More than 30,000 tomato plants are tested each year at Hurricane, Utah, where the curly top disease regularly occurs in severe form. From the vast population of tomato lines tested, only three wild species, Lycopersicon glandulosum, L. peruvianum var. dentatum, and L. peruvianum var. humifusum have exhibited any very marked resistance to the disease. These species have been crossed with varieties stemming from L. esculentum in many combinations of backcrosses and outcrosses. Some progress has already been made in the development of commercial varieties that are resistant to curly top but much work yet remains to be done before a good resistant variety can be released for commercial use.

Verticillium wilt is known to be a cool climate disease as contrasted with fusarium wilt, which thrives in relatively high temperatures. Therefore, verticillium wilt causes severe damage to tomato crops principally in those Western States where the growing season is generally cool.

Since verticillium wilt is caused by a soil-borne fungus, the best solution to the problem is the development of wilt-resistant varieties. Wilt-resistant wild tomatoes from South America have been successfully crossed with commercial varieties and lines have been selected from the progenies possessing a degree of resistance superior to that of the wild parent. Some of these lines produce fruit of excellent size and color but field trials have indicated that these selections are too late in maturity and the yields are too low. The best of these lines are being outcrossed to free-fruiting earlier varieties in an effort to combine earliness and good fruit production with disease resistance.

At the Cheyenne Horticultural Field Station, breeders are developing tomato varieties with capacity to set a good crop of fruit under adverse climatic conditions that prevail in certain areas in the West and Southwest. They are also cross breeding to incorporate in these varieties resistance to bacterial canker, leaf mold, tobacco mosaic, and fusarium wilt.

The Department is by no means alone in the program of breeding tomatoes for disease resistance. Many State or Territorial experiment stations have active tomato breeding projects with disease resistance an important goal. An outstanding example of success in developing multiple disease resistance and superior yield for unusually difficult growing conditions is the work of the Hawaii Agricultural Experiment Station. W. A. Frazier and associates have developed individual varieties that are resistant to fusarium, spotted wilt, and gray leaf spot and also are far more productive in the Territory than any varieties that originated on the mainland. Also active in the field of tomato disease resistance are some commercial seed firms and food processors who have shown a universal willingness to cooperate with the Federal and State laboratories.

One of the ultimate goals of tomato breeders is the development of varieties combining resistance to the principal diseases of each region in one variety. The Charleston laboratory is already working on the combination of resistance to fusarium wilt, collar rot, gray leaf spot, leaf mold and septoria leaf spot. However, many generations of cross breeding and testing will be required before a finished variety possessing all these superior attributes will be available to the tomato industry.

THE AUTHORS

William S. Porte is a pathologist in the Bureau of Plant Industry, Soils, and Agricultural Engineering. He has been doing research on disease resistance of tomatoes in the Department for more than 25 years.

C. F. Andrus, a pathologist in the same Bureau, has been with the Department 18 years and at present is handling vegetable disease work and tomato breeding at the Regional Vegetable Breeding Laboratory at Charleston, S. C.