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Postsymptom Activity of Ergosterol Inhibitors Against Apple Powdery Mildew. Jan Cimanowski, Chairman, Plant Pathology Section, Institute of Pomology and Floriculture, Skierniewice, Poland. Michael Szkolnik, Professor, Department of Plant Pathology, New York State Agricultural Experiment Station, Geneva 14456. Plant Dis. 69:562-563. Accepted for publication 17 December 1984. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/PD-69-562.

Ergosterol synthesis-inhibiting fungicides triadimefon, etaconazole, bitertanol, and triforine were compared with sulfur, dinocap, and benomyl for postsymptom activity against powdery mildew on McIntosh apple seedlings in the greenhouse. After a dilute spray to seedlings with abundantly sporulating mildew lesions, spore samples were taken at 1, 10, and 20 days using the transparent adhesive tape technique. Water and fungicide treatments greatly reduced the numbers of total and of newly formed nonwrinkled spores compared with unsprayed leaves. There was no reduction in the total number of spores between water- and fungicide-sprayed treatments at 1, 10, or 20 days after sprays except for a reduction with triforine in the 1-day count. Counts of apparently normal, nonwrinkled spores 1 day after spraying showed no significant difference among treatments except for a reduction by triforine compared with water; this difference was not evident 10 and 20 days after treatment. The greatest postsymptom reduction in the production of new viable spores was noted at 20 days, with significantly lower counts on leaves sprayed with triadimefon, etaconazole, or sulfur.