Evolution of fungicide resistance in Botrytis cinerea in protected crops in Greece

Isolates of Botrytis cinerea Pers.: Fr. from different crops and localities in Greece were compared with reference isolates from France and Israel and classified into various groups on the basis of their sensitivity to dicarboximides and benzimidazoles. Some isolates had a low resistance to both fungicides (spore germination and mycelial growth occurred on medium containing 3 micrograms iprodione ml-1 or 1 microgram carbendazim ml-1). These isolates were insensitive to phenylcarbamates (spore germination and mycelial growth on 100 micrograms diethofencarb ml-1). Many isolates had a high resistance to benzimidazoles (spore germination and mycelial growth on medium containing 100 micrograms carbendazim ml-1); these isolates showed negative cross-resistance to phenylcarbamates (spore germination and mycelial growth inhibited on 1 microgram diethofencarb ml-1). Some isolates exhibited reduced sensitivity to dichlofluanid (spore germination on medium containing 1 microgram fungicide ml-1). Infections caused by isolates resistant to dicarboximides or to both dicarboximides and benzimidazoles were uncommon at the beginning of each new crop cycle. However, both disease incidence and frequencies of infection by such isolates increased substantially as the season progressed and then declined after cessation of fungicide sprays. In contrast, infections caused by isolates that showed high resistance to benzimidazoles but were sensitive to dicarboximides were frequent throughout the growing season. There were no infections caused by strains resistant to both benzimidazoles and phenylcarbamates at sites treated with carbendazim plus diethofencarb. The reduced efficacy of this mixture could be due to its inferior activity in vitro against isolates exhibiting high resistance to benzimidazoles and negatively correlated cross-resistance to diethofencarb.