Characterization and fitness of Fusarium graminearum mutants resistant to fluazinam

Fusarium head blight (FHB), predominately caused by ascomycete Fusarium graminearum sensu stricto, is a devastating fungal disease of cereal crops worldwide. In this study, baseline sensitivity of F. graminearum s. str. to fluazinam was established using 100 isolates that were collected from Anhui and Jiangsu provinces of China in 2009 and 2018. The EC50 values of fluazinam on mycelial growth inhibition ranged from 0.0211 to 0.1097 mu g/mL and the mean EC50 was 0.0601 +/- 0.0191 mu g/mL, which indicated that fluazinam had a strong inhibitory effect on mycelial growth of F. graminearum s. str. To acquire fluazinam-resistant (Flu(R)) mutants, one wild-type strain (PH-1, with assembled genome) and another four fluazinam-sensitive (Flu(S)) isolates randomly selected from the above-mentioned population were domesticated with fluazinam or mutagenized by ultraviolet light in vitro. As a result, 45 Flu(R) mutants were obtained with resistance frequencies of 0.68% (domestication) and 8 in 10(-8) (mutagenesis), respectively. Of the 45 Flu(R) mutants, 11 were selected to determine the biological fitness. Based on data of radial mycelial growth, asexual and sexual reproduction, response to environmental stresses, pathogenicity and deoxynivalenol production, all the tested mutants suffered from a fitness penalty. Additionally, slightly positive cross-resistance was observed between the combination of fluazinam and tebuconazole, pyraclostrobin, or pydiflumetofen, but not phenamacril or carbendazim. Therefore, a low-to-moderate risk of development of F. graminearum s. str. resistant to fluazinam was suggested, indicating that fluazinam could be a good alternative fungicide against the FHB in the field.