Pan America

Foreign stocks of Lycopersicon pimpinellifolium, a wild, small-fruited species known as currant tomato, have been a fertile source of parent lines that are immune, resistant, or tolerant to wilt and other important diseases, including leaf mold, foliage blights, and mosaic.

One of these stocks, P. I. 79532, was discovered growing in a cane field near Trujillo, Peru, within sight of the Pacific Ocean. It was tested on soil that had been artificially loaded with the fusarium wilt fungus, and its healthy green foliage stood out like an emerald in the bright September weather in contrast to hundreds of browned, wilt-sick varieties. Repeated inoculation tests established the practical immunity of this tomato from attack by the fusarium wilt fungus. A series of reciprocal crosses was made between inbred lines of Marglobe, a widely used canning and shipping variety, and the almost immune wild line of Red Currant.

The fruits of Red Currant are only about a quarter of an inch in diameter. They are thin-walled, usually two-celled and filled with greenish watery pulp and numerous seeds. The breeding problem involving this cross was the isolation and development of a line combining the wilt resistance of Red Currant with the large, fleshy, heavy-walled, scarlet Marglobe fruits. The size of fruit in the progeny of the new hybrid was increased by successively backcrossing it three times to the Marglobe parent. Each backcrossed generation and the subsequent generations of line selections were rigidly tested to determine their complete resistance to attack by a highly virulent strain of the fusarium fungus.

In all the wilt tests of parent lines selected in 1938 or later, 95 to 100 percent of the samples tested were entirely free from any evidence of fusarium wilt infection. The variety Pan America, developed from this cross, was released by the Department in 1941 by distributing stock seed to commercial tomato seed growers and all interested research institutions. It is widely used by tomato breeders as a wilt-resistant parent in developing varieties that withstand two or more diseases and are adapted to special regional conditions. Commercial growers and home gardeners, especially in localities having soils heavily infested with the fusarium wilt fungus, are successfully producing crops practically free from wilt by using Pan America.

Essential Techniques Devised

Before a breeding program can be carried to a successful conclusion, technical procedures must be developed that will produce essentially uniform results in all successive tests conducted with the same variety or strain. In order to test effectively the resistance of a tomato line to a disease, the organism causing the disease must be thoroughly studied. The more the breeder knows concerning the life history of the agent causing the disease and the nature of the inheritance in the tomato of resistance to its attack, the more effectively he can breed resistant varieties. In recent years workers in the Department and in State research institutions have devoted much of their time and effort to this phase of the work. Several workers have studied cultures of the tomato wilt fungus, Fusarium oxysporum f. lycopersici, from diverse regions of the United States and found striking differences in their capacity to produce wilt in tomato plants. They have isolated the most virulent strains and these are now used in testing the resistance of tomato lines.

F. L. Wellman, working at the Plant Industry Station at Beltsville, devised a laboratory and greenhouse technique for the study of both the capacity of the tomato wilt organism to produce tomato wilt and the relative wilt resistance of tomato strains and varieties. By maintaining- a greenhouse soil and air temperature of about 80° F., he obtained conditions that enable tomato breeders to test the wilt resistance of a population of 3,000 tomato plants each month during the fall and winter with only 72 square feet of bench space. In previous field tests, usually fewer than 3,000 plants per acre were tested each year and the breeder could grow and test only one generation of tomato plants a year.

Similarly, workers at the United States Regional Vegetable Breeding Laboratory at Charleston, S. C., developed an effective laboratory and greenhouse technique for testing large populations of tomato seedlings for resistance to tomato leaf spot diseases. They also determined the nature of inheritance of resistance to the collar rot phase of Altenaria solani and to septoria leaf spot. These are important contributions in speeding up breeding for resistance to these damaging tomato foliage diseases.