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Disease Control and Pest Management

Evaluation of Two Triazole Fungicides for Postinfection Control of Apple Scab. R. D. Kelley, Graduate research assistant, Department of Botany and Plant Pathology and the Pesticide Research Center, Michigan State University, East Lansing 48824; A. L. Jones, professor, Department of Botany and Plant Pathology and the Pesticide Research Center, Michigan State University, East Lansing 48824. Phytopathology 71:737-742. Accepted for publication 30 December 1980. Copyright The American Phytopathological Society. DOI: 10.1094/Phyto-71-737.

The triazole fungicides 1-((2-(2,4-dichlorophenyl)-4-ethyl-1,3-dioxolan-2-yl)methyl)-IH-1,2,4-triazole (CGA-64251) and bitertanol effectively controlled apple scab under greenhouse and orchard conditions. In greenhouse studies, postinfection scab control by CGA-64251 at 18.7 μg/ml and bitertanol at 299.6 μg/ml applied 2 and 3 days, respectively, after inoculation were similar to fenarimol at 41.9 μg/ml, fenapanil at 617.9 μg/ml, and an organic mercury fungicide at 93.8 μg/ml. Fungicides applied 4.0, 4.5, 5.0, and 5.5 days after inoculation did not prevent establishment of lesions, but did inhibit their development and sporulation from them. In orchard studies, two sprays 7 days apart of CGA-64251 at 18.7 μg/ml or bitertanol at 299.6 μg/ml, applied to lesions either late in the incubation period or starting 2 days after they were visible, prevented spore production in chlorotic lesions and suppressed conidial development in sporulating lesions. Fruit infections also were suppressed. Fungicides applied at 7-day intervals suppressed scab development better than those applied at 14-day intervals. Leaves sprayed with CGA-64251 were smaller, thicker, puckered, darker green, and had more layers of palisade cells than those from unsprayed trees; whether the net effect was detrimental or beneficial remains to be determined. These therapeutic fungicides should be tested further in apple scab control experiments designed to identify infection periods and to suppress established epidemics of apple scab.