Strains of Botrytis cinerea resistant to dicarboximide and benzimidazole fungicides in New Zealand vineyards

Abstract

In a survey of New Zealand vineyards at harvest 1985, isolates of Botrytis cinerea resistant to benzimidazole and to dicarboximide fungicides were common. The mean frequency of resistance in the major vine‐growing districts ranged from 8 to 41% for benzimidazoles, and from 51 to 59% for dicarboximides. All benzimidazole‐resistant isolates showed high levels of resistance (EC50 greater than 100 mg/l carbendazim based on radial growth response), and all dicarboximide‐resistant isolates showed low levels of resistance. Two subgroups of dicarboximide‐resistant isolates were recognized, distinguished in the first instance by their osmotic response. Low‐level resistant isolates, which formed a dense margin on osmotically amended medium, exhibited an EC50 for mycelial growth on iprodione of c. 3‐2 mg/l; ultra‐low‐level resistant isolates, which formed a fibrillose margin on osmotically amended medium identical to that of sensitive isolates, exhibited an EC50 of c. 1‐3 mg/1. In agar culture, radial growth rate, and conidial and sclerotial production of both subgroups were similar to those of sensitive isolates. Virulence (lesion size) and conidial production on grape berries were highest in sensitive isolates, intermediate in ultra‐low‐level dicarboximide‐resistant isolates, and lowest in low‐level dicarboximide‐resistant isolates. Evidence is presented indicating that ultra‐low‐level dicarboximide‐resistant strains have progressively replaced low‐level dicarboximide‐resistant strains in the vineyard population. The presence of dicarboximide‐resistant strains was linked with a partial loss of fungicide efficacy.