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Sensitivity of Monilinia fructicola from Peach Farms in China to Four Fungicides and Characterization of Isolates Resistant to Carbendazim and Azoxystrobin

Brown rot of peach caused by Monilinia fructicola can cause considerable preharvest and postharvest losses in China. Fungicides are increasingly utilized to minimize such losses. Eighty isolates of M. fructicola were collected from commercial peach orchards located in five provinces in China and the sensitivity to carbendazim, azoxystrobin, tebuconazole, and boscalid was determined. Resistance to carbendazim was detected only in the Yunnan province in 15 of 16 isolates. Characterization of carbendazim-resistant isolates revealed stable resistance, no fitness penalty, and negative cross resistance to diethofencarb. Resistant isolates produced disease symptoms on detached fruit sprayed with label rates of formulated carbendazim and possessed the amino acid mutation E198A in β-tubulin. Resistance to azoxystrobin was detected in 3 of 10 isolates from Fujian. In contrast to carbendazim resistance, however, azoxystrobin resistance was unstable, associated with a fitness penalty, and not associated with mutations in the target gene cytochrome b. The concentration at which mycelial growth is inhibited 50% (EC₅₀) values of the azoxystrobin-sensitive isolates were 0.02 to 1.94 μg/ml, with a mean value of 0.54 μg/ml. All isolates were sensitive to tebuconazole, with a mean EC₅₀ value of 0.03 μg/ml. The EC₅₀ values for boscalid were 0.01 to 3.85 μg/ml, with a mean value of 1.02 μg/ml. Our results indicate that methyl benzimidazole carbamates (MBCs), quionon outside inhibitors, demethylation inhibitor fungicides, and succinate dehydrogenase inhibitors are likely to be very effective in controlling brown rot in many peach production areas in China, but that resistance to MBCs is emerging.