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Late Leaf Spot, Southern Stem Rot, and Peanut Yield Responses to Rates of Cyproconazole and Chlorothalonil Applied Alone and in Combination. A. K. Culbreath, Associate Professor; The University of Georgia, Coastal Plain Experiment Station, Tifton 31793-0748. T. B. Brenneman, Department of Plant Pathology, K. Bondari, Statistical and Computer Services, The University of Georgia, Coastal Plain Experiment Station, Tifton 31793-0748, K. L. Reynolds, Department of Plant Pathology, The University of Georgia, Athens 30602, and H. S. McLean, Sandoz Agro Inc., Cordele, Ga. 31015. PLANT DIS. 79:1121. Accepted for publication 31 July 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/PD-79-1121.

Field tests were conducted in Plains, Ga., in 1990 and 1991 and in Tifton, Ga., in 1991 to determine the effects of cyproconazole (0, 12.3, 24.6, 49.3, 73.9, and 98.7 g/ha) and chlorothalonil (0, 210, 420 and 630, g/ha), applied alone and in tank mixes, on late leaf spot (Cercosporidium personalum), southern stem rot (Sclerotium rolfsii), and yields of peanut (Arachis hypogaea). Asymptotic curvilinear reductions in late leaf spot and area under the disease progress curve were observed with increasing rates of cyproconazole within all levels of chlorothalonil. Late leaf spot ratings decreased linearly with increasing rates of chlorothalonil within cyproconazole rates of 24.6 g/ha or lower in 1990 and 49.3 g/ha or lower in 1991. Quadratic curvilinear reductions in the incidence of stem rot with increasing rates of cyproconazole were observed in both years. Chlorothalonil had no effect on incidence of stem rot. Yield increased linearly or curvilinearly with increasing rates of cyproconazole, with yields converging with the higher rates of cyproconazole within all rates of chlorothalonil. In 1990 and 1991, increases in yield with increasing rates of chlorothalonil diminished as rates of cyproconazole increased. When applied with 420 or 630 g/ha of chlorothalonil, rates of cyproconazole required to provide adequate leaf spot control were much lower than rates required for control of stem rot control in fields with moderate to heavy disease incidence. In field experiments conducted in Tifton in 1992 and 1993, tank mixes of 34 g/ha of cyproconazole and 420 g/ha of chlorothalonil provided control of leaf spot that was superior to that obtained with the standard recommended rate (1.26 kg/ha) of chlorothalonil alone. Incidence of stem rot and pod yields were similar for those two treatments.

Keyword(s): EBI fungicides, fungicide resistance management