Rapid development of fungicide resistance by Sclerotinia homoeocarpa on turfgrass

Dollar spot, caused by Sclerotinia homoeocarpa, is the most prevalent and economically important turfgrass disease in North America. Increasing levels of fungicide resistance, coupled with tightening environmental scrutiny of existing fungicides, has left fewer options for managing dollar spot. More knowledge about S. homoeocarpa populations is needed to improve dollar spot management strategies, especially with respect to minimizing the development of fungicide resistance. Population diversity of S. homoeocarpa was examined using inter-simple sequence repeat markers and vegetative compatibility assays. Two subgroups were found in S. homoeocarpa field populations on both fairway and putting green turfgrass at a research field in Wisconsin. These subgroups were genetically different, vegetatively incompatible, and had different fungicide sensitivities. The frequency of the two genetic subgroups differed significantly between the fairway and putting green, but was uniform within the fairway or within the green. Population dynamics of S. homoeocarpa in response to two systemic fungicides (thiophanate-methyl and propiconazole) were assessed based on in vitro fungicide sensitivity. Dynamics of S. homoeocarpa populations depended on the presence of fungicide-resistant isolates in the initial populations before fungicide applications and changed rapidly after fungicide applications. Shifting of the population toward propiconazole resistance was gradual, whereas thiophanate-methyl resistance developed rapidly in the population. In conclusion, field populations of S. homoeocarpa containing genetically distinct, vegetatively incompatible groups were different on turfgrass that was managed differently, and they were changed rapidly after exposure to fungicides.