Are you an organic farmer? Do you keep bees? If not, do you have a friend who keeps bees? If you are, and do, breeding potatoes is easy.
Get seed tubers of about twenty good potato varieties (or varieties you want to cross and improve). Plant a row of each variety, with about
ten plants in each row. Put a beehive admidst the rows.
If Colorado beetles are likely to devour the plants before they set fruit, either hand-pick off the egg custers and beetles or protect with Bt or natural pyrethrum. Protect against blight with copper. The object is to harvest the true fruits that look like small tomatoes. These contain true seeds which are the result of pollination. The bees will increase cross-pollination. Avoid insecticides that will kill the bees during flowering.
Some varieties may not flower, or may not set fruit, but if about half of them have fruits, that will do. Aim to harvest 50-100 fruits. Each fruit will contain up to 300 true seeds. Every true seed will be genetically different from every other seed.
When the fruits are ripe and soft like a ripe tomato, harvest them. Keep the fruits of each variety separate. Put the fruits in a kitchen blender, cover with water, and blend just enough to break up the fruits and liberate the seeds.
Leave this mixture in a plastic bowl to 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 on a paper towel to dry. Store them in an air-tight jar with silica gel in your refrigerator. They will keep for several years if necessary.
Come spring, prepare a plot up to half an acre to a fine, weed-free tilth by repeated rotovating. Using a small hand seed drill, sow the seeds from each of the original varieties in separate rows. (This is necessary in the first breeding cycle only, 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). Let the blight and the beetle 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 all the others. In the early breeding cycles they will be very susceptible. It may be necessary to protect them with a fungicide and insecticide to ensure their survival. Pull out all the others because they might produce undesirable pollen. Leave the bees to cross-pollinate all the selected plants. Harvest the fruits, mix them, and prepare the seeds. These will become the parents of the second breeding cycle. Aim once again for 50-100 fruits. Store the seeds. Come spring, sow them. Repeat the whole process each year for several years, discarding any plants that have poor tubers.
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 quantitative and 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 of perfection is reached.
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. Do not be put off by these low yields of seedlings.
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 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 your resistances will be durable resistances.