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Comparison of the RNAs and Some Physicochemical Properties of the Seed-Transmitted Tobacco Streak Virus Isolate Mel 40 and the Infrequently Seed-Transmitted Isolate Mel F. M. H. Walter, Former graduate student, Department of Plant Pathology, Washington State University, Pullman 99164-6430, Present address: Department of Plant Pathology, Throckmorton Hall, Kansas State University, Manhattan 66506-5502; S. D. Wyatt(2), and W. J. Kaiser(3). (2)Associate professor, Department of Plant Pathology, Washington State University, Pullman 99164-6430; (3)research plant pathologist, U.S. Department of Agriculture, Agricultural Research Service, Western Regional Plant Introduction Station, Washington State University, Pullman 99164-6402. Phytopathology 85:1394-1399. Accepted for publication 23 May 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/Phyto-85-1394.

The viral genetic basis of symptom severity and seed transmission was investigated with tobacco streak virus (TSV) pathotype I isolate Mel 40 and pathotype II isolate Mel F. These isolates differ in physico-chemical properties and RNA secondary structure. Electrophoresis of RNA from the mild, infrequently seed-transmitted TSV isolate Mel F revealed many minor RNA species not detected in the severe, seed-transmitted isolate Mel 40. Inoculations with gel-purified Mel F genomic RNAs 1, 2, and 3 produced atypically severe symptoms on Chenopodium quinoa and bean (Phaseolus vulgaris). Severe symptoms were correlated with the absence of minor RNAs in the inoculum. Subisolate Mel FS arose as a rare seed transmission event of Mel F in bean. It was transmitted to approximately 30% of progeny seedlings but appeared otherwise indistinguishable from Mel F. Non-seed-transmitted Mel F encapsidated one minor RNA, designated RNA F5, which was not detected in seed-transmitted Mel FS viral RNA. Six heterologous RNA combinations from Mel F and Mel 40 were inoculated on C. quinoa by using gel-purified genomic RNAs. Only one pseudorecombinant isolate was sufficiently infectious for transfer and characterization, causing a slow-spreading infection that was not seed transmitted. Infectivity of TSV Mel 40 and Mel F appears to be dependent on homologous RNAs.