Increased Resistance to Triadimefon and to Benomyl in<i>Sphaerotheca fuliginea</i>Populations Following Fungicide Usage over One Season

Abstract

Fungicide sensitivity of Sphaerotheca fuliginea was monitored during powdery mildew epidemics in research plots and production fields of pumpkin. Benomyl-resistant isolates (insensitive to 200 μg/ml) were detected in all populations before benomyl was used. The only triadimefon-resistant isolate (insensitive to 100 μg/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 μg/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 μg/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.