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Etiology

Effects of a Necrosis-Inducing Isolate of Alfalfa Mosaic Virus on Stand Loss in Tomatoes. D. A. Knorr, Graduate student, Plant Pathology Department, University of California, Riverside; F. F. Laemmlen(2), and W. O. Dawson(3). (2)Farm advisor, Cooperative Extension, Court House, El Centro, CA 92243; (3)Associate professor, Plant Pathology Department, University of California, Riverside. Phytopathology 73:1554-1558. Accepted for publication 6 June 1983. Copyright 1983 The American Phytopathological Society. DOI: 10.1094/Phyto-73-1554.

Alfalfa mosaic virus (AMV) causes a severe disease of tomatoes that reduces survival of direct-seeded tomato plants in fields adjacent to alfalfa in the Imperial Valley of California. Disease symptoms in tomato plants are stunting, bronzing, and necrosis followed by death. The disease is caused by an unusual strain of AMV that can be transferred mechanically from alfalfa or diseased tomatoes to various host plants, but not to tomatoes. The disease was reproduced in tomato plants by grafting with infected tobacco scions or by inoculating with aphids. The virus was localized in necrotic areas of infected tomato plants, but not always in bronzed or senescent tissues. A previously unreported vector of AMV, the blue alfalfa aphid, and the pea aphid, transmitted the virus experimentally and are probably the principal vectors in nature. Alfalfa was a reservoir of the virus. Patterns of disease in tomato fields indicate the necrosis-inducing strain of AMV spreads from alfalfa into tomatoes with no secondary spread from tomatoes to tomatoes. Results of field and greenhouse trials using commercial tomato cultivars indicated no immunity was present, but suggest that field resistance is associated with lack of infection. Cultivars that were most tolerant in greenhouse trials generally became infected less often during field trials.