Detection and Fitness of Dicarboximide-Resistant Isolates of Alternaria alternata from Dendrobium officinale, a Chinese Indigenous Medicinal Herb

Black spot, caused by Alternaria alternata, poses a severe threat to the industry of Dendrobium officinale, a Chinese indigenous medicinal herb. Dicarboximide fungicides (DCFs) have been intensively used to control this disease for decades in China, and offer excellent efficacy. The resistance of phytopathogenic pathogens against DCFs are reportedly selected in fields; however, the DCF resistance of A. alternata from D. officinale is not well understood. The isolates of A. alternata with low procymidone resistance (Pro^LR) were detected in the commercial orchards of D. officinale in China in 2018 and biochemically characterized in this study. The result showed that the Pro^LR isolates were selected in the commercial orchards with a resistance frequency of 100%, and no significant difference in mycelial growth, sporulation, and virulence was observed among the Pro^LR and procymidone-sensitive (Pro^S) isolates. A positive cross-resistance pattern was exhibited between procymidone and iprodione. Results of amino acid sequence alignment of AaOS-1 from the tested isolates showed that all of the Pro^LR genotypes could be categorized into two groups, including group I (mutations at AaOs-1) and group II (no mutation). Under procymidone (5.0 µg/ml) treatment conditions, the AaOs-1 expression levels increased in the Pro^S isolates and ranged from approximately 2.94- to 3.69-fold higher than those under procymidone-free conditions, while the AaOs-1 expressions of the Pro^LR isolates were significantly lower than those in the Pro^S isolates under the same conditions. The data indicated that the mutations at AaOs-1 are involved in the DCF resistance of A. alternata selected in the D. officinale orchards.