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Potato Fertility Scores
These scores are meant to give you some idea, at a glance, of how easy a potato variety is to use for breeding/seed production. Tests are performed outdoors in a mild climate, so they will not necessarily hold up under more difficult conditions, but they should usually have relative value. In harsh climates, the heat or drought tolerance of the variety may be more important than its fertility, but a low fertility variety is never likely to be more fertile in a difficult climate. I sometimes repeat these tests and always report the best result obtained. These scores lack one important factor, which is the number of seeds produced per berry. After all, who cares how many berries you get if they have no seeds? If a variety has unusually low seed counts, that will be noted in the description.
Female Fertility
The female fertility score is the number of mature berries obtained when 10 freshly opened flowers of the variety are pollinated with a heavy coating of mixed pollen. This tells us both how easily a plant forms berries and how well it holds them. In some cases, I note a female fertility of <1, which means that the test gave an FF0, but I have strong evidence that the variety does produce berries. True FF0 varieties are rare outside of wild species hybrids like S. juzepczukii, but varieties that produce no flowers are scored as FF0.
Male Fertility
The male fertility score is the number of mature berries obtained when 10 emasculated flowers of a FF10 variety are pollinated with the pollen obtained from 10 flowers. This reflects both how much pollen a variety produces and how viable it is, which is perhaps not the most satisfying metric, but it is one that is easy to measure. Many varieties that have low MF scores have perfectly viable pollen but don’t produce much. Conversely, you can often assume that a variety that produces a lot of pollen is very fertile, but this isn’t always the case. There are varieties that produce a lot of pollen that is mostly non-viable. MF0 varieties are either male sterile or nearly so. Varieties that produce no flowers are also scored as MF0.