Breeding Healthy Potatoes

by F. J. STEVENSON and ROBERT V. AKELEY

OUR standard kinds of potatoes are hard to beat in yield and quality. Our problem is how to overcome diseases and insects.

The diseases are commonly caused by fungi, viruses, and bacteria. Fungous diseases include late blight, early blight, and common scab. The worst of the virus diseases are mild mosaic, latent mosaic, rugose mosaic, leaf roll, net necrosis, yellow dwarf, and spindle tuber. Of the bacterial diseases, the most troublesome are ring rot, brown rot, and black leg.

Any one of them could eliminate the potato as a leading food crop of North America. We have control measures that allow us to grow even the most susceptible varieties, but the cost of the preventives and cures amounts to many millions of dollars a year, and they have by no means eliminated all losses. If a small fraction of the expense and effort now consumed in fighting disease had been put into the production of disease-resistant varieties, potato growing would be much more profitable.

State experiment stations and the Department organized the national potato-breeding program in 1929 and have distributed to growers 25 new varieties of potatoes. Some of them resist late blight, or virus diseases, or common scab. Much has been done, but there is much more to do.

Potato plants are propagated asexually and sexually. In commercial practice, the new crop is grown from tubers or pieces of tubers. That is a vegetative or asexual method of reproduction. A variety can be grown vegetatively for years without visible change, and any attempt to make a selection within it is usually futile. Occasionally bud mutations occur, and a few new potato varieties have been obtained by selecting bud sports. When such changes do occur they are for the most part of minor importance, because most of them have been color changes, for example, from red tubers to "splashed" or to white. Changes from white to red or from white to russet have also been known to occur. Even though only a few varieties have come as the result of bud mutations, this is still a source of variation that the potato breeder cannot ignore. But because it is quite impractical to make much improvement by selecting tubers of a variety with the hope of getting something new, the plant breeder must use true seed and seedlings to get more variations and combinations of characters.

True seed is the result of the fusion of male and female gametes, or germ cells; that is the first step in sexual reproduction. The seed is found in the fruits or seed balls that grow on the potato vines and look very much like small tomatoes. A seed ball may contain 200 or more seeds which will produce plants that are quite different from each other. Most seedlings are undesirable from the commercial standpoint, but occasionally a new variety is produced that excels the old in one or more important characters. Many people who live in the South or where the climate is hot and dry have never seen a seed ball growing on a potato plant, and not a few are quite surprised when they first discover one. In cool, long-day climates, on the other hand, some sorts produce many seed balls.

The first potato-breeding work in this country started nearly 100 years ago, when late blight was destroying potato crops in many European countries and causing large losses in this country. Blight was the cause of the famine in Ireland in 1845, which, because of the failure of the potato crop, brought sickness and distress to many people and caused the death of at least a million persons. The cause of the disease was not known then, but many persons thought it was the result of a loss of vigor in the plants, brought about by growing the crop year after year from tubers. They also believed that the vigor could be restored and the disease eliminated by growing plants from true seed.

Acting upon this belief, a number of people in Europe and the United States began growing seedlings. As a result, many new varieties -were produced. A few of them are among the best varieties we have today.

The blight problem remained unsolved, however, because the trouble was not a result of the loss of vigor but was due to a parasitic fungus that attacks the leaves and stems of the potato plants and often kills them before they have a chance to mature. Susceptible seedlings are just as severely attacked as the commonly grown susceptible varieties.

In 1910, when potato breeding was actively undertaken by the Department, resistance to late blight was still considered the most important character for which to work, but the project was just under way when the virus diseases took first place in the breeding work, and blight resistance was once more thrown into the background. It was not until 1932 that Department workers could again attend to the problem.

Varieties of potatoes showing different degrees of resistance to blight were introduced from foreign countries and used as parents of new seed-lines. A few seedlings with an intermediate type of resistance were produced by crossing two susceptible American varieties. A much larger proportion of intermediates was obtained when moderately resistant varieties, like Ekishirazu from Japan or President from Holland, were used as parents. Selections from the so-called W races, which were introduced from Germany, showed resistance to late blight, but were low in yield and market quality.