Other Breeding Projects

A similar breeding program is being followed in the combination of practical immunity to fusarium wilt with resistance to gray leaf spot, to septoria leaf spot, and to leaf mold, respectively.

Gray leaf spot resistance was found primarily in the South American collections of currant tomato, Lycopersicon pimpinellifolium, and the slow incorporation of large fruit size has retarded this program. However, several tomatoes resistant to gray leaf spot and to fusarium wilt and of acceptable commercial size have already been produced. Some of these carry a combination of earliness and productivity that is popular in certain commercial varieties.

Resistance to septoria leaf spot is now known to occur in many collections of the wild species Lycopersicon hirsutum and L. peruvianum, but at the Regional Vegetable Breeding Laboratory, resistance to septoria was originally found in a primitive type segregated from the Australian variety Targinnie Red, which proved to be wholly fertile in crosses with domestic varieties. These crosses are well advanced, but none of the lines is sufficiently fixed as to the factors of productivity, size, and quality.

Leaf mold, caused by Cladosporium fulvum, is a serious disease on greenhouse tomatoes. Four physiologic races of this fungus that have been isolated exhibit distinct differences in pathogenicity. All varieties of the common tomato are more or less susceptible, but Red Currant is highly resistant. Resistance to one strain of the leaf mold fungus is already available in the forcing varieties Globelle, Bay State, and Veto-mold. Breeders are progressing in their development of other varieties resistant to other races of the leaf mold fungus. The cross Vetomold X Pan America has yielded some promising types at Charleston.

During the winter of 1946 an epidemic of late blight caused by Phytophthora infestans on field tomatoes in southern Florida provided an exceptional opportunity for a field test of resistance to this disease. All of the commercial varieties observed were very susceptible, but among 15 advanced breeding lines from the Vegetable Breeding Laboratory under propagation at Homestead three were highly tolerant to late blight. All of the blight-resistant lines possess near-immunity to fusarium wilt; one of them is also resistant to collar rot and to early blight.

The Department breeders working at various locations, and State investigators, have devoted much effort toward using the resistance to 411 various diseases found in Lycopersicon hirsutum and L. peruvianum species, which are closely related to the common tomato and possess high resistance to a number of tomato diseases, but have no economic value unless their resistance can be combined with other desirable characters by hybridizing them with horticultural varieties. A selected clonal line of L. hirsutum possessing high resistance to the ordinary tobacco mosaic virus has been crossed with several commercial varieties at Beltsville. Some of these progeny lines have shown high tolerance when subjected to heavy inoculations with mosaic virus.

Hybrids have also been made between Lycopersicon peruvianum and L. esculentum. Although they are practically sterile in the first hybrid generation, they have been outcrossed to several popular canning varieties. These outcrosses are now under test by both Federal and State breeders to determine their resistance to tomato leaf spot, to leaf mold, to root knot, and to late blight caused by Phytophthora infestans.

Work is in progress in cooperation with the Utah Agricultural Experiment Station to develop tomatoes resistant to curly top and to verticillium wilt, two very serious diseases in certain western regions. Curly top, also known as western yellow blight, is caused by a virus which is disseminated by the sugar-beet leafhopper, a native of the wastelands of the semiarid West. Literally thousands of tomatoes, including all available commercial varieties and wild related species, have been extensively tested by Department workers in regions where curly top is prevalent.