Epidemiology and Seed Transmission of Two Tobacco Streak Virus Pathotypes Associated with Seed Increases of Legume Germ Plasm in Eastern Washington. Walter J. Kaiser, Research Plant Pathologist, Agricultural Research Service, U.S. Department of Agriculture, Western Regional Plant Introduction Station, Washington State University, Pullman 99164-6402. Stephen D. Wyatt, and Robert E. Klein. Associate Professor, Department of Plant Pathology, Washington State University, Pullman 99164-6430; and Assistant Plant Pathologist, Irrigated Agriculture Research and Extension Center, Washington State University, Route 2, Box 2953-A, Prosser 99350-9687. Plant Dis. 75:258-264. Accepted for publication 4 September 1990. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1991. DOI: 10.1094/PD-75-0258.

Two pathotypes of tobacco streak virus (TSV), designated I and II, were isolated from naturally infected bean (Phaseolus vulgaris), chickpea (Cicer arietinum), adzuki bean (Vigna angularis), fenugreek (Trigonella foenum-graecum), alfalfa (Medicago sativa), and white sweet clover (Melilotus alba) at Central Ferry, WA. The two pathotypes could be distinguished by their host reactions, serology, and by SDS-polyacrylamide gel electrophoresis. Pathotype I infected cowpea (Vigna unguiculata), but pathotype II did not. Pathotype I isolates were similar, if not identical, to the red node strain of TSV in bean, whereas pathotype II isolates induced green to yellow mosaics in this host. Alfalfa and white sweet clover were overwintering hosts of both pathotypes. Pathotype I isolates were seed transmitted in naturally infected Black Turtle Soup (BTS) bean at a rate of 3.8%. Isolates of pathotypes I and II were seed transmitted in six of nine and five of nine mechanically inoculated bean cultivars, respectively, at rates of 0.9–15.1% and 0.5–2.4%. Inoculation of BTS bean in the greenhouse with pathotypes I and II reduced seed yields by 14–49%, and seed transmission ranged from 0.5 to 32.8% depending on the virus isolate and method of inoculation. In greenhouse trials, an isolate of pathotype I was seed transmitted in artifically infected adzuki bean (29%) but not in chickpea or fenugreek. Seed transmission of TSV was not detected in naturally infected alfalfa from Central Ferry. Inoculation of chickpea at Central Ferry with isolates of pathotypes I and II reduced seed yields at prebloom by 77–96% and at full bloom by 25–71%. Contrary to results from greenhouse tests, seed transmission of pathotypes I and II in prebloom-inoculated, field-grown chickpeas ranged from 1.1 to 11.1%. BTS bean trap plants placed in screened cages at biweekly intervals at Central Ferry became infected with both pathotypes of TSV. Transmission of TSV to bean plants in the cages differed greatly depending on the type of screen covering the cages. Maximum infection of 83% occurred on 9 July in cages with screens with larger openings (500 μm), compared with 11% of the trap plants in cages with screens with smaller openings (110 μm). Results suggest that an insect vector smaller than alate aphids is responsible for field spread of TSV to beans.