Abstract
Isolates of Sclerotinia homoeocarpa were collected from bentgrass putting greens where iprodione had failed to control the dollar spot disease. These isolates were highly resistant, as assessed by hyphal growth on fungicideamended PDA, to dicarboximide fungicides and PCNB, showing 300 to 1, 500 times higher in EC50 values as compared with normal isolates. Based on the sensitivity to iprodione, these isolates could be classified into three groups showing MIC values of 4.0-5.0, 200-500, and 1, 000-1, 600 ppm, respectively. All isolates belonging to any group were equally sensitive to benomyl and propiconazole. The use of these two chemicals was quite effective to control the disease development in the inoculation experiments with the iprodioneresistant isolates and creeping bentgrass seedlings.
Successive transfer of the resistant isolates on iprodione-amended and iprodione-free PD A for about one year exhibited that the level of resistance was maintained on the former medium but was gradually reduced on the latter medium.
The use of benomyl alone for the control of the dollar spot disease for one year completely depressed the appearance of iprodione-resistant strains. It was thus suggested that the alternative use of fungicides having different modes of action may effective to suppress the outbreak of resistant strains.
