Prevalence of H6Y mutation in β-tubulin causing thiophanate-methyl resistant in Monilinia fructicola from Fujian, China.

Brown rot disease broke out in stone fruit orchards of Fujian, China in 2019, despite pre-harvest application of methyl benzimidazole carbamate (MBC). To determine the reason, a total of 44 Monilinia fructicola strains were collected from nectarine, plum and peach fruits in this study, among which 79.5% strains were resistant to thiophanate-methyl, indicated by discriminatory dose of 5 μg/mL. The resistance of these strains was confirmed by treating detached peach fruit with label rates of formulated thiophanate-methyl which only completely inhibit infection of the sensitive strains, but not the resistant strains. Further analysis of the mechanism of MBC resistance revealed that all resistant strains carry a H6Y mutation in β-tubulin protein Tub2, which was only reported previously in the M. fructicola strains from California, USA, and do not display obvious fitness penalties, as no significant defects in mycelial growth rate, sporulation, conidia germination, aggressiveness on detached peach fruit and temperature sensitivity was detected. In addition, we found that diethofencarb, the agent for managing MBC-resistance strains, was unable to inhibit growth of the H6Y strains. Taken together, our study, for the first time, identified a mutation form of H6Y in the β-tubulin protein of M. fructicola in China, rendering the strains wide resistance to thiophanate-methyl. This mechanism of M. fructicola gaining resistance to MBC fungicides needs to be fully considered, when designing management strategies to control brown rot disease in stone fruit orchards.