Sensitivity of Botrytis cinerea from greenhouse vegetables to DMIs and fenhexamid

Two hundred isolates of Botrytis cinerea were collected from greenhouse vegetables between 2003 and 2006 to determine their baseline sensitivity to triadimefone, penconazole, tebuconazole and fenhexamid. Mean values of 50% effective concentrations (EC 50 ) of inhibiting growth were 4.853±5.102, 0.41±0.215, 0.19±0.099 and 0.36±0.891 mg l −1 , respectively (mean±SD). Individuals of B. cinerea in the population differed by a factor (EC 50 of the least sensitive isolate/EC 50 of the most sensitive isolate) of 6625, 20, 603 and 1800, respectively. Naturally fenhexamid-resistant isolates were detected with an unexpected high frequency of 10% although the pathogen population had never been exposed to this fungicide. The resistance level (mean EC 50 of resistant isolates / mean EC 50 of sensitive isolates) was 19.5. These naturally resistant isolates also were resistant in vivo , and there was no significant difference in growth rate, conidial production or pathogenicity ability between naturally resistant and wild sensitive isolates. These results indicated that there was a potential risk of practical resistance if fenhexamid was applied alone. Negative cross-resistance was observed between fenhexamid and tebuconazole in 90% of the naturally resistant isolates. Moreover, an obvious synergism of the antifungal activity of fenhexamid by tebuconazole was demonstrated in some of the naturally fenhexamid-resistant isolates.