Risk Assessment on Resistance to Pyraclostrobin of Colletotrichum gloeosporioides

Objective  Drug sensitivity to and cross-resistance of pyraclostrobin with other fungicides of Colletotrichum gloeosporioides that caused anthracnose in soybeans in Fujian Province were investigated.

Methods Sensitivity to pyraclostrobin of 112 strains of C. gloeosporioides isolated from 5 regions in the province in 2023 was determined using the mycelial production rate method. Resistant mutants of the pathogen were obtained by fungicide domestication to determine their genetic stability, fitness, and possible cross-resistance to 4 fungicides.

Results  The EC₅₀ of pyraclostrobin for the 112 C. gloeosporioides strains ranged 0.0682 µg·mL⁻¹ to 1.0309 µg·mL⁻¹ with a coefficient of variation of 15.12 and averaged (0.203 6±0.121 5) µg·mL⁻¹ as the sensitivity baseline. The distribution of the sensitivity frequency was in a continuous unimodal function consistent with the continuous skewed normal distribution. Induced from two wild C. gloeosporioides, 4 moderate resistant strains and one low resistant strain were secured at a frequency of 8.3×10⁻⁴. The mutants stably inherited the drug resistance traits but differed from their parents in temperature sensitivity and were slightly lower on mycelial growth rate, sporulation ability, and pathogenicity. In the field, they might sacrifice other natural competitive advantages as a fitness cost in exchange for the drug resistance. Pyraclostrobin showed no cross-resistance with carbendazim, difenoconazole, and prochloraz but did with the same type of fungicide, picoxystrobin.

Conclusion Although the C. gloeosporioides isolated in Fujian were relatively high in sensitivity to pyraclostrobin, they represented merely a moderate drug resistance risk. Nonetheless, in practice, it was plausible to blend pyraclostrobin or apply alternatively with fungicides such as carbendazim, difenoconazole, and prochloraz to delay such potential drawback.