Virulence, Legume Host Specificity, and Genetic Relatedness of Isolates of Fusarium oxysporum from Red Clover. B. C. VENUTO, Louisiana State University Agricultural Center, Department of Agronomy, Baton Rouge 70803-7511. R. R. SMITH, U.S. Dairy Forage Research Center, USDA-ARS, and Department of Agronomy, University of Wisconsin-Madison 53706; and C. R. GRAU, Department of Plant Pathology, University of Wisconsin-Madison 53706. Plant Dis. 79:406-410. Accepted for publication 19 January 1995 . 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, 1995. DOI: 10.1094/PD-79-0406.

Fusarium oxysporum is the most prevalent fungal pathogen recovered from symptomatic red clover (Trifolium pratense) plants in Wisconsin and contributes to stand loss and reduced productivity. Three field isolates and 44 single-conidium isolates of F. oxysporum were recovered from red clover plants with vascular wilt symptoms and assessed for virulence, host specificity, and source of variation in host reaction. In addition, genetic relatedness of selected isolates was determined by vegetative compatibility groups (VCG). Twenty-one populations of red clover, two populations of alfalfa (Medicago saliva), and one population each of alsike clover (Trifolium hybridum), ladino clover (Trifolium repens), and birdsfoot trefoil (Lotus corniculatus) were tested for their reaction to isolates of F. oxysporum. Host populations differed significantly in their reaction to specific field isolates or single-conidium isolates. Isolates differed significantly in their ability to elicit reactions both among and within host species. VCGs were not useful in predicting host reaction because isolates from distinct groupings elicited similar host reactions. The reaction of red clover to F. oxysporum is attributable to genetic diversity in isolate virulence as well as to variation within the host population for resistance