During the period January‐March 1989, 15 greenhouses at 12 sites in Israel were surveyed for the presence of fungicide‐resistant strains of Botrytis cinerea, using a fungicide‐amended Botrytis‐selective medium. Resistance to benzimidazoles (BenR) and to dicarboximides (DicR) was frequent in most sites. Resistance to carbendazim + diethofencarb (BenR NPCR) was found in all eight sites in which a mixture of these fungicides had been used against grey mould, but not in other sites. A new phenotype of multiple fungicide resistance was found among these isolates. The new phenotype, designated BenR DicR NPCR, combines the three previously described characteristics of resistance to benzimidazole, dicarboximide and N‐phenylcarbamate fungicides. It was found only in cucumber greenhouses that had been sprayed with the fungicide mixture carbendazim + diethofencarb against grey mould. Isolates of this phenotype were pathogenic in artificial inoculation of cucumber cotyledons treated with carbendazim, iprodione or carbendazim + diethofencarb.
Based on the ‘negative cross‐resistance’ phenomenon between N‐phenylcarbamates and benzimidazole fungicides, the fungicidal mixture of carbendazim + diethofencarb (MBC + NPC) was introduced, to control Botrytis cinerea phenotypes which are either sensitive to benzimidazoles and resistant to NPC (wild‐type: BenS NPCR). or resistant to benzimidazoles and sensitive to NPC(BenR NPCS). At one out of four sites where the MBC + NPC mixture was used in commercial cucumber greenhouses, grey mould control failed and a new phenotype of B. cinerea was found. The new phenotype was resistant to benzimidazoles, as was 100% of the population screened in the four sites, but retained resistance to NPC. Accordingly, the new phenotype was designated BenR NPCR. It was pathogenic on cucumber seedlings.
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