Reduced-Rate Fungicide Mixtures to Delay Fungicide Resistance and to Control Selected Turfgrass Diseases. P. L. Sanders, Associate Professor, Department of Plant Pathology, The Pennsylvania State University, University Park 16802. W. J. Houser, Research Technologist, P. J. Parish, Research Technologist, and H. Cole, Jr., Professor, Department of Plant Pathology, The Pennsylvania State University, University Park 16802. Plant Dis. 69:939-943. Accepted for publication 15 March 1985. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/PD-69-939.

Metalaxyl was applied singly, in alternation, or in combination with mancozeb or propamocarb to greenhouse-grown perennial ryegrass inoculated with populations of Pythium aphanidermatum having known proportions of metalaxyl-resistant (R):metalaxyl-sensitive (S) individuals. The populations were cycled repeatedly through inoculation, fungicide application, incubation, and harvest/assay. In populations with R:S ratios of 1:10 and 1:1,000, reduced-rate mixtures of metalaxyl with mancozeb or propamocarb were most effective in delaying increases in the R proportion. Half-rate mixtures of metalaxyl with propamocarb, mancozeb, or fosetyl Al provided excellent control of Pythium blight (P. aphanidermatum) on field-grown perennial ryegrass. Excellent suppression of dollar spot (Sclerotinia homoeocarpa) on field-grown creeping bentgrass was provided by half-rate, two-component mixtures of benzimidazoles, dicarboximides, and sterol biosynthesis inhibitors.