Geographic Distribution of Fungicide-Insensitive Sclerotinia homoeocarpa in New England.

Repeated applications of fungicides used to control dollar spot (Sclerotinia homoeocarpa F.T. Bennett) in turf may lead to the selection of strains with reduced fungicide sensitivity. The objective of this study is to determine the scope of S. homoeocarpa insensitivity to fungicides commonly used to control dollar spot of golf courses in New England. In 2007, 1,104 isolates of S. homoeocarpa were collected from 22 populations on 18 golf courses in southern New England. Additional isolates (n=408) were obtained from 205 golf courses in New England and New York. Isolates were evaluated in vitro for their sensitivity to iprodione, propiconazole, and thiophanate-methyl. Relative growths of baseline and a random sampling of isolates were determined using serial concentrations of fungicides previously shown to exhibit quantitative sensitivity. Data were subjected to probit analysis to determine the concentration resulting in 50% growth inhibition (EC₅₀). For thiophanate-methyl, isolates were grown at a single concentration and scored for the presence or absence of growth. The mean sensitivity of the baseline population to iprodione and propiconazole was 0.2985±0.0335 and 0.0016±0.0007 µg a.i. mL^-1, respectively. All baseline isolates were sensitive to thiophanate-methyl. Discriminatory concentrations developed from the random population were 0.01 and 0.4 µg a.i. mL^-1 for propiconazole and iprodione, respectively. Sensitivities of isolates to propiconazole and iprodione were estimated using growth at these single discriminatory concentrations (EC_50(D)). Mean EC_50(D) values of isolates evaluated to date (n = 266) ranged from 0.0009 to 0.1938 for propiconazole and from 0.2167 to 0.8289 for iprodione. For thiophanate-methyl, 169 isolates from three populations (100%) were sensitive, 82 isolates from two populations (100%) were resistant, and 47% of isolates from a sixth population (n = 15) were resistant. Preliminary results of indicate that S. homoeocarpa insensitivity to select dollar spot fungicides is present in varying degrees in New England.