Healthier Tomatoes

by WILLIAM S. PORTE and C. F. ANDRUS

THE TOMATO is a native tropical American plant, but for a long time it was without honor in its own hemisphere. Jefferson grew tomatoes in Virginia in 1781 and Washington is said to have used them in his army rations, but about a half century elapsed before they became generally regarded as a food in the United States.

Now tomatoes are grown on more than 800,000 acres in the United States, and the crop is valued at 166 million dollars, not counting the tomatoes grown in home gardens.

At first, when tomatoes were grown only in small patches, diseases were no problem. The few varieties planted had been brought from England and France. The early breeding, done by amateurs, was devoted to improving the size, shape, color, and production of the fruit. But at the beginning of this century the commercial tomato industry was concentrated in definite regions in which the incidence and severity of diseases were continually increasing. The Department recognized that the control of tomato diseases had become a national problem, and in 1915 started a project for breeding disease-resistant varieties.

The work was under the leadership of F. J. Pritchard, whose name all tomato growers know. His first job was to do something about fusarium wilt, which is caused by a soil-infesting fungus. During the next 10 years a series of wilt-tolerant varieties enabled growers to produce profitable crops on soils lightly infested with the fungus.

The new kinds and the practice of crop rotation reduced damages from wilt. But the disease was still causing an estimated annual loss of more than a million dollars to commercial crops and uncalculated losses in home gardens before the war—because of the expansion of the crop and the gradual infestation of more soil areas and because of the natural development of more virulent races of the fusarium fungus. At the same time, the foliage blights and virus diseases also were taking a graduated toll of the crop, despite a program of field sanitation that had been developed in the Department to control leaf spot diseases.

Field sanitation programs are not very successful because the measures are not generally and properly applied. On the other hand, any disease resistance or immunity bred into a variety of tomato becomes available to all growers wherever the resistant line is used. That is the great advantage of breeding for disease resistance over other methods of disease control, and is a reason why the Department has increasingly emphasized and expanded its breeding work for disease control. For such efforts we must have varieties possessing the genetic factors for immunity or high resistance to all the important diseases.

Plant explorers, in recent years, have searched Peru and adjacent South American countries in which the tomato is native, and have imported hundreds of varieties of Lycopersicon esculentum (the edible tomato) and numerous lines of related species such as L. hirsutum, L. peruvianum, and L. pimpinellifolium. They have also obtained a large number of exotic varieties from Europe, Asia, Africa, and Australia, in their search for new and superior genetic factors for disease resistance.

The introductions are tested by breeders in the Department and in the State and Territorial stations. The tests determine the resistance of each strain to various diseases, so that the breeders can isolate superior resistant plants for use as parents in the development of stronger varieties. Because varieties that tolerated fusarium wilt were not entirely satisfactory when cropped on soils heavily infested with the wilt fungus, the introduction of new parental material stimulated a search for stocks that showed definitely higher degrees of resistance to the wilt. From 1935 to 1941 a project at Beltsville was directed chiefly to the development of commercially acceptable varieties possessing higher resistance to wilt than those already in use.