Sensitivity detection and resistance risk analysis of Phytophthora capsici on tomato to azoxystrobin in Jiangxi Province

In order to determine the sensitivity of Phytophthora capsici to azoxystrobin, the mycelia growth rate method was used to measure the EC₅₀ value of azoxystrobin to 58 strains of P. capsici collected from the major tomato producing areas in Jiangxi Province from 2018 to 2019. Four resistant mutants were obtained from susceptible strain (YD5) by UV irradiation, and their biological characters were investigated. The sensitivity of susceptible strain (YD5) and four resistant mutants to dimethomorph and metalaxyl was determined by mycelial growth rate method, and the cross-resistance of azoxystrobin to dimethomorph and metalaxyl was also detected. The results showed that the mycelial growth of 58 P. capsicistrains was affected by azoxystrobin, with a mean EC₅₀ value of (0.8606±0.3318) μg/mL and ranged from 0.1867 μg/mL to 1.6239 μg/mL. The sensitivity of 58 strains to azoxystrobin was fitted with a normal distribution function, which indicated that P. capsiciwas sensitive to azoxystrobin. Four resistant mutants were obtained by induction, and the resistance of them could be inherited stably. The pathogenicity and zoospore germination rates of the mutants were not significantly different from those of their parent, but the sporulation decreased significantly. There was no cross-resistance between azoxystrobin and dimethomorph, metalaxyl respectively. It was suggested that P. capsici had a moderate resistance risk to azoxystrobin. So once resistant strains appear in the field, measures should be taken immediately to control the spread of the flora. Meanwhile, the sensitivity of P. capsici to azoxystrobin should be monitored, and mixed or rotated application of fungicides with different mechanism should be considered to avoid or postpone the development of resistance.