Control of Dollar Spot of Creeping Bentgrass Caused by an Isolate of Sclerotinia homoeocarpa Resistant to Benzimidazole and Demethylation-Inhibitor Fungicides

Abstract

The effects of fungicides were studied on two isolates of Sclerotinia homoeocarpa that differed in fungicide sensitivity. Concentrations of benzimidazole (benomyl and thiophanate-methyl), demethylation inhibitor (fenarimol, propiconazole, and triadimefon), and nitrile (chlorothalonil) fungicides required to inhibit mycelial growth in vitro by 50 and 90% (effective concentration; EC_50,90) were significantly greater for isolate S088 than for isolate S084. No differences were observed in the EC_50,90 values of a pyridylaniline (fluazinam) or dicarboximide (iprodione) fungicide. In field tests conducted on creeping bentgrass, S088 had a significantly shorter incubation period than S084 in plots treated with propiconazole applied at 0.2 or 0.8 kg a.i. ha^-1 in 1994 or 0.4 kg a.i. ha^-1 in 1995, thiophanate-methyl applied at 1.5 or 3.0 kg a.i. ha^-1 in 1994 and 1995, or a tank-mix of propiconazole and iprodione applied at 1.6 + 0.8 kg a.i. ha^-1 in 1994. No differences in incubation periods occurred in plots treated with chlorothalonil or iprodione in either year, or with fluazinam in 1995 and 1996. Fewer days were required to reach 5% disease severity in turf inoculated with isolate S088, compared to isolate S084, in plots treated with propiconazole, thiophanate-methyl, or certain tank-mixes containing propiconazole, in 1994 and 1995. Several treatments, including propiconazole, thiophanate-methyl, and fluazinam, resulted in suppression of disease caused by isolate S084 to less than 5% severity for more than 21 days, while only fluazinam applied at 1.6 or 3.2 kg a.i. ha^-1 provided more than 21 days of disease suppression in turf inoculated with either of the isolates tested. Slope coefficients from linear regressions between incubation period or days to 5% disease and concentrations of fungicide applied in 1996 were significantly different for isolate S084 compared to isolate S088 in plots treated with propiconazole, but not in plots treated with fluazinam.