If you’re an organic farmer and you keep bees, or have a friend who keeps bees, then breeding disease resistant potatoes is easy.
Just get seed tubers of about twenty good potato varieties, and plant a row of each variety. If necessary, protect against Colorado beetle with Bt or natural pyrethrum, and protect against blight with copper.
The object is to harvest the true fruits, which are like small tomatoes, and which contain true seeds that are the result of pollination.
The bees will ensure that the potato flowers get mostly cross-pollinated — some varieties may not flower, or may not set fruit, but if half or more of them have fruits, that will do.
You should aim to harvest 50–100 fruits. Each fruit will contain up to 300 true seeds. And every true seed will be genetically different from every other seed.
Collecting the seeds
When the fruits are ripe (soft like a ripe tomato) harvest them, keeping the fruits of each variety separate.
Put the fruits in a kitchen blender, cover with tomato juice or water, and blend just enough to break up the fruits and liberate the seeds. Again, keep the seeds of each variety separate from the others.
Leave the seed mixtures in plastic containers, and let them ferment for twenty four hours. The seeds will sink and the fruit debris will float. With several washings, you will have clean seeds.
Drain them through a coffee filter and spread them out to dry. Don’t expose them to heat!
Store the seeds in your refrigerator, in air-tight jars with silica gel. They will keep for several years if necessary.
The screening process
Come spring, prepare a suitable size plot (up to half an acre) to a fine, weed-free tilth by repeated rotavating.
Using a small hand seed drill, sow the seeds from each of the original varieties in separate rows.
It’s only necessary to keep the varieties separate in the first breeding cycle, to ensure a suitably wide genetic base. In all subsequent breeding cycles, you will work with a single screening population, and a single gene-pool, and you won’t have to separate varieties into different rows.
Let the blight and the beetle (and everything else) do their worst. The most susceptible seedlings will die and disappear.
Mark the most resistant with a prominently painted stick. These may look quite susceptible in the early breeding cycles but that’s O.K. — they will be more resistant than the others.
Remove all of the others before they flower, because they might otherwise produce undesirable pollen. Leave the bees to cross-pollinate all the selected plants.
Harvest the fruits, aiming again for 50–100 of them. Mix them together and prepare the seeds. These will become the parents of the second breeding cycle.
Store the seeds. Come spring, sow them. Repeat the whole process for several years.
Developing disease resistance
You will be amazed at how quickly resistance to both beetle and blight will accumulate.
This happens because of a genetic phenomenon called transgressive segregation. It produces the so-called horizontal resistance, which is controlled by many genes of small effect (polygenes).
This kind of resistance is durable. It does not break down to new races of the blight fungus in the way that single-gene (so-called vertical) resistances do.
Because horizontal resistance is durable, a good variety need never be replaced, except with a better variety. This means that breeding for horizontal resistance is cumulative and progressive until an eventual ceiling is reached.
That ceiling is called perfection.
Any plant that looks really good may be kept and propagated vegetatively as a potential new variety.
Remember that potato plants grown from true seeds produce fewer and smaller tubers than those grown from seed tubers. Don’t be put off by these low yields of seedlings.
But a word of warning. Some of your original parent varieties may be carrying single genes for vertical resistance to blight. This resistance is useless because it will fail quite quickly, now that both mating types of the blight fungus are present in North America.
So, if you see a plant that appears to be completely free of blight, check it carefully. If it shows necrotic flecks that are just visible to the naked eye, it has a functioning vertical resistance. Weed it out to prevent it from producing undesirable pollen that is carrying detrimental genes.
There are no single-gene resistances to Colorado beetle or to any other North American pests and diseases of potatoes.
So all of your resistances will be durable resistances.