Baseline sensitivity and resistance risk assessment of Stemphylium solani to fluxapyroxad

Fluxapyroxad, a broad-spectrum and high-efficiency succinate dehydrogenase inhibitor (SDHI) fungicide, can control plant pathogens on many crops. However, no data about the fungicidal activity of fluxapyroxad against Stemphylium solani and its resistance risk are available. In this study, the mycelial growth inhibition method was used to determine the sensitivity of 99 S. solani strains to fluxapyroxad. The EC50 values ranged from 0.0183 to 0.3017 μg/mL, with an average EC50 value of 0.0839 ± 0.0051 μg/mL. The results indicated that fluxapyroxad exhibited an excellent inhibitory efficacy against S. solani. Six resistant mutants were obtained by laboratory fungicide taming. The resistance ratio of each mutant was greater than 50 and the resistance of the mutants to fluxapyroxad was steadily inheritable through 10 successive culture transfers to fungicide-free PSA plates. We found that fluxapyroxad resistance not only affected mycelial growth in S. solani, but was also temperature dependent. Moreover, fluxapyroxad resistance caused markedly decreased virulence of S. solani on detached tomato leaves, indicating that the resistant mutants suffered a fitness penalty. Cross-resistance analysis showed that no cross-resistance was observed between fluxapyroxad and fludioxonil, iprodione, azoxystrobin or fluazinam, while a positive cross-resistance was observed between fluxapyroxad and pydiflumetofen. We concluded that the resistance of S. solani to fluxapyroxad was moderate risk. Therefore, fluxapyroxad is recommended as a potential candidate for controlling tomato gray leaf spot caused by S. solani. •Fluxapyroxad exhibited an excellent in vitro antifungal activity against Stemphylium solani.•The resistance of Stemphylium solani to fluxapyroxad was classified as moderate risk.•There was a positive cross-resistance between fluxapyroxad and pydiflumetofen against Stemphylium solani.•Fluxapyroxad is recommended as a potential candidate for controlling tomato gray leaf spot caused by Stemphylium solani.