Botrytisfragariae, a New Species Causing Gray Mold on Strawberries, Shows High Frequencies of Specific and Efflux-Based Fungicide Resistance

Abstract

ABSTRACT Botrytis cinerea causes pre- and postharvest decay of many fruit and vegetable crops. A survey of German strawberry fields revealed Botrytis strains that differed from B. cinerea in diagnostic PCR markers and growth appearance. Phylogenetic analyses showed that these strains belong to an undescribed species in Botrytis clade 2, named Botrytis fragariae sp. nov. Isolates of B . fragariae were detected in strawberry fields throughout Germany, sometimes at frequencies similar to those of B. cinerea , and in the southeastern United States. B . fragariae was isolated from overwintering strawberry tissue but not from freshly infected fruit. B . fragariae invaded strawberry tissues with an efficiency similar to or lower than that of B. cinerea but showed poor colonization of inoculated nonhost plant tissues. These data and the exclusive occurrence of this fungus on strawberry plants indicate that B . fragariae is host specific and has a tissue preference different from that of B. cinerea . Various fungicide resistance patterns were observed in B . fragariae populations. Many B . fragariae strains showed resistance to one or several chemical classes of fungicides and an efflux-based multidrug resistance (MDR1) phenotype previously described in B. cinerea . Resistance-related mutations in B . fragariae were identical or similar to those of B. cinerea for carbendazim (E198A mutation in tubA ), azoxystrobin (G143A in cytB ), iprodione (G367A+V368F in bos1 ), and MDR1 (gain-of-function mutations in the transcription factor mrr1 gene and overexpression of the drug efflux transporter gene atrB ). The widespread occurrence of B . fragariae indicates that this species is adapted to fungicide-treated strawberry fields and may be of local importance as a gray mold pathogen alongside B. cinerea . IMPORTANCE Gray mold is the most important fruit rot on strawberries worldwide and requires fungicide treatments for control. For a long time, it was believed to be caused only by Botrytis cinerea , a ubiquitous pathogen with a broad host range that quickly develops fungicide resistance. We report the discovery and description of a new species, named Botrytis fragariae , that is widely distributed in commercial strawberry fields in Germany and the southeastern United States. It was observed on overwintering tissue but not on freshly infected fruit and seems host specific on the basis of its occurrence and artificial infection tests. B . fragariae has also developed resistance to several fungicides that is caused by mutations similar to those known in B. cinerea , including an efflux-based multidrug resistance. Our data indicate that B . fragariae could be of practical importance as a strawberry pathogen in some regions where its abundance is similar to that of B. cinerea .