Abstract
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 isolated 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 isolated was determined by vegetative compatibility groups (VCG). Twenty-one populations of red clover, two populations of alfalfa (Medicago sativa), 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 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
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