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Sensitivity of Sclerotinia homoeocarpa Isolates to Propiconazole and Impact on Control of Dollar Spot

In response to reports of reduced efficacy of propiconazole for control of dollar spot, isolates of Sclerotinia homoeocarpa were collected from several locations in Georgia and tested for sensitivity to propiconazole and other demethylation-inhibiting (DMI) fungicides. Two discriminatory concentrations of propiconazole (0.02 and 0.2 μg ml^-1) were used to detect lower in vitro sensitivity in two populations that had been exposed repeatedly to propiconazole than in four nonexposed populations. Mean 50% effective concentration (EC₅₀) values for a nonexposed population (baseline) and a DMI-exposed population were 0.0049 and 0.0283 μg ml^-1, respectively. Positive correlations were significant among log₁₀ EC₅₀ values for propiconazole, fenarimol, and myclobutanil but not between triadimefon and any of the other three fungicides, indicating cross-resistance relationships in this pathogen may not be universal among the DMIs. In greenhouse experiments, propiconazole-treated bentgrass was inoculated with seven isolates of S. homoeocarpa differing in sensitivity to propiconazole. Incubation period decreased and relative area under the disease progress curve and disease severity 28 days after inoculation increased linearly with increasing log₁₀ EC₅₀ value of the isolate. Results of this study confirm a significant relationship between in vitro sensitivity of S. homoeocarpa and in planta control efficacy of propiconazole and provide evidence of field resistance to propiconazole in S. homoeocarpa in Georgia.