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

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 Botrytisfragariae sp. nov. Isolates of Bfragariae were detected in strawberry fields throughout Germany, sometimes at frequencies similar to those of B. cinerea, and in the southeastern United States. Bfragariae was isolated from overwintering strawberry tissue but not from freshly infected fruit. Bfragariae 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 Bfragariae is host specific and has a tissue preference different from that of B. cinerea Various fungicide resistance patterns were observed in Bfragariae populations. Many Bfragariae 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 Bfragariae 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 Bfragariae indicates that this species is adapted to fungicide-treated strawberry fields and may be of local importance as a gray mold pathogen alongside B. cinereaIMPORTANCE 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 Botrytisfragariae, 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. Bfragariae 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 Bfragariae could be of practical importance as a strawberry pathogen in some regions where its abundance is similar to that of B. cinerea.

Botrytis pseudocinerea Is a Significant Pathogen of Several Crop Plants but Susceptible to Displacement by Fungicide-Resistant B. cinerea Strains.

Botrytis cinerea is one of the most important pathogens worldwide, causing gray mold on a large variety of crops. Botrytis pseudocinerea has been found previously to occur together with B. cinerea in low abundance in vineyards and strawberry fields. Here, we report B. pseudocinerea to be common and sometimes dominant over B. cinerea on several fruit and vegetable crops in Germany. On apples with calyx end rot and on oilseed rape, it was the major gray mold species. Abundance of B. pseudocinerea was often negatively correlated with fungicide treatments. On cultivated strawberries, it was frequently found in spring but was largely displaced by B. cinerea following fungicide applications. Whereas B. cinerea strains with multiple-fungicide resistance were common in these fields, B. pseudocinerea almost never developed resistance to any fungicide even though resistance mutations occurred at similar frequencies in both species under laboratory conditions. The absence of resistance to quinone outside inhibitors in B. pseudocinerea was correlated with an intron in cytB preventing the major G143A resistance mutation. Our work indicates that B. pseudocinerea has a wide host range similar to that of B. cinerea and that it can become an important gray mold pathogen on cultivated plants.

Spread of Botrytis cinerea Strains with Multiple Fungicide Resistance in German Horticulture.

Botrytis cinerea is a major plant pathogen, causing gray mold rot in a variety of cultures. Repeated fungicide applications are common but have resulted in the development of fungal populations with resistance to one or more fungicides. In this study, we have monitored fungicide resistance frequencies and the occurrence of multiple resistance in Botrytis isolates from raspberries, strawberries, grapes, stone fruits and ornamental flowers in Germany in 2010 to 2015. High frequencies of resistance to all classes of botryticides was common in all cultures, and isolates with multiple fungicide resistance represented a major part of the populations. A monitoring in a raspberry field over six seasons revealed a continuous increase in resistance frequencies and the emergence of multiresistant Botrytis strains. In a cherry orchard and a vineyard, evidence of the immigration of multiresistant strains from the outside was obtained. Inoculation experiments with fungicide-treated leaves in the laboratory and with strawberry plants cultivated in the greenhouse or outdoors revealed a nearly complete loss of fungicide efficacy against multiresistant strains. B. cinerea field strains carrying multiple resistance mutations against all classes of site-specific fungicides were found to show similar fitness as sensitive field strains under laboratory conditions, based on their vegetative growth, reproduction, stress resistance, virulence and competitiveness in mixed infection experiments. Our data indicate an alarming increase in the occurrence of multiresistance in B. cinerea populations from different cultures, which presents a major threat to the chemical control of gray mold.