Detection of Demethylation Inhibitor (DMI) Resistance in Sclerotinia homoeocarpa Populations. R. C. Golembiewski, Department of Botany and Plant Pathology and the Pesticide Research Center, Michigan State University, East Lansing 48824. J. M. Vargas, Jr., A. L. Jones, and A. R. Detweiler, Department of Botany and Plant Pathology and the Pesticide Research Center, Michigan State University, East Lansing 48824. Plant Dis. 79:491-493. Accepted for publication 1 February 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/PD-79-0491.

Sclerotinia homoeocarpa is an important turfgrass pathogen on golf courses in Michigan and Ohio. Isolates were collected from golf courses on which sterol demethylation inhibitor (DMI) fungicides had never been used and from courses where DMI fungicides were used but where little or no control of dollar spot was achieved. The mean relative growth value on DMI-amended media of 150 isolates from golf courses treated with DMI fungicides was two to three times higher than for 150 isolates from golf courses with no history of DMI treatments. Isolates with high relative growth values for triadimefon also had similarly high values for fenarimol and propiconazole. The difference in growth on DMI-amended media of the S. homoeocarpa isolates from untreated courses compared to treated courses indicated that resistance to DMI fungicides had developed. Dollar spot control on a golf course in southeast Michigan infected by DMI-resistant S. homoeocarpa was not obtained with triadimefon, fenarimol, or propiconazole. Chlorothalonil applied at 191 ml/100 m2 was the only chemical that provided acceptable disease control for the duration of the study (1991 through 1993). A triadimefon concentration of 2 µg ml-1 was an effective monitoring dose for S. homoeocarpa populations.