Protectant and After-Infection Activity of Fungicides Against Botryosphaeria obtusa on Apple. L. F. Arauz, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616. T. B. Sutton, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616. Plant Dis. 74:1029-1034. Accepted for publication 18 June 1990. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/PD-74-1029.

Reduction of radial growth of Botryosphaeria obtusa on fungicide-amended potato-dextrose agar medium was determined. EC₅₀ values (µg/ml) were as follows: benomyl, 0.032; bitertanol, 0.043; flusilazole, 0.045; mancozeb, 10.26; myclobutanil, 0.426; penconazole, 0.132; and tebuconazole, 0.036. Six of the fungicides were tested for their protectant and eradicant activity against B. obtusa. When apple (Malus domestica) seedlings were inoculated 7 days after fungicide application, mean disease severities (lesions per 100 cm²) were as follows: benomyl, 4.6; bitertanol, 4.6; flusilazole, 20.7; mancozeb, 0.4; penconazole, 11.6; tebuconazole, 0.9; and control (no fungicide), 101.0. Only tebuconazole gave satisfactory disease control when plants were inoculated 14 days after fungicide application. Flusilazole, penconazole, and tebuconazole reduced the severity of frogeye leaf spot by more than 50% when applied 48 hr after inoculation or earlier. Tebuconazole and benomyl reduced the severity of black rot on detached apple fruit when applied 96 hr after inoculation or earlier. The application of 4.8 cm of simulated rain did not reduce the protectant activity of mancozeb and benomyl against B. obtusa but did significantly reduce the efficacy of tebuconazole and penconazole.