Resistance of ginseng gray mold pathogen, Botrytis cinerea, to boscalid and pyraclostrobin fungicides in China

Gray mold caused by Botrytis cinerea severely threatens the yield of ginseng (Panax ginseng). Various categories of fungicides have been utilized to control gray mold on this crop. In this study, the resistance of 102 isolates of B. cinerea from 11 commercial ginseng growing regions in China to fungicide was examined. A total of 32.4% were resistant to boscalid with EC50 values that ranged from 12.26 to 235.87 μg/ml, and 94.1% were resistant to pyraclostrobin with EC50 values that ranged from 5.88 to 487.72 μg/ml. Except for sdhA and sdhD, the amino acid substitutions of P225F, P225L, N230I, H272Y, and H272R in the sdhB subunit from 24 (4 S and 20 R), five (1 S and 4 R), one (S), 1 (R), and eight (4 S and 4 R) strains and the concurrent amino acid substitutions of G85A + I93V + M158V + V168I in the sdhC subunit from five (4 S and 1 R) strains were identified. A G143A substitution in cytochrome b was identified from 96 isolates that were resistant to pyraclostrobin and three that were sensitive to it. The Bcbi-143/144 intron was identified from the other three isolates sensitive to pyraclostrobin, but it was absent in the isolates that harbored the G143A mutation. The results showed that the populations of B. cinerea on ginseng have developed strong resistance to pyraclostrobin. Therefore, it is not recommended to continue using this fungicide to control gray mold on P. ginseng. Boscalid is still effective against most isolates, however, to prevent fungicide resistance, it is recommended to use mixtures with other categories of fungicides.