Organization for More Effective Breeding

Because of the greater need to get varieties adapted to all regions of the country, the Department has undertaken research at the Cheyenne Horticultural Field Station in Wyoming, the laboratory at Charleston, and a number of other field stations and laboratories.

Since 1938 the workers at these institutions have been testing a large group of wild tomatoes and related species from South America in addition to unfamiliar varieties from other parts of the world. From this group of varieties and species they have isolated original sources of resistance to several important diseases including collar rot (Alternaria solani), gray leaf spot, (Stemphylium solani), septoria leaf spot, (Septoria lycopersici), and mosaic. They have also found new sources of resistance to fusarium wilt, leaf mold, and root knot. Through information from other sources, stocks of tomatoes with resistance to anthracnose (Colletotrichum phomoides) have also been accumulated. With few exceptions, the original resistant sources have been commercially worthless types. Long breeding programs involving carefully designed cross pollinations, selection for many generations, and repeated disease tests are necessary before the new disease-resistant varieties will approach acceptable size and quality.

The foliage disease called early blight, caused by the fungus Alternaria solani, is considered by tomato growers and research men to be one of the most generally harmful diseases of tomatoes, perhaps second only to fusarium wilt. It attacks both stems and foliage of tomato plants. When this fungus infects young tomato stems at the soil line, it produces a black lesion or canker known as collar rot, which either stunts the growth of the plant or causes it to break off entirely. The foliage infections of early blight cause spotting, withering, and dropping of leaves when humid weather favors the spread and growth of the fungus.

Starting in 1938, the workers at the laboratory at Charleston vigorously attacked the problem of breeding early-blight-resistant tomatoes. While no tomatoes were found to be outstandingly resistant to this disease, two very hopeful factors emerged. One is that, in many localities, early blight often has been confused with damage caused by other diseases, and the development of resistance to other foliage diseases such as septoria leaf spot, gray leaf spot, and leaf mold, will definitely reduce losses that formerly might have been entirely attributed to early blight. The second factor is the discovery of a high degree of resistance to the collar rot or stem canker phase of infection by Alternaria solani and the knowledge that tomatoes resistant to the stem canker phase, while not highly resistant to leaf infection, are significantly less susceptible to it.

Resistance to collar rot was found in several wild types from South America, in a number of western European forcing varieties, and in a few of the less familiar domestic varieties. The domestic varieties contributed size and the European forcing varieties productivity, in crosses with Rutgers and Marglobe, and the collar rot problem was found to be relatively easy of solution. Several new tomato lines of commercial grade have been produced, among which probably could be found one or more with a range of adaptation equivalent to that of such popular varieties as Rutgers and Marglobe. However, we believe resistance to collar rot in a new variety is no longer sufficient in itself because it is evident that new varieties should also possess Pan America's resistance to the fusarium wilt fungus.

Fortunately, the combination of resistance to collar rot and Pan America's kind of resistance to fusarium wilt also proved to be relatively simple to accomplish. A large number of tomatoes resistant to both diseases have been produced, but none has yet been released as a new variety. After extensive field trials, one or more of these tomatoes will probably be found to be qualified for commercial production.