Sensitivity of Botryosphaeria dothidea from California pistachio to tebuconazole

Botryosphaeria blight of pistachio, caused by Botryosphaeria dothidea, is a devastating disease in California. Baseline sensitivity of B. dothidea to tebuconazole (Elite®) was determined using 105 wild-type single-spore isolates collected in 1999 and 2000 from a commercial pistachio orchard without a previous history of fungicide exposure in Glenn County, California. The 50% effective concentration (EC50) values of tebuconazole for these 105 isolates ranged from 0.019 to 0.159 μg/ml, and the mean EC50 was 0.080 μg/ml. In 2000, 277 single-spore isolates were collected from a fungicide trial orchard with 4 years of multi-sprays of tebuconazole in Glenn County, California. The EC50 values of these 277 isolates ranged from 0.037 to 0.291 μg/ml. The mean EC50 (0.099 μg/ml) of these 277 isolates was significantly higher (F=62.2, P<0.0001) than that of wild-type isolates. The isolate PB22 from the fungicide trial orchard showed reduced-sensitivity to tebuconazole with an EC50 of 0.291 μg/ml significantly higher (F=114.21, P<0.0001) than those of 381 other tested isolates. PB22 also showed low sensitivity to propiconazole and benomyl. PB22 retained relatively high virulence on pistachio, but the efficacy of tebuconazole in controlling PB22 was not significantly different from that its efficacy against sensitive isolates.