VIRULENCE, LEGUME HOST-SPECIFICITY, AND GENETIC RELATEDNESS OF ISOLATES OF FUSARIUM-OXYSPORUM FROM RED-CLOVER

Fusarium oxysporum is the most prevalent fungal pathogen recovered fro m symptomatic red clover (Trifolium pratense) plants in Wisconsin and contributes to stand loss and reduced productivity. Three field isolat es and 44 single-conidium isolates of F. oxysporum were recovered from red clover plants with vascular wilt symptoms and assessed for virule nce, host specificity, and source of variation in host reaction. In ad dition, genetic relatedness of selected isolates was determined by veg etative compatibility groups (VCG). Twenty-one populations of red clov er, two populations of alfalfa (Medicago sativa), and one population e ach of alsike clover (Trifolium hybridum), ladino clover (Trifolium re pens), and birdsfoot trefoil (Lotus corniculatus) were tested for thei r reaction to isolates of F. oxysporum. Host populations differed sign ificantly in their reaction to specific field isolates or single-conid ium isolates. Isolates differed significantly in their ability to elic it reactions both among and within host species. VCGs were not useful in predicting host reaction because isolates from distinct groupings e licited similar host reactions. The reaction of red clover to F. oxysp orum is attributable to genetic diversity in isolate virulence as well as to variation within the host population for resistance.