Sixty isolates of the plant pathogenic fungus Sclerotinia sclerotiorum (Lib.) de Bary and six isolates of Sclerotinia minor Jagger were evaluated in a controlled environment for virulence on leaves excised from 8-week-old dandelion plants. Significant negative correlations were obtained between the relative virulence of the isolates and the dry weights of dandelion plants inoculated in a controlled environment; and positive correlations were detected between the relative virulence of isolates and reduction in number of dandelion plants in turfgrass swards infested with inoculum of the isolates. In August 1988, an 80.7% reduction in the number of dandelion plants was detected in a turfgrass sward treated in 1987 with four applications of heat-killed seed of perennial ryegrass (100 g m −2 application –1 ) infested with isolate R30 of S. sclerotiorum , followed by six applications at the same rate in 1988. Populations of dandelions in untreated swards increased by 22.2% during the same period. Heat-killed seed of perennial ryegrass (100 g m −2 ) infested with isolate R30 applied simultaneously with dandelion seed (25 g m −2 ) onto a sward of Kentucky bluegrass reduced the establishment of dandelion seedlings by 85.5%. Necrosis or discoloration did not develop on Kentucky bluegrass, creeping bentgrass, annual bluegrass, or quackgrass treated with inoculum of Sclerotinia. Sclerotia of Sclerotinia spp. were found, on occasion, in crowns but not on the foliage of diseased dandelions.