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Relation of Dosage Rates, Nutrition, Air Temperature, and Suscept Genotype to Side Effects of Systemic Fungicides on Turfgrasses. Bobby G. Joyner, Formerly Graduate Assistant, Department of Plant Pathology and Physiology, Virginia Polytechnic Institute and State University, now Plant Pathologist, Diagnostic Laboratory, Chem-Lawn Corp., 1052 Crupper Ave., Columbus, Ohio 43229; Houston B. Couch, Professor of Plant Pathology, Department of Plant Pathology and Physiology, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061. Phytopathology 66:806-810. Accepted for publication 29 December 1975. DOI: 10.1094/Phyto-66-806.

Field trials and laboratory-based experiments were conducted to determine the effects of benomyl [methyl 1- (butylcarbamoyl)-2-benzimidazole carbamate], thiophanate-methyl [dimethyl 4,4-o-phenylenebis-(3-thioallophanate)], thiophanate-ethyl [diethyl 4,4-o-phenylenebis-(3-thioallophanate)], and thiabendazole [2-(4-thiazolyl)benzimidazole] on seven nontarget diseases of turfgrass. Also studied was the relation of dosage rates, air temperature, nutrition, and suscept genotype to the phytotoxicity of these compounds to turfgrasses. The fungicide treatments did not affect the incidence of zonate eyespot (caused by Helminthosporium giganteum) of hairy crabgrass (Digitaria sanguinalis), Helminthosporium leaf spot (caused by H. sorokinianum) of Seaside bentgrass (Agrostis palustris), or Pythium blight (caused by P. ultimum) of Manhattan ryegrass (Lolium perenne). However, the levels of red leaf spot (caused by Helminthosporium erythrospilum) of Seaside bentgrass, Helminthosporium leaf spot of Merion and common Kentucky bluegrass (Poa pratensis), and Helminthosporium blight (caused by H. dictyoides) of K-31 tall fescue (Festuca arundinacea) were reduced. Benomyl, thiophanate-methyl, and thiophanate-ethyl reduced the severity of Pythium blight of Seaside bentgrass. The incidence of rust (caused by Puccinia coronata) on Manhattan ryegrass increased after applications of thiabendazole and thiophanate-methyl. Single applications of 279 mg active ingredient/m2 of either benomyl, thiophanate-methyl, or thiophanate-ethyl, induced chlorosis of field-grown Penncross bentgrass 36-72 hours from the time of treatment. The treated areas were also characterized by light yellow rings 30-60 cm in diameter which persisted for approximately 21 days. In the laboratory-based studies, symptoms of benomyl-induced phytotoxicity developed sooner, were more severe, and persisted longer in bentgrass, Kentucky bluegrass, and perennial ryegrass grown under low nitrogen nutrition. Thiabendazole was more phytotoxic to bentgrass grown under high nitrogen nutrition. Bentgrass and Kentucky bluegrass grown at 35 C were more severely injured by benomyl than plants produced at 22 C. In descending order of susceptibility to benomyl-induced phytotoxicity, certain cultivars were ranked as follows: Merion Kentucky bluegrass > Manhattan ryegrass > Highland bentgrass > Pennfine ryegrass > Penncross bentgrass > common Kentucky bluegrass.

Additional keywords: benzimidazole fungicides, phytotoxicity, turfgrass diseases, disease control, disease susceptibility.