Characterization of Fludioxonil-Resistant Strains of Colletotrichum gloeosporioides from Eastern China

Ripe rot caused by Colletotrichum spp. poses a devastating threat to the grape industry worldwide. C. gloeosporioides is the main pathogen of grape ripe rot in eastern China. Fludioxonil has been used extensively to control disease on grape in China for more than 20 years. However, it remains unclear whether the strains of C. gloeosporioides in Chinese vineyards are resistant to fludioxonil. Of the 68 C. gloeosporioides isolates collected from the commercial vineyards during 2016 to 2018, 52 showed low resistance to fludioxonil (Flu^LR) with a resistance frequency of 76.47%. To further characterize strains with moderate or high resistance to fludioxonil, six induced highly fludioxonil-resistant (Flu^IHR) mutants were obtained by repetitive exposure to fludioxonil in vitro. In comparison with the fludioxonil-sensitive (Flu^S) isolates, most of the low fludioxonil-resistant (Flu^LR) and Flu^IHR strains hardly exhibited biological fitness penalty based on mycelial growth, sporulation, and virulence. Analysis of EC₅₀ values of several fungicides showed a strong correlation between fludioxonil and procymidone but no correlation between fludioxonil and fluazinam or prochloraz. Sequence alignment of CgOs-1 indicated that amino acid substitutions in Flu^IHR strains could be categorized into three Flu-resistant genotypes, CgOs-1^T50A, CgOs-1^T50A&A300T, and CgOs-1^T50A&T654K, while no amino acid substitutions were found in CgOs-1 from the Flu^LR or Flu^S isolates. To our knowledge, this is a detailed report characterizing fludioxonil-resistant strains of C. gloeosporioides selected from commercial vineyards and laboratories, which will provide a valuable reference for the control of grape ripe rot.