Increase in Incidence and Severity of Target Turfgrass Diseases by Certain Fungicides. Houston B. Couch, Professor, Department of Plant Pathology, Physiology and Weed Science, Virginia Polytechnic Institute and State University, Blacksburg 24061. Brian D. Smith, Laboratory Specialist, Department of Plant Pathology, Physiology and Weed Science, Virginia Polytechnic Institute and State University, Blacksburg 24061. Plant Dis. 75:1064-1067. Accepted for publication 1 April 1991. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-1064.

Field and laboratory studies were conducted to determine if in a situation of fungicide resistance, the use of the fungicide in question might increase the severity of the target turfgrass disease. Applications of either a mixture of cycloheximide and thiram or triphenyltin hydroxide to tall fescue (Festuca elatior var. arundinacea) colonized by a strain of Rhizoctonia solani resistant to these fungicides increased the incidence of Rhizoctonia blight. Applications of benomyl, thiophanate-methyl, and thiophanate-ethyl to creeping bentgrass (Agrostis palustris) colonized by a benzimidazole-resistant strain of Sclerotinia homoeocarpa resulted in significant increases in the incidence and severity of Sclerotinia dollar spot. The benzimidazole-induced increase in Sclerotinia dollar spot was detected within 7 days of the second fungicide application and continued for an additional 28 days in the absence of additional fungicide applications. Treatments of creeping bentgrass with triphenyltin hydroxide also exhibited an increase in Sclerotinia dollar spot. This increase was first detected 3 wk after the third and final fungicide application and continued for 36 days.