Dollar spot (caused by Sclerotinia homoeocarpa) is the most economically important turfgrass disease in North America. This disease is primarily controlled by fungicide applications on golf courses; however, fungicide resistance has been confirmed in three of the four systemic fungicide classes commonly used to control dollar spot. The main objective of this study was to evaluate S. homoeocarpa sensitivity to multiple chemical classes and cross-resistance among active ingredients within the same class; in particular, the association between the fungistatic effect of demethylation inhibitors (DMIs) and plant growth regulators (PGRs). Fifty-eight isolates were selected arbitrarily from four locations in the United States and assayed for in vitro sensitivity to six DMI, two dicarboximide, one carboximide, and one benzimidazole fungicide as well as three type II PGRs. A series of concentrations for each active ingredient was used to determine the mean 50% effective concentration (EC50) values and correlation coefficients were calculated for all active ingredients. The EC50 values of all active ingredients from the DMI class were highly correlated (P < 0.0001) to each other as well as to the one dicarboximide (iprodione) and two PGRs (flurprimidol and paclobutrazol). Isolates resistant to thiophanatemethyl had significantly higher EC50 values than sensitive isolates for all active ingredients assayed except for boscalid. Findings showed that multiple and cross-resistance has developed in S. homoeocarpa and that the two PGRs have a fungistatic effect on this pathogen similar to that of DMI fungicides. The high correlation of in vitro sensitivities among PGRs and DMI fungicides further suggests that PGRs may contribute to the selection of DMI-resistant isolates or facilitate decreased sensitivity to DMI fungicides in the field.
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