotherapy to achieve high rates of virus free plants.
Chemotherapy involves exposing the plant to an antiviral that disrupts the viral replication process. Plant antivirals are usually alternate forms of the RNA bases (adenine, cystine, guanine, and uracil) that the virus will use as building blocks, but that result in a break or error in the final product. For example, the most common plant antiviral, Ribavirin, is able to substitute for guanine or adenine in RNA and can then pair with either cytosine or uracil. Because there are now two possibilities for pairing instead of just one, errors (mutations) are introduced into the RNA strand and, since mutations almost always lead to dysfunction, viral reproduction is inhibited. Although the plant requires the same nucleic acid building blocks, plants have error correction mechanisms, so they are able to survive treatment, though growth rate is reduced.
Thermotherapy involves growing the plants at temperatures that are high enough to interfere with virus replication, but that aren’t quite high enough to kill the plants. The most common temperatures are 42C (107F), although this is often too high for potatoes, and 37C (99F). Thermotherapy also includes cryotherapy, in which shoot tips are exposed to the extreme cold of liquid nitrogen, which is a highly effective technique, but challenging to work with at a small scale. These techniques are usually applied in tissue culture, because the smaller amount of plant material in culture is more sensitive to these therapies. Every virus responds differently. Some are very sensitive to one therapy technique or the other. Some are resistant to both. Sensitive viruses may easily be cleared in one pass. Resistant viruses may only yield a small number of clean plantlets and may require more than one round of therapy.
This can be an expensive and time-consuming process if the viruses that you want to remove are recalcitrant. In that case, you need to culture and treat large numbers of meristems and then test all of them to find the ones that have come through the process clean. If ELISA tests are available for the virus that you are removing, you can probably manage the cost, but if you need to resort to PCR testing, the cost of testing your cultured plantlets could easily run to thousands of dollars. This is usually the case only with rare viruses though. Most common viruses can be tested with ELISA and you can run hundreds of tests for a few hundred dollars. In a best case scenario, the process can take as little as three months. In the worse case, it can take years and you might never be successful.
Where I have successfully cleaned viruses, I note the protocol below in the virus descriptions. These are the main treatments that I use:
| Treatment 0 (establishment) | Culture nodal segments for six weeks on standard M&S + vitamins, 20mg/L sucrose, 2ml/L PPM, 7mg/L salicylic acid (pre-autoclave), 100mg/L ascorbic acid. In this preparatory step, PPM helps to reduce any endogenous contamination and salicylic acid helps the plantlets to later resist stress conditions, particularly thermotherapy. Studies have shown that it is more effective to expose the cultures to SA before the stress event than during. Ascorbic acid is used to moderate browning in explants that have been sterilized and can be omitted when subculturing. |
| Treatment 1 (chemotherapy) | Starting from established culture (T0), undertake a series of three, six week apical bud subcultures on standard M&S + vitamins medium with 20mg/L sucrose and 80mg/L ribavirin (pre-autoclave) at room temperature. Although slow (at least 24 weeks when starting from induction), this has become my preferred treatment for most viruses, as most plants tolerate it well, no refined technique is necessary, and the rate of success is high. Longer subculture periods appear to be even more effective, so if you aren’t in a hurry, it is fine to extend them as long as the plantlets are still growing well. |
| Threatment 2 (chemotherapy + thermotherapy) | Starting from established culture (T0), culture apical buds on standard M&S + vitamins medium with 20mg/L sucrose and 30mg/L ribavirin (pre-autoclave) at 99F (37C) for 4 weeks, then subculture on media with 80mg/L ribavirin at room temperature for six weeks. This is my preferred treatment for viruses that are resistant to Ribavirin. Thermotherapy greatly increases the clearance rate, but also reduces the survival rate. Plantlets that don’t clear can be taken back to Treatment 1 and will usually be clear after that. |
| Treatment 3 (meristem culture + chemotherapy) | Culture meristems from any source on standard M&S + vitamins medium with 20mg/L sucrose and 30mg/L ribavirin (pre-autoclave) at room temperature for six weeks. Meristem culture usually works when nothing else does, but the technique requires a lot of practice, good equipment, good vision, and a steady hand. The clearance rate is high but survival rate is often low. Because of the effort involved and low rate of success, I reserve meristem culture for only the most difficult viruses. |
Alternative Treatments
One of the most common questions that I receive is whether an easier and cheaper way exists to de-virus potatoes. The answer is generally no, but there are exceptions. For the most part, you will probably be wasting effort trying to clean varieties in non-standard ways, but if you read the available research about a given virus, paying close attention to its temperature sensitivity and mode of infection within the plant, you might be able to devise a low tech approach. The following methods are known to work for some viruses:
Tuber Electrotherapy
There are some studies that show promising results with electrotherapy for some viruses, which involves passing a small current at high voltage through sprouts, cuttings, or even tubers. While the virus clearance rate is much lower than for meristem culture, this is technically less demanding, so the lower odds of success may be an acceptable trade. I haven’t seen a great explanation for how electrotherapy works, but it does heat the samples, so it is possible that it is just a faster acting form of thermotherapy. Electrotherapy is generally done with an electrophoresis power supply, set to variable voltage and fixed current generally between 10 and 35 mA, with treatment periods of 5 to 20 minutes. Tubers tend to die at the high end of current and duration, but that is also where the virus reduction is usually greatest. Tolerance appears to differ between varieties, so you will probably need to experiment to determine how much the plants that you are treating can withstand. Probes can be inserted at either end of a tuber and connected directly to the supply, or the tubers can be immersed in electrolyte solution (like salt water) with electrodes at either end of the bath. At the end of treatment, some tubers may be virus free, but you will have to test to confirm. I have found electrotherapy at the mid to high range to be effective for clearing PVY. I have had no success clearing PVS with electrotherapy.
Tuber Selection
Some viruses do not uniformly infect tubers. You could grow all the tubers from an infected plant and then test each plant to see if you get lucky. This approach may work with many viruses in the first year of infection. It generally becomes much less useful in systemically infected plants grown from infected tubers, but some viruses are particularly inefficient and may still turn up some clean tubers. This approach only makes sense when there is a serological test for the virus, since it would get pretty expensive with genetic testing.
Tuber Thermotherapy
Some viruses are so vulnerable to heat that you can apply heat directly to tubers in order to clean them. This usually involves putting the tubers in an incubator for several weeks. Some of the tubers usually will die, but some of those that survive may be clean. Five weeks at about 100 degrees F is probably a good place to start. I have noted in the virus descriptions below when research has determined a specific protocol. You may have to reduce the temperature or duration a bit if the tubers all die. You will have to test the survivors to determine if any of the tubers are clean. You could potentially save yourself some time and money this way, although this approach has not been tested with many viruses and probably works only with those that are unusually vulnerable. This approach has proven ineffective with the common potato viruses PVX and PVY, but it does work with PLRV.
Long Term Cold Storage
Some viruses can be reduced or possibly eliminated by long-term, low temperature storage. Keeping tubers at minimum survivable temperature, about 34 degrees F over the course of two or even three years appears to improve their health. Not all tubers will usually survive such long storage periods, but those that do grow normally. Plants grown from tubers that have received this treatment often appear symptom free, but this is a “common knowledge” practice and I have never found a study that tested this method, so I am not sure whether it truly eliminates viruses or which viruses it might be effective for. It is cheap and easy, so if you don’t have any better options, it seems like it would be worth a try.
Viruses with Potential for Alternative Treatment
Research has shown some success with the treatments listed in this table. That doesn’t mean that they won’t work for other viruses, but you will be breaking new ground if you attempt them.
| Alfalfa Mosaic Virus (AMV) | Tuber selection, tuber thermotherapy |
| Andean Potato Latent Virus (APLV) | Tuber selection |
| Cucumber Mosaic Virus (CMV) | Tuber selection |
| Potato Leafroll Virus (PLRV) | Tuber selection, tuber thermotherapy |
| Potato Mop Top Virus (PMTV) | Tuber selection |
| Potato Virus S (PVS) | Tuber selection |
| Potato Virus Y (PVY) | Tuber selection, tuber electrotherapy |
| Potato Yellow Vein Virus (PYVV) | Tuber selection |
| Tobacco Mosaic Virus (TMV) | Tuber selection |
| Tobacco Rattle Virus (TRV) | Spontaneous recovery, tuber selection |
| Tobacco Streak Virus (TSV) | Tuber selection |
| Tomato Black Ring Virus (TBRV) | Tuber selection, tuber thermotherapy |
| Tomato Spotted Wilt Virus (TSWV) | Tuber selection |
Virus Transmission
Plant viruses are transmitted between plants by one or more of the following means.
Contact Transmission
Contact between an infected and uninfected plant lead to transmission. Usually, contact occurs between foliage, but it is also possible for transmission to occur through root contact in some cases. Contact can also occur through intermediary objects (fomites), particularly with highly transmissible viruses. They can spread on pots, tools, or even the hands of the gardener. The extremely transmissible Tobacco Mosaic Virus (TMV) is known to spread on pots years after being contaminated in some cases. There are cases of smokers transmitting the virus from cigarettes to susceptible plants. Potato Virus X used to be the most widespread potato virus as a consequence of cutting seed tubers with an unsterilized knife, spreading the virus to each seed piece in the process. Not all viruses are able to spread by contact.
Insect and Animal Transmission
Insects that move from infected to uninfected plants may transmit the virus by their feeding activity. Aphids are one of the most well known virus spreading insects. Usually, the virus is transmitted by infected residues from feeding, so the insect does not continue to transmit the virus for very long after leaving the infected plant. In some cases, known as persistent transmission, the insect itself can be infected with the virus. The classic case for this kind of transmission is that if Potato Leafroll Virus (PLRV) by aphids. The aphid actually becomes infected with PLRV and therefore can transmit the virus to plants that it feeds on for the rest of its life. In addition to insects, other animals that feed on plants are able to spread infection by the same means, by chewing infected plants and passing residue to uninfected plants, but animals are not usually considered to be a major source of virus transmission. Most viruses can be spread in at least the non-persistent manner by insects.
Transmission by Fungi and Fungus-like Organisms
Fungi that infect and feed on the tissues of infected plants can transmit viruses in some cases. Similarly to the case with insects, the virus usually does not reproduce in the fungus and is simply moved from one plant to another through fungal structures, including spores. Potato Virus Y, Potato Virus X, Potato Spindle Tuber Viroid, and other viruses are know to be transmitted by the late blight organism in a non-persistent manner. Also as with insects, there are some organisms in the plant virus can replicate, resulting in persistent transmission. The most well known case with potatoes is that of Potato Mop-Top Virus (PMTV), which can be acquired and indefinitely transmitted by the powdery scab organism.
Transmission through Pollen and Seeds
The majority of viruses are not transmitted through seeds or pollen or, if they are, only do so at very low rates. There are exceptions. Some viruses are able to infect seeds and/or pollen efficiently. When pollen is infected with a virus, it then has two possibilities for transmission: horizontally and vertically. In horizontal transmission, when the infected pollen comes into contact with the flower of an uninfected plant, it is able to infect that plant in its current generation. In vertical transmission, the pollen transmits the virus to the seed that is formed when it pollinates the flower, infecting the next generation. Some pollen transmitted viruses can infect the plant both horizontally and vertically.
Virus Resistance
There are several kinds of virus resistance in plants. Tolerance is often the result of the combination of many genes with more limited effect (horizontal resistance) and is more difficult for pathogens to overcome. Resistance is often the result of a single or a few individual genes with strong effect (vertical resistance) and is easier for pathogens to overcome. These are extreme simplifications, but make a good starting point to learn from.
Tolerance
The plant is able to survive infection with the virus and is either asymptomatic or has mild symptoms. Virus replication may be limited compared to plants with no resistance or it may be normal. Because the virus survives, the plant is able to transmit the virus.
Tolerance is a mixed blessing. For potatoes that will be field propagated for long periods of time without being refreshed from a clean source, tolerance means maintaining productivity. Unfortunately, it can also make it more difficult to identify infected plants, resulting in propagation from infected tubers. Unlike outright resistance, tolerance exerts less mutation pressure on the virus, so the virus is less likely to become more pathogenic.
Extreme Resistance
The plant acts to limit the spread of the virus and generally remains asymptomatic with, at most, localized and short-lived areas of infection. Tubers often remain uninfected in plants with extreme resistance. Resistance exerts greater mutation pressure on the virus and so is best coupled with an effort to rogue out infected plants to limit the opportunities for the virus to mutate and overcome resistance.
Hypersensitive Resistance
Cells infected with the virus die, leading to obvious necrotic symptoms. The reaction may be localized, in which case the plant will survive, or it may be systemic, in which case the plant may not survive or may survive but fail to produce. The major benefit of hypersensitivity is that it makes infected plants easy to identify. Although the plants may not pass the virus into the tubers due to the containment provided by the hypersensitive reaction, it is still better to rogue out affected plants, limiting the potential spread of the virus.
Potato Virus Profiles
Virus profiles are based on research but also on my experiences and I am not a plant pathologist, so you certainly shouldn’t consider them authoritative. I have done a lot of testing of varieties that came from private collections though, so I probably have an uncommon perspective. I have seen things that don’t always agree with the literature. The screening recommendations in these profiles are not intended for casual growers, but for people who are maintaining collections for the long term without access to replacement virus-free stock. Because of that, they are considerably more conservative than the standards used in most seed potato certification programs.
I have limited this list to viruses that have been observed to transmit naturally in potatoes. Plant viruses can often be inoculated into additional hosts that they do not have the opportunity to infect naturally. I assume that viruses that infect wild potatoes also can infect domesticated potatoes. 86 viruses and viroids are currently listed, with 49 known to be present in the USA. The number of known potato infecting viruses is likely to increase and probably by a lot. New viruses continue to be discovered, particularly with increasing use of high throughput sequencing. In fact, we are probably now entering the golden age of virus discovery. A recent survey in Peru turned up a large number of new viruses in potato that have not yet been described. The more comprehensive and inexpensive the tools become and the more the search widens, the more viruses we are likely to uncover. If I have missed any viruses that infect potatoes naturally in the field, please let me know.
Present in USA is “yes” if the virus has been documented in any plant in the wild in the USA, “no” if there is no such evidence.
Frequency in Potato (USA) is my assessment of the literature in combination with my experience. “Frequent” viruses are usually those that are common in field grown potatoes and are widespread. “Infrequent” viruses are typically those that are not a problem for agriculture, but tend to turn up on the fringes and may be more likely in collections. “Rare” viruses may be common in a small area and absent elsewhere or widespread but only occasionally present in potatoes. “Unknown” means that, although the virus has been noted to be present in the USA, there is no information available about how often it infects potato. “Unknown/not detected” means that the virus is not known to be present in the USA.
Seed Transmitted is my assessment of whether the variety might be transmitted in true potato seeds. “Confirmed in potato” means that there has been at least one study that conclusively demonstrated infection in TPS. “Possibly in potato” usually means that the virus is known to transmit through seed in other species or belongs to a genus that has members that are known to spread by seed. “Unlikely in potato” means that there is no reason to believe that the virus can spread through seed, but also no study that demonstrates that it does not. “Vegetative only” means that there is good evidence that the virus does not spread through TPS. “Unknown” means that there is not enough evidence for me to even take a guess.
Color: Red indicates that a virus is serious, widespread, or both, and I recommend testing for it if you can. Yellow indicates that a virus is mostly a nuisance, although it may be widespread. I recommend testing for these if you have seen them before, if you are in an area where they are more common, or if you are made of money. Green indicates that a virus is either not particularly damaging or too rare to test for unless you know that you have exposure. Gray indicates that the virus is not known to be present in the USA.
Alfalfa Mosaic Virus (AMV, genus Alfamovirus)
Not a common virus in potatoes from what I can tell. It is one of those that turns up from time to time in well-traveled potatoes. ‘Black Irish’ and ‘Peruanita’ both tested positive here for AMV. The primary symptom is known as calico and produces large blotches of yellow on the leaves, with clear boundaries between the yellowed and normal areas. In some potato varieties, there may also be tuber necrosis, beginning at the proximal end and spreading in storage. Primarily spread by aphids and also by contact or mechanical means. It may also be asymptomatic. A small amount of transmission though seed has been observed in some potato relatives, so it seems likely that this virus infects TPS to some degree. Tubers are infected unreliably, so it may be possible to recover disease free tubers from infected plants. Field tests are available. Not worth screening unless you know it is in your collection. You are at greater risk if you grow potatoes near alfalfa, since alfalfa tends to be heavily infected, even through seed.
Recommended treatment: Treatment 2.
In one study (Kaiser 1980), tubers stored at 98.6 F (37 C) for 5 to 10 weeks were completely clean of AMV.
More info: DPVWeb, EPPO, Ephytia (Pictures), Cropwatch (Pictures), Plantwise
Andean Potato Latent Virus (APLV, genus Tymovirus)
This virus is found in wild and domesticated potatoes in the Andes and is present in some heirloom ulluco varieties, which is the main reason why I no longer grow them. We test every known host here for APLV and have never seen it in a potato or in anything other than those ulluco varieties, so I don’t think it spreads easily. In the Andes, it is primarily spread by contact and mechanical means, but is also reportedly spread by flea beetles. Symptoms range from mild to severe mosaic. Reports are mixed on seed transmission, which was observed to take place at a low rate in the Andes, but was not observed in a greenhouse study. It is also said to infect tubers unreliably, so it may be possible to recover clean plants from tubers of infected plants. This virus is a very good reason not to grow the old ullucos or to keep them absolutely isolated from potatoes if you insist on it. No field tests. You should screen for this if you have potential contact between potatoes and heirloom ullucos. This virus was reclassified as a strain of Eggplant Mosaic Virus (EMV) and then returned to its own species, so you may need to search under both names for information. APLV is a quarantine virus in the USA.
Recommended treatment: Treatment 1.
More info: DPVWeb, EPPO, Plantwise
Andean Potato Mild Mosaic Virus (APMMV, genus Tymovirus)
A virus of limited distribution in the central Andes. This virus was once considered a strain of APLV, but has since been broken out into its own species. See APLV for more details.
More info: EPPO
Andean Potato Mottle Virus (APMoV, genus Comovirus)
Probably not present in North America or at least very rare if so. Causes mild to severe mottle (light and dark patches) and deformation of the leaves. In some cases, this may lead to systemic necrosis. Spreads by plant to plant contact. No field tests. Not worth screening unless you know it is in your collection. APMoV is a quarantine virus in the USA.
More info: DPVWeb (Pictures), EPPO, Plantwise (Pictures)
Arabis Mosaic Virus (ArMV, genus Nepovirus)
This virus is probably very rare in potato, reported only once to date. In potato, it appears to be asymptomatic in some cases, causing mild chlorosis in others. Nothing is known about how it spreads in potato, but Nepoviruses are typically nematode transmitted and sometimes also contact or pollen/seed transmitted as well. Field tests are available, but this is probably not worth screening for unless you know that you have another source of infection. ArMV is a quarantine virus in the USA.
More info: DPVWeb, EPPO, Monger (2020)
Arracacha Virus B (AVB, genus Cheravirus)
This virus is considered economically unimportant in potato, but it is among those that worry me the most because it is symptomless, hard to test for, and would be bad news since it transmits through seed. It infects several Andean tuber crops and is also capable of infecting potatoes. It is normally asymptomatic in potato, oca, and arracacha. The O strain that is somewhat common in oca in the Andes can infect potatoes and is known to spread through TPS. It spreads from pollen to seed without infecting the parent plant. The virus is also transmissible by plant to plant contact. I am not aware of any positive tests in North America. There are no field tests available. There used to be ELISA reagents available for this virus, but they have been off the market for several years now. Unfortunately, I also don’t know of any labs that offer PCR testing for Cheraviruses, so there is really no option for testing, short of sequencing, which is very expensive. It would probably be a good idea to screen for this once if you also grow heirloom ocas or arracachas of unknown provenance, but you may not be able to find someone who can do the testing. AVB is a quarantine virus in the USA.
More info: DPVWeb, EPPO, Plantwise
Bean Common Mosaic Virus (BCMV, genus Potyvirus)
A common virus worldwide that is very rarely found in potatoes. Transmits through pollen and seed in some species, but there is no information about whether this occurs in potato and it seems unlikely. It will be caught by potyvirus screening, which I recommend, even though it is unlikely that you will ever encounter this potyvirus in potatoes. BCMV is a quarantine virus in the USA.
Bean Leaf Roll Virus (BLRV, genus Luteovirus)
A wiespread virus which has only rarely been found in potatoes. Primarily aphid transmitted, persistently in somes species. Symptoms have not been reported for potato. No field tests. Not worth screening unless you know it is in your collection.
Beet Curly Top Virus (BCTV, genus Curtovirus)
I have never seen a positive test for this virus, but it is fairly common in other hosts and occasionally makes it to potato, where it causes green dwarf disease. The symptoms may appear similar to herbicide poisoning. Leaves turn yellow, curl, and dwarf shoots form, before the plant ultimately dies. Spread primarily by leafhoppers. No field tests. Not worth screening unless you know it is in your collection.
More info: DPVWeb, EPPO, Plantwise
Beet Ringspot Virus (BRSV, genus Nepovirus)
There is not a lot of information available about this virus. It has been reported in the USA, but is not common. It may be asymptomatic or may produce rings, leaf necrosis, shoot stunting, and distorted leaves. Spread by nematodes. It has been reported to infect potato pollen but not to spread through seed, although the presence in pollen doesn’t exactly fill me with confidence in that finding. No field tests. Not worth screening unless you know it is in your collection. This may be more properly a strain of Tomato Black Ringspot Virus and is at least capable of recombination with it. The experts are still sorting that out.
More info: EPPO, Harrison (1959)
Beet Western Yellows Virus (BWYV, genus Polerovirus)
This virus is closely related to PLRV, but is more commonly a disease of beets and closely related species. It causes the same general symptoms in potato as PLRV. It probably does not spread in potatoes as easily as PLRV, but it appears to have received relatively little study. Spread by aphids, who transmit it persistently. No field tests. Not worth screening unless you know it is in your collection.
More info: DPVWeb, Duffus (1981)
Bell Pepper Alphaendornavirus (BPEV, genus Alphaendornavirus)
This is a widespread virus in bell peppers, recently detected in potato for the first time in Colombia. Endornaviruses generally do not cause symptoms and nothing has been written about symptoms on potato. Viruses of this type spread vertically, through seed, but probably do not spread plant to plant by other means. There are no tests for this virus and it is probably both rare and non-pathogenic, so not really worth worrying about.
More info: EPPO, Mejia (2020)
Belladonna Mottle Virus (BeMV, genus Tymovirus)
This virus is mostly restricted to Europe and has very rarely been found in potato. Virtually nothing is known about symptoms in potato. No field tests. Not worth screening unless you know it is in your collection.
Capsicum Chlorosis Virus (CaCV, genus Orthotospovirus)
This virus is mostly restricted to Asia and has very rarely been found in potato. No information is available about symptoms. No field tests. Not worth screening unless you know it is in your collection.
More info: EPPO
Cauliflower Mosaic Virus (CaMV, genus Caulimovirus)
This very common and widespread virus has rarely been detected in potato, so it probably isn’t a matter of much concern. No information is available about symptoms. The virus is probably spread by aphids. It is a pararetrovirus, so it can include part or all of its genome in the host genome. No field tests. Not worth screening unless you know it is in your collection.
Cherry Leaf Roll Virus (CLRV, genus Nepovirus)
This virus was detected in Solanum acaule from Peru. Symptoms may include leaf chlorosis and necrotic lesions, but domesticated potatoes were asymptomatic when the virus was introduced to them. The virus is probably transmitted primarily by nematodes, but infected S. acaule through true seed. This virus is fairly common in rhubarb, so that could be a source of infection in mixed gardens. No field tests. Not worth screening unless you know it is in your collection. CLRV is a quarantine virus in the USA.
More info: DPVWeb, EPPO, Crosslin (2010)
Cherry Rasp Leaf Virus (CRLV, genus Cheravirus)
This virus has only been detected in potato in a germplasm collection, so very little is known about it as a potato virus. Present in the USA in plants other than potato. Probably spread by nematode. No field tests. Not worth screening unless you know it is in your collection.
More info: DPVWeb, EPPO, Thompson (2004)
Chilli Leaf Curl Virus (ChiLCV, genus Begomovirus)
This virus is currently restricted to Asia. It is not a common potato pathogen, but it has been found in potatoes in Pakistan, where it causes stunting, apical leaf curl, and leaf distortion. No field tests. Not worth screening unless you know it is in your collection.
More info: EPPO, Mubin (2009)
Chrysanthemum Stunt Viroid (CSVd, genus Pospiviroid)
A widespread viroid, known primarily from ornamental plants. It was recently detected in potato in Russia. It appears to be asymptomatic in potato. Screen for this viroid by testing for the Pospiviroid genus, which will also catch the much more important Potato Spindle Tuber Viroid.
More info: DPVWeb, EPPO, Matsushita (2019)
Colombian Potato Soil-borne Virus (CPSbV, genus Pomovirus)
A recently described virus from Colombia, closely related and similar to Potato Mop Top Virus. Symptoms in potato are unclear, but may be similar to PMTV. Probably spread primarily by soil fungi. No field tests. Not worth screening unless you know it is in your collection.
More info: EPPO, Gil (2016)
Cucumber Mosaic Virus (CMV, genus Cucumovirus)
An extremely common, worldwide virus with the widest host range of any virus. It is not particularly common in potatoes. May be asymptomatic or cause mild to moderate mosaic. It may also cause leaf blistering and wavy margins. I have only seen it once, in the variety ‘Raudar Islenskar’, which appeared asymptomatic to me. Infection rarely reaches the tubers, so healthy plants can probably be recovered from the tubers of infected plants. It is rare enough that I no longer routinely test potatoes for CMV. Primarily insect transmitted, but also by plant to plant contact. Field tests are available. Not worth screening unless you know it is in your collection or perhaps only on acquisition.
More info: DPVWeb, EPPO, Plantwise
Cucurbit Yellow Stunting Disorder Virus (CYSDV, Crinivirus)
This virus is present in some parts of the USA but is very rarely found in potato. No field tests. Not worth screening unless you know it is in your collection.
More info: EPPO
Eggplant Mottled Dwarf Virus (EMDV, genus Nucleorhabdovirus)
Mostly found in the Mediterranean and Middle East. Appears to be incidental in potatoes, causing foliage yellowing, reduced yield, and internal browning of tubers. Spread primarily by leafhoppers. Probably not a concern in North America. No field tests. Not worth screening unless you know it is in your collection.
More info: DPVWeb, EPPO, Plantwise
Groundnut Bud Necrosis Virus (GBMV, genus Orthotospovirus)
A recently described strain of this virus causes leaf spotting, stunting, and stem necrosis of potato in India. Spread primarily by thrips. Little information is available about this virus, but it is probably not a concern outside of India and surrounding countries at this point. No field tests. Not worth screening unless you know it is in your collection.
More info: EPPO, Jain (2004)
Groundnut Ringspot Virus (GRSV, genus Orthotospovirus)
Little has been written about this virus with regard to potatoes, but it does occasionally infect potato. It probably does not infect tubers or at least does not do so uniformly. This virus is present in a few states along the east coast, but is otherwise absent from the USA. No field tests. Not worth screening unless you know it is in your collection.
More info: EPPO
Impatiens Necrotic Spot Virus (INSV, genus Orthotospovirus)
This is a widespread virus with a wide host range, including many vegetables and ornamentals. It appears to be incidental in potatoes, occurring primarily in greenhouses. Field tests are available. It is common in dahlias and I believe that I have observed spread from dahlias to potatoes. Primarily spread by thrips. Not worth screening unless you know it is in your collection. This virus has been reclassified as a strain of Tomato Spotted Wilt Virus, TSWV-I. You will still find that most potato related literature references INSV.
Lucerne Australian Latent Virus (LALV, genus Nepovirus)
Little has been written about this virus and, although it is reported to infect potatoes, it does not appear to ever have become a serious concern. Probably largely asymptomatic in potato. The virus is known only from Australia and New Zealand. Presumably spread by nematodes. May spread through true seed in some species. No field tests. Not worth screening unless you know it is in your collection. LALV is a quarantine virus in the USA.
Papaya Leaf Crumple Virus (PaLCrV, genus Begomovirus)
A very poorly known virus, apparently restricted to Asia. It has rarely been found in potato. No field tests. Not worth screening unless you know it is in your collection.
More info: EPPO
Papaya Mosaic Virus (PapMV, genus Potexvirus)
This virus occurs in the northern Andes and the USA. In the Andes, it occasionally infects potatoes in the field. In the USA, it primarily infects papaya in the Southeast, but it also is (or was) present in some heirloom varieties of mashua, oca, and ulluco. In papayas, it causes the confusingly named “pawpaw disease,” which has nothing to do with North American pawpaw (Asimina triloba). There is little information about symptoms in potato, probably indicating that it is either asymptomatic or causes mild mosaic, as is the case for ulluco. In potato, it has only been detected in small amounts in a field survey in the Andes, so I don’t think it infects potato easily. Attempts to experimentally inoculate plants with this virus were unsuccessful. Probably spreads by plant to plant contact, as is usually the case for potexviruses. Field tests are available. It would be a good idea to screen for this virus if you have grown other Andean tubers near to potatoes, or perhaps if you live in an area with a lot of papaya cultivation, but it is otherwise unlikely. I have tested our entire potato collection for PapMV with no positive results.
Recommended treatment: Treatment 1.
More info: DPVWeb, EPPO, Plantwise, Salazar (2006)
Papaya Ringspot Virus (PRSV, genus Potyvirus)
This virus is present in the USA but has very rarely been found in potato. It will be caught by potyvirus screening, which I recommend, even though it is unlikely that you will ever encounter this potyvirus in potatoes.
Pepino Mosaic Virus (PepMV, genus Potexvirus)
This virus was restricted to the Andes until about 20 years ago, when it began to spread in greenhouse tomatoes worldwide. It may actually be very common in potatoes in the Andes, according to a germplasm survey in which 14% of plants were infected. Symptoms range from nothing to mild mosaic or mottle to systemic necrosis. Spreads primarily by plant to plant contact. Can occur on the seed coat, so TPS should be surface sterilized. One study found .02% seed transmission even after sterilizing infected tomato seeds, which may indicate that the virus can infect the interior of the seed as well. Field tests are not available. Not worth screening unless you know that you have exposure.
More info: DPVWeb (Pictures), EPPO, Plantwise
Potato Aucuba Mosaic Virus (PAMV, genus Potexvirus)
A widespread but rare virus. It was known to infect several varieties in North America, but appears to have been mostly eliminated. It would not be surprising if it lurks in some older varieties in collections. It has also recently been detected infecting the ornamental Solanum jasminoides in the UK. Symptoms are yellow spotting of leaves and tuber necrosis and may appear similar to the symptoms of PMTV. Spreads by aphid and by plant to plant contact. No field tests. Not worth screening unless you know it is in your collection. Two strains of this virus were originally known as Potato Virus F and Potato Virus G.
More info: DPVWeb (Pictures), EPPO
Potato Black Ringspot Virus (PBRSV, genus Nepovirus)
A widespread virus in South America, but apparently not economically important. It also infects oca, although I am not aware of any positive tests in North America. Symptoms are necrotic spotting and yellowing. Probably spreads by plant to plant contact. No field tests. ELISA tests for the closely related TRSV will also detect PBRSV, so I wouldn’t be surprised if the TRSV field tests (which are also immunological) can also detect it. Not worth screening unless you know it is in your collection. There is some uncertainty with this virus. Recent reports have indicated that the TRSV strains infecting potato are actually PBRSV and, therefore, TRSV may not infect potato at all. If reports of TRSV infecting potato in North America are correct, then that would seem to suggest that PBRSV may be more widespread than believed. Also known as Andean Potato Calico Virus. PBRSV is a quarantine virus in the USA.
More info: DPVWeb (Pictures), EPPO (Pictures), Plantwise, Richards (2014)
Potato Latent Virus (PotLV, genus Carlavirus)
An uncommon and apparently mostly or entirely asymptomatic virus that infected several varieties in North America as recently as the late 1990s. It is probably out of the commercial crop, but it wouldn’t be surprising if it still lurks in collections. No field tests. Not worth screening unless you know it is in your collection.
More info: EPPO
Potato Leafroll Virus (PLRV, Polerovirus)
PLRV is one of the worst common potato viruses. If you trade tubers, you are almost certain to get this virus at some point. It is common in seed potatoes and even occurs occasionally in certified tubers. It spreads easily and typically causes varieties to run out within several years. Symptoms begin with mild yellowing and tip reddening and proceed to leaf rolling. In the first year of infection, these symptoms are mostly restricted to the upper part of the plant. Leaves may become brittle and leathery. Tuber net necrosis may also occur. Symptoms in Andean potatoes and some wild potatoes are different, lacking leaf rolling, but instead showing stunting, marginal yellowing of leaves, and a more upright habit.
Most plants are symptomatic and the symptoms are pretty easy to see, so roguing works well as long as you are quick about it. Unfortunately, the major symptom, leaf rolling, may be common in some environments even when the virus is not present. Leaf rolling is a stress reaction in potatoes and it is common in some varieties under drought conditions. Relying on symptoms alone is likely to result in the destruction of many healthy plants. That is not a big deal if you are a monocrop farmer, but it can be very difficult to deal with if you are managing a diverse collection. Unfortunately, there is not a field test available for this one, so you will need to use ELISA to test for it. I found it so surprising that there were no field tests for PLRV that I contacted one of the major suppliers of test kits, Agdia, and asked why. It turns out that the antibody used for detection simply doesn’t work well in the lateral flow type kits used for field tests. This is probably the best reason to learn to perform ELISA tests. Primarily spread by aphids, which are persistently infected. Some tubers may remain uninfected in the first year. I think this is a virus that most collectors should screen for.
Following advice from several sources, I have found that heating tubers to 100 F (37.8 C) for 25 days is sufficient to free them of the virus, although many tubers do not survive the process. In one study (Kaiser 1980), tubers stored at 98.6 F (37 C) for 3 to 6 weeks were completely clean of PLRV, so it may be possible to reduce the number of days and get greater survival.
Recommended treatment: Treatment 2
More info: DPVWeb (Pictures), EPPO, Plantwise (Pictures)
Potato Necrosis Virus (PoNV, genus Alphanecrovirus)
A rare virus, recently described in the UK, and probably not present in North America. No tests available. Not worth screening unless you know it is in your collection.
More info: Monger (2018)
Potato Mop-top Virus (PMTV, genus Pomovirus)
Mop top is a fairly bad virus in potatoes, causing sometimes severe yellowing, significant yield depression, foliage deformation, and tuber damage. Internal brown arcs, known as “spraing,” may occur in the tubers, a symptom that also occurs in TRV. It is transmitted by the powdery scab fungus, which is very common, although the virus itself is not widespread in North America. Once the virus has been introduced to the powdery scab population in your soil, it can survive for years even without any potato planting. One of the common recommendations for mop top control is to permanently abandon infected potato fields. Because of that, if you are a serious collector with limited land, I highly recommend testing any new varieties before you grow them in your soil. Introducing new varieties from cuttings may reduce the chance of inoculating your soil with powdery scab from the tubers. Grow in pots and test before intermingling with your collection. Unfortunately, there is no field test available for PMTV, so you must use ELISA. Additionally, plants may test negative even when the tubers are infected, since infection occurs below ground. You must test plants grown from suspect tubers, rather than testing the parent plant. Not all of the tubers of infected plants are typically infected, so it may be possible to recover infected varieties by growing out multiple tubers and testing them. PMTV is a quarantine virus in the USA. I’m not sure how that works when it is so widespread.
Recommended treatment: Treatment 1.
More info: DPVWeb (Pictures), EPPO, Plantwise
Potato Rough Dwarf Virus (PRDV, genus Carlavirus)
A recently described virus that currently appears to be restricted to South America. No specific tests available, but there are PCR group tests for carlaviruses. Not worth screening unless you know it is in your collection. It is possible that PRDV and PVP are strains of the same virus.
More info: EPPO
Potato Rugose Stunting Virus (PotRSV, genus Torradovirus)
A recently described virus that appears to be restricted to Peru. It has been detected in customs interceptions outside Peru, which is a good reminder of why we shouldn’t carry potatoes across borders. The name is tentative, so it could change. This appears to be the first Torradovirus known to infect potato. Other viruses in the same family are known to transmit through seed.
More info:
Potato Spindle Tuber Viroid (PSTVd, genus Pospiviroid)
Although included here among the viruses, PSTVd is actually a viroid. In fact, it was the first viroid discovered. A virus is essentially a strand of RNA or DNA, encoding a few genes, and packaged in a shell that is able to attach to cells and invade them. Once inside the cell, the virus uses the cellular machinery to build the proteins and nucleic acids as instructed by the viral genes. In contrast, a viroid is nothing more than a short, circular segment of RNA. It doesn’t make proteins because it doesn’t even have genes to make proteins. When a cell becomes infected, it will replicate the viroid RNA. How viroids produce symptoms in plants when they don’t make any proteins was a mystery for quite a while, but it appears that they may produce small, interfering RNA that interrupts normal gene expression in the plant cell. I won’t be surprised if there are more potato-infecting viroids discovered in the near future. There are several closely related viroids in the genus Pospiviroid and surveys of tomato and pepper seeds in international commerce have turned up as many as six different viroids that infect seeds of those crops.
PSTVd is probably the top concern of regulators in the USA and so it should be one of your top concerns as well. You don’t want to be the person who finds out the hard way that PSTVd is present in your collection. Symptoms include stunting and leaf twisting or curling, but the symptoms may be absent in cool weather. Tubers may be unusually pointed at the ends and it has been reported that russeting and tuber color are reduced, so reds become pinks and purples become lighter blue. Eyes may also become more pronounced and there may be interior tuber necrosis. The symptoms are not consistent enough for effective roguing of this viroid, but they may be sufficient to alert you to the possibility that it is present. This viroid spreads easily by contact and by chewing insects. Worse, it infects pollen and true potato seed. It can transmit by pollen to both the pollinated plant directly (horizontal transmission) and to the seeds that result from pollination (vertical transmission). It has also recently been shown that it can infect the late blight organism and be transferred by it to other plants. It has a number of alternative hosts and so could become established easily.
PSTVd has been present in North America for a long time, but appears to have been eliminated from the commercial potato crop. It is probably still present in wild and weedy Solanaceae. It occasionally turns up in collections and breeding programs. The latter may indicate that there are still some infected seeds in gene banks, or it may just be transmission from wild reservoirs. (As of 2021, the US Potato Genebank has reported that some seed stocks that previously tested negative may actually be positive for PSTVd, possibly as a result of testing plants that had not matured long enough to develop detectible levels of the viroid.) Unfortunately, PSTVd requires relatively expensive nucleic acid hybridization or PCR testing to detect, so most people are not going to be able to afford to screen for this routinely. Be on the lookout for the characteristic change in tuber shape to more of a football shaped tuber (unless the variety already has that shape, in which case you’re out of luck). PSTVd is a quarantine virus in the USA.
PSTVd responds poorly to warm temperature thermotherapy, but it has been found to decline at lower temperatures, so growing at a temperature of 46F/8C or less may result in clean plants in time. It should also be vulnerable to conventional chemotherapy.
More info: DPVWeb (Pictures), EPPO, Plantwise
Potato Virus A (PVA, genus Potyvirus)
PVA is a very common mosaic virus that may be asymptomatic, particularly in mild climates. I have seen it often in heirloom varieties and even certified seed tubers and test for it routinely. May be asymptomatic or may cause mild mosaic, but is usually associated with a substantial decrease in yield, as much as 40%. Strains with more severe symptoms exist. Resistance to this virus is conferred by the Rysto gene from Solanum stoloniferum, which is is better known for providing PVY resistance. Primarily spread by aphids. Field tests are available. Collectors should screen for this one using a potyvirus group test.
Fuentes (2021) divides PVA into three strain groups: PVA-A comprised of strains restricted to the Andes, PVA-W comprised of strains found world-wide, and PVA-T comprised of strains found in tamarillo. There are also recombinant strains that cross groups.
Recommended treatment: Treatment 1
More info: DPVWeb (Pictures), EPPO
Potato Virus B (PVB, genus Nepovirus)
A recently discovered virus, infecting potatoes in Peru. It causes calico symptoms. No tests available, so no potential for screening at this point, short of PCR testing for the Nepovirus genus.
More info: EPPO, De Souza (2017b)
Potato Virus H (PVH, genus Carlavirus)
PVH is a recently described virus, restricted to Asia, where it has been found to commonly infect potato and pepino. Take note, pepino collectors – pepinos from China could be infected. It appears to be largely asymptomatic in potatoes, which is common for carlaviruses. It may be asymptomatic or cause mild mottle and leaf curl. It can be spread mechanically. Other vectors are unknown. No specific tests available, but there are PCR group tests for carlaviruses. Not worth screening unless you know it is in your collection.
More info: EPPO
Potato Virus M (PVM, genus Carlavirus)
PVM is said to be a very common potato virus, but I have only seen it once. Symptoms may range from mild mosaic strong mosaic with crinkle and leaf distortion. It may also be asymptomatic, particularly at temperatures above 75 F (24 C). Spread by aphids and plant to plant contact. Resistance to this virus was found in Solanum boliviense and has been introduced to some domesticated varieties. There is evidence that this virus cannot replicate at temperatures higher than 28 C. No field test available. Probably worth screening if you can do your own ELISA, but otherwise I’d say it isn’t worth the additional expense unless you suspect it.
Recommended treatment: Treatment 1
More info: DPVWeb (Pictures), EPPO
Potato Virus P (PVP, genus Carlavirus)
A recently described virus that currently appears to be restricted to South America. No specific tests available, but there are PCR group tests for carlaviruses. Not worth screening unless you know it is in your collection. It is possible that PVP and PRDV are strains of the same virus.
More info: EPPO
Potato Virus S (PVS, genus Carlavirus)
PVS is a very common mosaic virus that is often asymptomatic, or nearly so, particularly in mild climates. Even when asymptomatic, in some varieties it can result in a substantial decline in yield. Modern potato varieties typically show very little decline in yield, so there is relatively little effort to control this virus and it is allowed in most certification programs after the pre-nuclear generation. Symptoms can include leaf rugosity, particularly in the early growth, very mild mosaic, yellowing and/or bronzing of the lower leaves, and wavy leaf margins. It may produce a yield loss of 10 to 20%. In my experience, the symptoms tend to be most visible when the tubers are first sprouting. Co-infection with PVS can intensify the symptoms of other viruses like PVX and PVY and can also reduce resistance to late blight. Some symptoms may be specific to strain. The Andean strain, PVSA, may have more damaging symptoms than the ordinary strain, PVSO, including defoliation and premature senescense. There is also a strain in the EU known as Southern Potato Latent Virus (SoPLV) that may have more severe symptoms.
I have seen PVS often in heirloom varieties and even commonly in certified seed tubers and test for it routinely. I also got a positive test in ad hoc tubers supplied by the USDA potato introduction station. One thing that I have noticed but never seen discussed is a decline in flowering and berrying in varieties infected with this virus. That has probably not been studied since it would only be important in breeding programs. It may be possible to recover uninfected tubers in the year the plant is first infected, particularly if infection occurs later in the season. PVS is primarily spread by plant to plant contact, but also by aphids. I have heard that the Andean strain spreads more easily through aphids, but I don’t know how common that strain is in the USA. Field tests are available. This virus is challenging to clean, so prevention is better than cure. I recommend screening for this virus regularly, particularly if sexual fertility is important to you.
A resistance gene, Ns, is found in some accessions of Solanum tuberosum subsp. andigenum and is located on chromosome eight (Marczewski 2002).
Recommended treatment: Treatment 1
More info: DPVWeb, EPPO, Plantwise (Pictures)
Potato Virus T (PVT, genus Tepovirus)
PVT is native to the Andes, where it infects potatoes and several other Andean tuber crops. It is often asymptomatic, but may produce a mild mottle. Incidence in germplasm collections of potato, mashua, oca, and ulluco is less than 10%, which means that it probably doesn’t spread very easily by contact, but it is also known to spread through true seed and pollen. Transmission occurs from pollen to seed without infecting the parent plant. It can also be spread mechanically but is not known to be spread by insects. While it is not considered an economically important virus in the Andes, viruses that transmit through true seed in potatoes are always quite concerning, since potato germplasm is often preserved as true seed. No field test available. If you have Andean tubers of unknown origin, it would probably be a good idea to screen for this at least once. PVT is a quarantine virus in the USA.
More info: DPVWeb, EPPO, Plantwise
Potato Virus U (PVU, genus Nepovirus)
An uncommon virus in South America, causing leaf necrosis and distortion. Appears to be transmitted by nematodes of the genus Longidorus in addition to contact and mechanical means. No specific tests available, but there are PCR group tests for nepoviruses. Not worth screening unless you know it is in your collection. PVU is a quarantine virus in the USA.
More info: EPPO
Potato Virus V (PVV, genus Potyvirus)
An uncommon potyvirus, found in South America and western Europe. Most varieties react in the hypersensitive fashion, with extensive necrosis. Transmitted by aphids. Not worth screening for specifically, unless you know it is in your collection. However, you should be screening for potyviruses, which will catch this as well. Some recent work suggests that this virus may be more widespread outside the Andes than previously recognized. PVV is a quarantine virus in the USA.
More info: DPVWeb (Pictures), EPPO
Potato Virus X (PVX, genus Potexvirus)
PVX is probably the most common virus infecting uncertified heirloom varieties in the USA. I have seen it often in heirloom varieties and even certified seed tubers and test for it routinely. It can be asymptomatic, particularly in mild climates and when it is the only virus present. When symptoms are present, it is usually a mild mosaic. In combination with other viruses, the symptoms can become severe. In unusually vulnerable varieties, both top necrosis and tuber necrosis may occur. Yield reduction can be as much as 20%. Primarily spread by plant to plant contact or contact with equipment. It may also be spread by chewing insects, by the late blight organism, and by the powdery scab organism. Field tests are available. This virus is challenging to clean, so prevention is better than cure. I recommend screening for this virus regularly.
A resistance gene, Rx, is found in some wild potatoes and has been introduced to domesticated potatoes through crosses. Some cultivars are reported to be highly resistant, appearing healthy and producing normal yields even when heavily infected.
Recommended treatment: Treatment 1
More info: DPVWeb, EPPO, Plantwise
Potato Virus Y (PVY, genus Potyvirus)
PVY is probably the most serious of the common potato viruses in the United States, although you could make a case for Potato Leafroll Virus as well. It spreads easily by aphid and it is common in heirloom varieties and even relatively common in certified seed potatoes. The common symptoms are strong mosaic and rugose (rough) leaf surfaces and leaf crinkling. There are necrotic strains that are even more damaging and the damaging strains are becoming more common because the milder strains were easier to recognize in the field. In addition to leaf necrosis, tubers can develop necrotic rings and internal spots. Brown rings on the surface of the tuber are a pretty certain sign of PVY. Symptoms may be even more severe in plants that carry over infection from tubers, including leaf drop and premature senescence. Yield reductions of half or more are common and plants with this virus will typically run out within a few years. The virus is spread primarily by aphids, but also by plant to plant contact. It can also be spread by the late blight organism. Uninfected tubers can often be recovered in the year of infection, particularly if infection takes place late in the growing season. If you test for nothing else, you should test for PVY. There are immunological tests for the Potyvirus group, which includes PVY, but also PVA and some lesser viruses, so that is the best approach if you don’t need to ID the specific Potyvirus. Happily, PVY does not spread through true seed. Collectors should probably screen for this virus at least once comprehensively and whenever adding new acquisitions.
Several wild species and some Andean domesticated potatoes carry resistance genes for PVY. There are two classes of resistance genes for PVY: Ry genes provide extreme resistance and Ny genes provide hypersensitive resistance. There are several variants of each found in different species. The most widely used of these is the Rysto gene from Solanum stoloniferum. Another resistance gene, Ryadg, is fairly common in Andean domesticated potatoes.
There are six common strains of PVY now in circulation. Ongoing recombination means that more will continue to develop. The general trend in the evolution of this virus is away from strains that have severe foliar symptoms and toward strains that have minimal foliar symptoms but cause more tuber damage and infect tubers more efficiently. This is probably because we have selected against the symptoms that are easiest to detect and rogue out in the field. Whoops. This makes it that much more important to test your varieties than to rely on clear symptoms for diagnosis.
| Strain | Type | Typical Symptoms |
| PVYC | AKA “Potato Virus C” | As PVYO but may not be aphid transmissible, possibly extirpated in North America |
| PVYO | Ordinary | Mottle, rugosity, minimal tuber damage |
| PVYN | Foliage Necrosis | Necrotic spots preceding mottle, minimal tuber damage |
| PVYNTN | Foliage and Tuber Necrosis | Mottle, rugosity, necrosis, tuber necrotic rings and spots |
| PVYN:O | Recombinant of N and O | Milder foliage symptoms but significant tuber necrotic rings and spots |
| PVYN-Wi | “Wilga” | Milder foliage symptoms but significant tuber necrotic rings and spots |
Tuber thermotherapy has been determined in several studies to be ineffective at clearing PVY.
Recommended treatment: Treatment 2
More info: DPVWeb (Pictures), EPPO (Pictures), Plantwatch
Potato Yellow Blotch Virus (PYBV, genus Potyvirus)
A virus recently discovered in Scotland, closely related to PVA. It doesn’t appear to be widespread, although it could be lurking out there, assumed to be one of the other common mosaic potyviruses. Not worth screening for specifically, unless you know it is in your collection. However, you should be screening for potyviruses, which will catch this as well.
More info: Nisbet (2019)
Potato Yellow Dwarf Virus (PYDV, genus Nucleorhabdovirus)
A virus now known only from the NE USA and nearby Canada. Outbreaks once occurred in major potato production areas of the USA and Canada, but not in the past 80 years. Infected plants are smaller, distorted, brittle, and severely yellowed in most cases. Growing tips die back and the plants become bushy and dwarfed. Tuber necrosis often occurs, appearing similar to heat necrosis. The virus is spread persistently by leafhoppers. No field tests available. Not worth screening unless you know it is in your collection, or possibly if you are in New Jersey, where it appears to have its greatest density. I recommend growing new, uncertified acquisitions from the NE USA in quarantine conditions until you have been able to check for symptoms, but I recommend that regardless of origin or potential virus exposure.
More info: DPVWeb, EPPO (Pictures), Plantwise
Potato Yellow Mosaic Virus (PYMV, genus Begomovirus)
An uncommon virus from South America, which has spread to the Caribbean, including Puerto Rico. It appears to be spread by whiteflies. The principal symptom is leaf yellowing. No field tests available. Not worth screening unless you know it is in your collection.
More info: EPPO
Potato Yellow Vein Virus (PYVV, genus Crinivirus)
A common and damaging virus in South America. The main symptom is bright yellow leaf veins. It has yet to spread outside of South America. Symptoms include bright yellow veins leading to generalized yellowing and malformed tubers with protruding eyes. Tubers are reportedly infected inefficiently, so it may be possible to recover healthy plants from the tubers of infected plants. No field or ELISA tests are available. Not worth screening unless you know it is in your collection.
Potato Yellowing Virus (PYV, genus Ilarvirus)
While this virus is only known to be found in potatoes in the Andes, it also infects yacon globally and it has recently been confirmed that the strain that infects yacon is capable of infecting potatoes (Silvestre 2020). The virus is otherwise restricted to South America. It appears not to be a particularly serious virus in potato, with a low rate of spread and mostly latent symptoms, but it can be transmitted in true seed in potato and that is something nobody wants. No field or ELISA tests are available and I am not sure how to advise on testing at this point. You should not grow heirloom yacon varieties of questionable status in close proximity to potatoes. PYV and Fragiaria chiloensis Latent Virus (FClLV) are strains of the same virus and FClLV is endemic to the United States. There is presently no way to differentiate the two without sequencing. PYV is a quarantine virus in the USA.
Red Clover Vein Mosaic Virus (RCVMV, genus Carlavirus)
A common virus in North America, but it has rarely been found in potato. No information is available about symptoms. No field or ELISA tests are available. Not worth screening unless you know it is in your collection.
Solanum Apical Leaf Curl Virus (SALCV, genus Begomovirus)
A poorly known virus limited to South America. Symptoms are similar to PLRV, but may be more severe. No field or ELISA tests are available. Not worth screening unless you know it is in your collection.
More info: Hooker (1983)
Sowbane Mosaic Virus (SoMV, genus Sobemovirus)
A widespread virus, nearly world-wide, but it does not appear to be a major problem. It may be asymptomatic. Other than potatoes, it commonly infects grape and mashua. It is seed transmitted readily in Amaranthaceae, but as far as I can tell, this hasn’t been confirmed in potato. Probably spreads mostly by contact. Leaf miners have been observed to transmit the disease from infected plants. It has been observed that the virus may transmit by contact between infected pollen and leaves. No field, ELISA, or PCR tests are available, so you really can’t screen for this one even if you wanted to.
More info: DPVWeb, EPPO, Plantwise
Tobacco Etch Virus (TEV, genus Potyvirus)
A widespread virus, particularly in peppers and tomatoes. Rare in potatoes and generally only seen where potatoes and more vulnerable Solanaceae are grown close together in enclosed conditions. Symptoms are mottling and mosaic. Probably spread by aphids. Field tests are available. I don’t recommend screening for TEV unless you know that you have exposure.
More info: DPVWeb, EPPO, Plantwise
Tobacco Mosaic Virus (TMV, genus Tobamovirus)
A virus of worldwide distribution with a huge host range. May cause mosaic symptoms in potatoes, but symptoms are usually mild and spread is slow. It may also cause symptoms similar to mop top. It is spread by contact, so roguing should be an effective means of control. It can also be spread by the late blight organism. Although it is not usually a major problem in potatoes, this virus is always worth taking seriously, because it is both very stable and very infectious. It can remain viable for years on surfaces or in plant debris and it can be challenging to control in greenhouses in particular. A 10% bleach solution is effective for disinfecting contaminated tools and surfaces. TMV is commonly transmitted on the seed coat, but not within the seed, so surface sterilization can help with control. Infection does not usually spread to the tubers, so it should be possible to recover healthy plants from the tubers of infected plants. Field tests are available. I don’t recommend regular screening for this one, but it would probably be worth testing any new acquisitions.
Recommended treatment: Treatment 1
More info: DPVWeb, EPPO, Plantwise
Tobacco Necrosis Virus A (TNV-A, genus Alphanecrovirus)
A common virus worldwide, with a wide host range. The two strains of this virus were recently raised to the rank of species. Both infect potato, but there doesn’t appear to be much information available about how they might differ as potato diseases. Infection in potatoes appears to be usually incidentally, although it has caused some problems in potato fields in western Europe. Aerial plants are asymptomatic, but tubers are blemished with raised, sunken, and cracked patches. No field tests or ELISA are available. Not worth screening unless you know it is in your collection.
Tobacco Necrosis Virus D (TNV-D, genus Betanecrovirus)
A common virus worldwide, with a wide host range. The two strains of this virus were recently raised to the rank of species. Both infect potato, but there doesn’t appear to be much information available about how they might differ as potato diseases. Infection in potatoes appears to be usually incidentally, although it has caused some problems in potato fields in western Europe and has recently been found in seed tubers in Colombia. Aerial plants are asymptomatic, but tubers are blemished with raised, sunken, and cracked patches. No field tests or ELISA are available. Not worth screening unless you know it is in your collection.
Tobacco Rattle Virus (TRV, genus Tobravirus)
A common virus, present in much of the northern hemisphere. It has a wide host range including many weedy species. It is not a major virus of potatoes, but can cause severe symptoms, including leaf yellowing and distortion, and spraing of the tubers. The foliage often shows no symptoms. Tubers may also be cracked or otherwise malformed. Transmitted mostly by nematodes, but also by contact with infected plants, debris, or contaminated tools. Because infection occurs underground, symptoms may not appear in infected plants. This virus reportedly infects tubers inefficiently and may disappear from plants over several seasons once they are removed from the source of infection. It is possible that this virus can transmit through true seed in potato, although I am not aware of any studies that demonstrate this. No field tests are available. It seems that you may not be able to avoid this virus if it is locally abundant and it is otherwise unlikely to become a serious problem in potatoes. Probably not worth testing for unless you observe symptoms. TRV is a quarantine virus in the USA.
Recommended treatment: Treatment 2
More info: DPVWeb, EPPO, Plantwise
Tobacco Ringspot Virus (TRSV, genus Nepovirus)
A common virus in North America that has spread to many potato production areas in Europe. It is generally not a particularly serious virus in potatoes, often asymptomatic and slow to spread. However, it transmits in true seed in many species and so probably also does in potato. Field tests are available. While it is not considered to be a major problem, I think that screening for this virus is a good idea, particularly if you are breeding or trading in TPS. You probably get the benefit of also testing for the rarer but more serious PBRSV, which is similar enough to be detected in immunological tests for this virus. There is some uncertainty with this virus. Recent reports have indicated that the TRSV strains infecting potato are actually PBRSV and, therefore, TRSV may not infect potato at all.
More info: DPVWeb, EPPO, Plantwise
Tobacco Streak Virus (TSV, genus Ilarvirus)
A widespread virus with a large range of hosts. It is often asymptomatic, but may cause mosaic and/or necrosis in potato. Although it is common in North America, it is generally not a serious problem in potatoes. It transmits though true seed in some species, so it might in potato as well. Not all of the tubers of infected plants are usually infected, so it may be possible to recover healthy plants from the tubers of infected plants. It is very common in dahlia in my experience, so it is probably a good idea to keep some separation between dahlias and potatoes. There is a potato strain (TSVP) found in Peru, which presumably infects potato more easily. Field tests are available. I don’t think it is worth screening for this virus unless you also have a dahlia collection or otherwise suspect it.
More info: DPVWeb, EPPO, Plantwise
Tobacco Vein Banding Mosaic Virus (TVBMV, genus Potyvirus)
A virus primarily restricted to China, Japan, and Taiwan, but it has also been detected in tobacco crops in the USA. It has been observed to infect potatoes in the field in China, causing mild mosaic and vein banding. The virus can be transmitted mechanically and appears to be spread primarily by aphids. No field test available, but it will probably be caught by Potyvirus group tests.
More info: EPPO, Geng (2014), Reddick (1992)
Tomato Black Ring Virus (TBRV, genus Nepovirus)
A widespread virus in Europe, but with little distribution elsewhere. It can naturally infect potatoes, but it not common. It produces a symptom complex in potatoes known as pseudo-aucuba, which includes yellow spots and rings on the leaves. Yield loss may be as high as 30%. Some tubers of infected plants may be free of the virus. Primarily transmitted by nematodes of the genus Longidorus. This virus infects seed in many species, so probably infects TPS as well. Tests are not widely available, other than PCR testing for the Nepovirus group. Closely related to Beet Ringspot Virus and capable of recombination with it. TBRV is a quarantine virus in the USA.
In one study (Kaiser 1980), tubers stored at 98.6 F (37 C) for 6 to 10 weeks were completely clean of TBRV.
Tomato Chlorosis Virus (ToCV, genus Crinivirus)
An increasingly widespread virus, appearing regionally in North and South America, Europe, Asia, and Africa. It is primarily a disease of tomatoes, but is spread by whiteflies and can be transmitted to potato. Potato symptoms appear to be yellowing and leaf curl. No field tests are available. Not worth screening unless you know it is in your collection.
Tomato Chlorotic Spot Virus (TCSV, genus Orthotospovirus)
This virus is present in a few small areas of the eastern USA, but is rarely found in potato anywhere. No information is available about symptoms in potato. No field tests are available. Not worth screening unless you know it is in your collection.
More info: EPPO, Webster (2015)
Tomato Leaf Curl New Dehli Virus (ToLCNDV, genus Begomovirus)
A recently discovered virus that has spread from Asia to Europe. It is not yet present in North America. It appears to be a greater concern for tomatoes and greenhouse plants, but a strain that infects potato is now widespread in India. Symptoms include severe mosaic with leaf curl and distortion of the new growth. Plants grown from tubers are stunted and show systemic infection. Yield and tuber size are substantially reduced. Primarily spread by whiteflies. Field tests are available. Not worth screening unless you know it is in your collection.
More info: EPPO, Kumar (2015) (Pictures)
Tomato Mosaic Virus (TMoV, genus Tobamovirus)
A very common and widespread tomato virus than can occasionally infect potatoes, causing a mild mosaic or no symptoms. It is seed transmitted in tomato, so possibly also in potato. Dry baking can eliminate or greatly reduce infection in seeds. There is no specific field test for this virus, but it is also detected by field and ELISA tests for Tobacco Mosaic Virus. Not worth screening unless you know it is in your collection.
More info: DPVWeb, EPPO, Plantwise
Tomato Mosaic Havana Virus (ToMHaV, genus Begomovirus)
A virus of limited distribution in the Caribbean and Central America. It has rarely been detected in potato. No information is available about symptoms. Field tests are not available. Not worth screening unless you know it is in your collection.
More info: EPPO
Tomato Mottle Taino Virus (ToMoTV, genus Begomovirus)
A virus probably restricted to Cuba, currently. Symptoms on potato include yellow leaf mottle, leaf distortion, and dwarfing. The virus is probably transmitted primarily by whiteflies. Yield reduction has been estimated between 20 to 50%. Field tests are not available. Not worth screening unless you know it is in your collection.
More info: EPPO, Cordero (2003)
Tomato Planta Macho Viroid (TPMVd, genus Pospiviroid)
Also known as Mexican Papita Viroid. This viroid is common in Mexico but is not known to be present in the USA. It has only rarely been detected in potato. Screening for the Pospiviroid genus will detect it and this would be the normal approach if you are screening for the much more common and damaging Potato Spindle Tuber Viroid.
More info: EPPO
Tomato Ringspot Virus (ToRSV, genus Nepovirus)
A common and widespread virus that has rarely been detected in potatoes. Like most Nepoviruses, this one is probably spread both by nematodes and through seed. Symptoms in potato have not been reported. Field tests are not available. Not worth screening unless you know it is in your collection.
Tomato Severe Rugose Virus (ToSRV, genus Begomovirus)
A virus probably restricted to Brazil, currently. Symptoms on potato include yellow mosaic with leaf distortion. The virus is probably transmitted primarily by whiteflies. Field tests are not available. Not worth screening unless you know it is in your collection.
More info: EPPO, Souza-Dias (2008)
Tomato Spotted Wilt Virus (TSWV, genus Orthotospovirus)
A virus with worldwide distribution and a wide host range. While it infects potatoes naturally, it is generally not a major concern. It is primarily spread by thrips. It is reported to spread through true seed in some plants, but has not been noted in potato. After the common potato field viruses, this is one of the most common viruses that I have found infecting potatoes from private collections. Symptoms on potato range from nothing to mosaic with chlorotic rings to necrotic spots on leaves and tubers and tuber malformation. Tuber size and yield is reduced. Not all of the tubers of infected plants are usually infected, so it may be possible to recover healthy plants from the tubers of infected plants. In addition, infection of sprouts on infected tubers has been reported to proceed slowly, so it may be possible to recover clean plants by removing and growing the young sprouts. Some varieties appear to show resistance to infection. Field tests are available. Collectors should probably screen for this virus, particularly if they are also growing other plants that are more seriously affected, since TSWV can persist in potato tubers.
Recommended treatment: Treatment 1
More info: DPVWeb, EPPO, Plantwise
Tomato Yellow Ring Virus (TYRSV, genus Orthotospovirus)
A virus that is probably restricted to Iran, currently. Also know as Tomato Yellow Fruit Ring Virus. Symptoms on potato may include leaf chlorosis, necrotic spots, and stem necrosis. The virus is probably spread by thrips. Field tests are not available. Not worth screening unless you know it is in your collection.
More info: EPPO, Golnaraghi (2008) (Pictures)
Tomato Yellow Leaf Curl Virus (TYLCV, genus Begomovirus)
A virus with a wide distribution in tomato production areas. It is extremely damaging to tomatoes. Transmitted by whiteflies. Infection in potato is generally incidental and the main concern is that this virus might persist in potatoes to infect more vulnerable crops. Field tests are not available. Not worth screening unless you know it is in your collection.
More info: DPVWeb, EPPO, Plantwise
Tomato Yellow Mosaic Virus (ToYMV, genus Begomovirus)
See Potato Yellow Mosaic Virus above. These viruses are either synonyms or closely related with similar details.
More info: EPPO
Tomato Yellow Vein Streak Virus (ToYVSV, genus Begomovirus)
A virus currently restricted to Brazil and Uruguay, where it primarily infects tomatoes, but is also found occasionally in potato. Potato symptoms include mottle and leaf distortion. Field tests are not available. Not worth screening unless you know it is in your collection.
More info: EPPO
Tomato Zonate Spot Virus (TZSV, genus Orthotospovirus)
A recently described and poorly known virus from China. It appears to be somewhat common in potato fields in Southern China. Symptoms on potato are similar to Tomato Spotted Wilt Virus and include chlorotic and necrotic spots on foliage and leaf tip necrosis. Field tests are not available. Not worth screening unless you know it is in your collection.
More info: EPPO, Huang (2015)
Watermelon Mosaic Virus (WMV, genus Potyvirus)
A widespread virus in the USA and globally, but rarely seen in potatoes. Symptoms on potato have not been reported. Field tests are available, but it isn’t worth screening for this one unless you have some reason to suspect it. Testing for the potyvirus group would be a better bet.
Wild Potato Mosaic Virus (WPMV, genus Potyvirus)
WPMV appears to naturally infect only some wild potato species and not domesticated potatoes, but it probably has the potential to infect domesticated potatoes if it can infect wild potatoes. This virus is unknown in potatoes outside South America, but it appears to commonly infect pepino. I see a lot of people trading pepino varieties of South American origin on the Internet, which almost certainly skirted quarantines, so there may be some risk in that crop. WPMV is related to PVY, which is a pretty damaging virus in potatoes, so there is probably good reason to be careful. Luckily, this is a potyvirus and the potyvirus group is one of the easiest to target with broad testing. If you grow pepinos of unknown origin, you should test them for potyviruses.
More info: EPPO
Zucchini Yellow Mosaic Virus (ZYMV, genus Potyvirus)
A virus that is widespread globally but rarely found in potato. Symptoms on potato have not been reported. Field tests are available, but it isn’t worth screening for this one unless you have some reason to suspect it. Testing for the potyvirus group would be a better bet.
Potato Virus Genera
Basic information about virus genera is presented here to help fill in the gaps. Description of symptoms or methods of transmission are not always known for newly discovered or uncommon viruses. The traits of the genus may help to give some clues about the behavior of lesser known viruses. Links are also provided to sources with more detailed and technical information.
Alfamovirus
Name derivation: Type virus Alfafa Mosaic Virus
The genus Alfamovirus is part of the family Bromoviridae, which includes two other genera that contain potato viruses: Cucumovirus and Ilarvirus. These are RNA viruses that are single stranded and positive sense. Virus replication takes place in the cytoplasm. Transmission is usually mechanical.
There is only one known potato virus in this genus: Alfalfa Mosaic Virus.
Alphaendornavirus
Name derivation: The alpha division of the old Endornavirus genus.
The genus Alphaendornavirus resulted from a division of the prior Endornavirus genus. It belongs to the family Endornaviridae, which includes no other genera that contain potato viruses. These are RNA viruses that are double stranded, positive sense, and unencapsulated. Virus replication takes place in the cytoplasm. Transmission occurs through gametes. Endornaviruses are persistent, asymptomatic viruses, although they do appear to have some impact on flowering, fruiting, and seed germination in some cases. They may also have beneficial features, such as moderating the effects of acute viruses.
There is one known potato virus in this genus: Bell Pepper Alphaendornavirus
More info: Viralzone
Alphanecrovirus
Name derivation: The alpha division of the old Necrovirus genus, named for Tobacco Necrosis Virus.
The genus Alphanecrovirus resulted from a division of the prior Necrovirus genus. It belongs to the family Tombusviridae, which includes no other genera that contain potato viruses. These are RNA viruses that are single stranded and positive sense. Virus replication takes place in the cytoplasm. Transmission can take place by contact or mechanical means or through true seed.
There are two known potato viruses in this genus: Potato Necrosis Virus and Tobacco Necrosis Virus A.
More info: Viralzone
Begomovirus
Name derivation: Type virus Bean Golden Mosaic Virus
The genus Begomovirus is part of the family Geminiviridae, which also includes one other genus that contains potato viruses: Curtovirus. These are DNA viruses, with two separate segments. Virus replication takes place in the nucleus. Transmission usually occurs by whitefly. In the USA, whitefly transmitted diseases are probably more of a problem in greenhouses than in the field, but as the climate warms and potentially becomes more humid, whitefly transmitted diseases may gain ground.
There are quite a few potato viruses in this genus, but none of them are widespread. These include: Potato Yellow Mosaic Virus, Solanum Apical Leaf Curling Virus, Tomato Leaf Curl New Dehli Virus, Tomato Mottle Taino Virus, Tomato Yellow Leaf Curl Virus, Tomato Severe Rugose Virus, Tomato Yellow Mosaic Virus, and Tomato Yellow Vein Streak Virus. Of these, Tomato Leaf Curl New Dehli Virus appears to be the only one that is economically important in potato.
Betanecrovirus
Name derivation: The beta division of the old Necrovirus genus, named for Tobacco Necrosis Virus.
The genus Betanecrovirus resulted from a division of the prior Necrovirus genus. It belongs to the family Tombusviridae, which includes no other genera that contain potato viruses. These are RNA viruses that are single stranded and positive sense. Virus replication takes place in the cytoplasm. Transmission can take place by contact or mechanical means or through true seed.
There is only one known potato virus in this genus: Tobacco Necrosis Virus D.
More info: Viralzone
Carlavirus
Name derivation: Type virus Carnation Latent Virus
The genus Carlavirus is part of the family Betaflexiviridae, which includes one other genus with viruses that infect potatoes: Tepovirus. These are RNA viruses that are single stranded and positive sense. Virus replication takes place in the cytoplasm. Transmission usually occurs by insect feeding.
There are several potato viruses in this genus: Potato Latent Virus, Potato Rough Dwarf Virus, Potato Virus H, Potato Virus M, Potato Virus P, and Potato Virus S. Potato Virus S is a widespread virus in potato that is economically important.
More info: Viralzone
Caulimovirus
Name derivation: Type virus Cauliflower Mosaic Virus
The genus Caulimovirus is part of the family Caulimoviridae, which includes no other genera with viruses that infect potatoes. These are DNA viruses that are double stranded. Virus replication take place in both cytoplasm and nucleus. Caulimoviruses are reverse transcribing viruses and they may result in endogenous viral elements (EVEs) being incorporated into the host genome. Unlike true retroviruses, the virus genome does not need to be incorporated into the host genome in order to function; EVEs appear to be added to the host genome incidentally as part of nuclear DNA replication and repair. Transmission occurs through aphids or by mechanical contact.
There is only one known virus in this genus that infects potatoes: Cauliflower Mosaic Virus.
More info: Viralzone
Cheravirus
Name derivation: Type virus Cherry Rasp Leaf Virus
The genus Cheravirus is part of the family Secoviridae, which includes two other genera that contain potato viruses: Comovirus and Nepovirus. These are RNA viruses that are single stranded and positive sense. Virus replication takes place in the cytoplasm. Transmission usually occurs by nematode feeding or through true seed.
There are two known potato viruses in this genus: Arracacha Virus B (more specifically, the oca strain AVB-O) and Cherry Rasp Leaf Virus.
Comovirus
Name derivation: Type virus Cowpea Mosaic Virus
The genus Comovirus is part of the family Secoviridae, which includes two other genera that contain potato viruses: Cheravirus and Nepovirus. These are RNA viruses that are single stranded and positive sense. Virus replication takes place in the cytoplasm. Transmission usually occurs by beetle or mechanical means.
There is only one known potato virus in this genus: Andean Potato Mottle Virus.
Crinivirus
Name derivation: Virus structure, crini = hairlike
The genus Crinivirus is part of the family Closteroviridae, which includes no other genera that contain potato viruses. These are RNA viruses that are single stranded and positive sense. Virus replication takes place in the cytoplasm. Transmission usually occurs by whitefly.
There are two known potato viruses in this genus: Potato Yellow Vein Virus and Tomato Chlorosis Virus.
More info: Viralzone
Cucumovirus
Name derivation: Type virus Cucumber Mosaic Virus
The genus Cucumovirus is part of the family Bromoviridae, which includes two other genera that contain potato viruses: Alfamovirus and Ilarvirus. These are RNA viruses that are single stranded and positive sense. Virus replication takes place in the cytoplasm. Transmission generally takes place by contact or mechanical means.
There is only one known potato virus in this genus: Cucumber Mosaic Virus.
Curtovirus
Name derivation: Type virus Beet Curly Top Virus
The genus Curtovirus is part of the family Geminiviridate, which also includes one other genus that contains potato viruses: Begomovirus. These are DNA viruses that are single stranded. Virus replication takes place in the nucleus. Transmission generally occurs by leafhopper.
There is only one known potato virus in this genus: Beet Curly Top Virus.
Ilarvirus
Name derivation: Symptom, isometric labile ringspot
The genus Ilarvirus is part of the family Bromoviridae, which includes two other genera that contain potato viruses: Alfamovirus and Cucumovirus. These are RNA viruses that are single stranded and positive sense. Virus replication takes place in the cytoplasm. Transmission generally takes place by contact or mechanical means.
There are two known potato viruses in this genus: Potato Yellowing Virus and Tobacco Streak Virus.
Nepovirus
Name derivation: Descriptive, nematode-transmitted polyhedral virus
The genus Nepovirus is part of the family Secoviridae, which includes two other genera that contain potato viruses: Cheravirus and Comovirus. These are RNA viruses that are single stranded and positive sense. Virus replication takes place in the cytoplasm. Transmission occurs through nematodes, mites, and thrips. Several viruses in this group are known to transmit through true seed in potato as well.
There are several known potato viruses in this genus: Beet Ringspot Virus, Cherry Leaf Roll Virus, Lucerne Australian Latent Virus, Potato Black Ringspot Virus, Potato Virus B, Potato Virus U, Tobacco Ringspot Virus, and Tomato Black Ring Virus.
Nucleorhabdovirus
Name derivation: Location and structure, reproduces in the nucleus, rhabdo = rod-shaped
The genus Nucleorhabdovirus is part of the family Rhabdoviridae, which includes no other genera that contain potato viruses. These are RNA viruses that are single stranded and negative sense. Virus replication takes place in the nucleus. Transmission occurs by planthoppers.
There are two known potato viruses in this genus: Eggplant Mottled Dwarf Virus and Potato Yellow Dwarf Virus.
Orthotospovirus
Name derivation: Type virus Tomato Spotted Wilt Virus
The genus Orthotospovirus is part of the family Bunyaviridae, which includes no other genera that contain potato viruses. These are RNA viruses that are single stranded and negative sense or ambisense. Virus replication takes place in the cytoplasm. Transmission occurs mostly by thrips.
There are several known potato viruses in this genus: Groundnut Bud Necrosis Virus, Impatiens Necrotic Spot Virus, Tomato Yellow Fruit Ring Virus, and Tomato Spotted Wilt Virus.
More info: Viralzone
Polerovirus
Name derivation: Type virus Potato Leafroll Virus
The genus Polerovirus is part of the family Luteoviridae, which includes no other genera that contain potato viruses. These are RNA viruses that are single stranded. Virus replication takes place in the cytoplasm. Transmission takes place mechnically or by aphid.
There are two known potato viruses in this genus: Beet Western Yellows Virus and Potato Leafroll Virus.
More info: Viralzone
Pomovirus
Name derivation: Type virus Potato Mop-top Virus
The genus Pomovirus is part of the family Virgaviridae, which includes two other genera that contain potato viruses: Tobamovirus and Tobravirus. These are RNA viruses that are single stranded and positive sense. Virus replication takes place in the cytoplasm. Transmission occurs primarily through a fungus.
There are two or three known potato viruses in this genus: Colombian Potato Soil-borne Virus, Potato Mop Top Virus, and possibly Soil Borne Virus 2, which is too poorly described as yet to include above.
Pospiviroid
Name derivation: Type virus Potato Spindle Tuber Viroid
The genus Pospiviroid is part of the family Pospiviroidae, which includes one other genus that contains potato viroids: Hostuviroid Like all viroids, these are small, circular strands of RNA with no protein encapsulation. Viroid replication takes place in the nucleus. Transmission takes place by contact, mechanical means, or through aphids. Viroid replication is more efficient in warm climates and symptoms are more severe.
There are three known potato viroids in this genus: Potato Spindle Tuber Viroid, Chrysanthemum Stunt Viroid, and Tomato Planta Macho Viroid. Only PSTVd is considered economically important in potato. In addition to these three viroids, at least three more have been determined to infect potato under experimental inoculation: Citrus Exocortis Viroid (CEVd) and Columnea Latent Viroid (CLVd) in this genus, and Hop Stunt Viroid (HSVd) in the genus Hostuviroid.
More info: Viralzone
Potexvirus
Name derivation: Type virus Potato X Virus
The genus Potexvirus is part of the family Alphaflexiviridae, which includes no other genera that contain potato viruses. These are RNA viruses that are single stranded and positive sense. Virus replication takes place in the cytoplasm. Transmission takes place by contact, mechanical means, or by insect. Potexviruses can be extraordinarily infectious, spreading mechanically through contact as minor as bumblebee foraging and liquid fertilizer recirculation. They are very difficult to manage under greenhouse conditions.
There are several known potato viruses in this genus: Papaya Mosaic Virus, Pepino Mosaic Virus, Potato Aucuba Mosaic Virus, and Potato Virus X. Potato virus X is an economically important virus.
More info: Viralzone
Potyvirus
Name derivation: Type virus Potato Y Virus
The genus Potyvirus is part of the family Potyviridae, which includes no other genera that contain potato viruses. These are RNA viruses that are single stranded and positive sense. Virus replication appears to mostly take place in the cytoplasm, but may also occur within the nucleus or organelles. Transmission takes place by mechanical means or by insect.
There are quite a few known potato viruses in this genus: Potato Virus A, Potato Virus V, Potato Virus Y, Potato Yellow Blotch Virus, Tobacco Etch Virus, Tobacco Vein Banding Mosaic Virus, and Wild Potato Mosaic Virus. Potato Virus Y may be the most economically important potato virus globally.
Sobemovirus
Name derivation: Type virus Southern Bean Mosaic Virus
The genus Sobemovirus is not currently assigned to a family, although it is noted to have similarities to the family Tombusviridae. These are RNA viruses that are single stranded and positive sense. Virus replication takes place in the cytoplasm. Transmission occurs by insect or through true seed.
There is only one known potato virus in this genus: Sowbane Mosaic Virus.
More info: Viralzone
Tepovirus
Name derivation: Type virus T Potato Virus
The genus Tepovirus is part of the family Betaflexiviridae, which includes one other genus that contains potato viruses: Carlavirus. These are RNA viruses that are single stranded and positive sense. Virus replication takes place in the cytoplasm. Transmission takes place by mechical means or through pollen and true seeds.
There is only one known potato virus in this genus: Potato Virus T.
More info: Viralzone
Tobamovirus
Name derivation: Type virus Tobacco Mosaic Virus
The genus Tobamovirus is part of the family Virgaviridae, which includes two other genera that contain potato viruses: Pomovirus and Tobravirus. These are RNA viruses that are single stranded and positive sense. Virus replication takes place in the cytoplasm. Transmission occurs by mechanical means.
There are two known potato viruses in this genus: Tobacco Mosaic Virus and Tomato Mosaic Virus.
Tobravirus
Name derivation: Type virus Tobacco Rattle Virus
The genus Tobravirus is part of the family Virgaviridae, which includes two other genera that contain potato viruses: Pomovirus and Tobamovirus. These are RNA viruses that are single stranded and positive sense. Virus replication takes place in the cytoplasm. Transmission occurs through nematodes or by mechanical means.
There is only one known potato virus in this genus: Tobacco Rattle Virus.
Torradovirus
Name derivation: From Torrado Disease, caused by Tomato Torrado Virus.
The genus Torradovirus is part of the family Secoviridae, which includes three other genera that infect potatoes: Comovirus, Cheravirus, and Nepovirus. These are RNA viruses that are single stranded and positive sense. Virus replication takes place in the cytoplasm. Transmission occurs mechanically, by insects or nematodes and most commonly by whitefly.
There is only one known potato virus in this genus: Potato Rugose Stunting Virus.
More info:
Tymovirus
Name derivation: Type virus Turnip Yellow Mosaic Virus
The genus Tymovirus is part of the family Tymoviridae, which includes no other genera that contain potato viruses. These are RNA viruses that are single stranded and positive sense. Virus replication takes place in the cytoplasm. Transmission takes place by contact, mechanical means, or beetles.
There are two known, closely related potato viruses in this genus: Andean Potato Latent Virus and Andean Potato Mild Mosaic Virus.
More info: Viralzone
William, I am lost for words. After having read this and doing my best to understand even the most basic things you are talking about, I marvel at your knowledge. As a retired professor for over 40 years I understand the amount of time you have put into your research and education regarding potatoes and viruses. Believe me, you are a professor and could be teaching in college with your incredible knowledge. Your writing style is clear and it has been my pleasure to read this and hopefully after several more readings I will understand some of what you are reporting. I was never good at science having majored in philosophy and literature. Peace and blessings to you. Jean
Thanks Jean. There is a lot to digest in this article and it is really not meant to be taken all at once. Writing is a big part of my learning process, so it is possible that some of this may not be as valuable for others to read as it was for me to write.