Competitive ability of multi-fungicide resistant Botrytis cinerea in a blackberry planting over three years

Botrytis cinerea isolates with multi-fungicide resistance have frequently been isolated from small fruit fields such as strawberries and blackberries. Individual B. cinerea isolates have been found resistant to up to seven chemical classes of fungicides. Fitness costs and less competitiveness have been observed in multi-fungicide resistant isolates, but this has not been examined under field conditions. In the spring of 2016, flowers of field-grown blackberries were either not inoculated or inoculated with B. cinerea isolates sensitive (OCCR), resistant to five or six chemical classes excluding phenylpyrroles (5CCR), or resistant to six or seven chemical classes including phenylpyrroles (6CCR/MDR1h). The experimental field was left unsprayed for the duration of this study and isolates of B. cinerea were collected from flowers and/or fruit in each of the three experimental years. Isolates collected in summer of 2016 revealed resistance phenotypes in each plot closely matching those of the respective inoculum, with 95% OCCR, 55% 5CCR, and 91% 6CCR/MDR1h isolates recovered from OCCR, 5CCR, and 6CCR/MDR1h inoculation plots, respectively. In the 2017 and 2018 isolate collections, 6CCR/MDR1h resistance phenotypes were found in plots inoculated and non-inoculated with this phenotype, indicating their persistence and movement between plots. Resistance phenotypes different from the inoculum were also recovered each year, indicating that the inoculum was successfully competing with a native Botrytis population. Despite the competition, 6CCR/MDR1h isolates were recovered in high frequency from all inoculated plots in 2018. G3pdh and mrrl sequences of 6CCR/MDR1h isolates collected in 2018 were identical to the sequences of the inoculum, indicating that these isolates likely descended from the inoculum. This study demonstrates that isolates carrying multi-fungicide resistant phenotypes, specifically 6CCR/MDR1h, are competitive in the absence of fungicide selection pressure.