Resistance mechanism of Phomopsis longicolla to fludioxonil is associated with modifications in PlOS1, PlOS4 and PlOS5.

Phomopsis longicolla, a causal agent of soybean root rot, stem blight, seed decay, pod and stem canker, which seriously affects the yield and quality of soybean production worldwide. The phenylpyrrole fungicide fludioxonil exhibits a broad spectrum and high activity against phytopathogenic fungi. In this study, the baseline sensitivity of 100 P. longicolla isolates collected from the main soybean production areas of China to fludioxonil were determined. The result showed that the EC values of all the P. longicolla isolates ranged from 0.013 to 0.035 μg/ml. Furthermore, 12 fludioxonil-resistance (Flu) mutants of P. longicolla were generated from 6 fludioxonil-sensitive (Flu) isolates. and the resistance factors (RF) of 12 Flu mutants were >3500. Sequence alignment showed that multiple mutation types were found in PlOS1, PlOS4 or/and PlOS5 of Flu mutants. All the Flu mutants exhibited fitness penalty in mycelial growth, conidiation, virulence and osmo-adaptation. Under fludioxonil or NaCl treatment condition, the glycerol accumulation was significantly increased in Flu isolates, but was slightly increased in Flu mutants, and the phosphorylation level of most Flu mutants was significantly decreased when compared to the Flu isolates. Additionally, positive cross-resistance was observed between fludioxonil and procymidone but not fludioxonil and pydiflumetofen, pyraclostrobin or fluazinam. This is first reported that the baseline sensitivity of P. longicolla to fludioxonil, as well as the biological and molecular characterizations of P. longicolla Flu mutants to fludioxonil. These results can provide scientific directions for controlling soybean diseases caused by P. longicolla using fludioxonil.