Abstract
Botrytis cinerea, the causal agent of gray mold disease, is one of the most important pathogens of strawberry. Its control in commercial strawberry fields is largely dependent on the application of fungicides during bloom and fruit maturation. The hydroxyanilide fenhexamid is one of the most frequently used fungicides in the southeast of the United States for gray mold control. It inhibits the 3-ketoreductase (Erg27) of the ergosterol biosynthesis pathway and, due to this site-specific mode of action, is at risk for resistance development. Single-spore isolates were collected from 11 commercial strawberry fields in North and South Carolina and subjected to a conidial germination assay that distinguished sensitive from resistant phenotypes. Of the 214 isolates collected, 16.8% were resistant to fenhexamid. Resistance was found in three of four locations from North Carolina and in four of seven locations from South Carolina, indicating that resistance was widespread. Mutations in Erg27 (T63I, F412S, F412C, and F412I) were associated with resistance, with F412S the predominant and most widespread mutation. In this study, mutations T63I and F412C in field isolates of B. cinerea are described for the first time. Detached fruit studies showed that field rates of Elevate 50 WDG (fenhexamid) controlled sensitive but not resistant isolates carrying any of the four mutations. Resistant isolates produced the same lesion size and number of sporulating lesions on fruit sprayed with Elevate 50 WDG as on untreated controls, showing the fungicide's loss of efficacy against those isolates. A rapid polymerase chain reaction method was developed to quickly and reliably distinguish isolates sensitive or resistant to fenhexamid in the Carolinas and to determine the mutation associated with resistance. The presence of fenhexamid-resistant strains in B. cinerea from strawberry fields in the Carolinas must be considered in future resistance management practices for sustained gray mold control.
