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Increased Resistance to Triadimefon and to Benomyl in Sphaerotheca fuliginea Populations Following Fungicide Usage over One Season. Margaret Tuttle McGrath, Department of Plant Pathology, Long Island Horticultural Research Laboratory, Cornell University, 3059 Sound Avenue, Riverhead, NY 11901-1098. Plant Dis. 80:633. Accepted for publication 19 February 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/PD-80-0633.

Fungicide sensitivity of Sphaeroiheca fuliginea was monitored during powdery mildew epidemics in research plots and production fields of pumpkin. Benomyl-resistant isolates (insensitive to 200 mg/ml) were detected in all populations before benomyl was used. The only triadimefon-resistant isolate (insensitive to 100 mg/ml) detected prior to treatment was near triadimefon-treated plants. Populations in nontreated fields did not change appreciably. Fungicide application had a large impact on fungicide resistance. Proportion of resistant isolates in research plots treated four times with triadimefon plus chlorothalonil shifted from 0 to 81% for triadimefon and from 30 to 69% for benomyl between 15 August and 19 September 1991; proportion shifted from 3 to 100% for triadimefon and from 10 to 44% for benomyl between 14 August and 17 September 1992. The shift occurred rapidly: proportion of resistant isolates shifted within 2 weeks from 0 to 96% for triadimefon and from 10 to 74% for benomyl following two applications of triadimefon in a commercial field in August 1992. Most isolates (87%) from fungicide-treated fields tolerated triadimefon at 100 or 200 mg/ml. These were considered triadimefon-resistant because fungicide performance declined during September. However, control was commercially acceptable. Of 122 isolates collected from fungicide-treated pumpkin, one was benomyl-resistant, triadimefon-sensitive; 20 were benomyl-sensitive, triadimefon-resistant; six were sensitive to both fungicides; and 95 were resistant to both with 77 of these insensitive to triadimefon at 200 mg/ml. In contrast, 57 of 92 isolates from nontreated populations were sensitive to both fungicides, none were resistant to both, and 32 were resistant to benomyl.