Susceptibility to Clover Yellow Vein Potyvirus in the United States Germ Plasm Collection of Subterranean Clover. M. R McLAUGHLIN, Research Plant Pathologist, USDA-ARS, Crop Science Research Laboratory, Forage Research Unit, P.O. Box 5367, Mississippi State, MS 39762. T. E. FAIRBROTHER, Research Animal Scientist, USDA-ARS, Crop Science Research Laboratory, Forage Research Unit, P.O. Box 5367, Mississippi State, MS 39762. Plant Dis. 78:665-667. Accepted for publication 29 March 1994. 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, 1994. DOI: 10.1094/PD-78-0665.

In spring 1991, natural infections of clover yellow vein virus (CYVV) caused severe disease in 60% of experimental accessions of subterranean clover (Trifolium subterraneum) grown in field trials at Mississippi State, Mississippi. A search for resistance to CYVV was initiated within the United States collection of subterranean clover germ plasm. Plants of each of 261 plant introduction (PI) lines and three cultivars were grown in the greenhouse during the winter and spring of 1991 -1992. Seedlings were mechanically inoculated with CYVV-Pratt and evaluated 21-28 days later. Initial DAS-ELISA confirmed an absolute correlation of symptoms with infection. Symplomless plants were reinoculated and reevaluated up to five times. Fewer than 2% of all tested plants were selected as resistant and held as first-generation parents (P1). Selected P1 plants (45 from 21 PI lines) were identified as var. yanninicum (one line), var. oxaloides (seven lines), and var. flagelliforme (13 lines) and allowed to produce first-generation self-pollinated (S1) seed. The S1 seed were grown and the S1 plants tested in the summer of 1992, following the same procedures used in the selection of the P1 plants. Six S1 plants from four PI lines were selected for second-generation (S2) seed production. The S2 seed were grown and the S2 plants tested in 1993, following procedures similar to those used for P1 and S1 tests. All S2 plants were susceptible and showed uniformly severe reactions consisting of rapid systemic wilt and plant death. We conclude that no heritable resistance to CYVV exists in the major portion of the germ plasm collection tested. The severe susceptible (hypersensitive) reaction identified in some lines may be a useful management tool in limiting spread of CYVV in the field.