Fungicide Resistance of and Isolates from Mandarin Fruit and Its Influence on Control of Postharvest Alternaria Rot.

Alternaria rot, caused by and , is one of the common postharvest diseases affecting mandarin fruit in California. Fungicide resistance profilings of and to quinone outside inhibitors (QoIs), fludioxonil, pyrimethanil, imazalil, and propiconazole were examined in this study. Of the 100 isolates of and , 40 were identified as resistant to QoI fungicides according to a PCR-restriction fragment length polymorphism based on a partial gene. Effective concentrations of fludioxonil that caused a 50% reduction in fungal growth relative to the control (EC) were 0.089 ± 0.020 and 0.101 ± 0.032 µg/ml for 43 and 19 isolates, respectively. EC values of pyrimethanil, imazalil, and propiconazole for the 70 isolates tested were 0.373 ± 0.161, 0.492 ± 0.133, and 1.135 ± 0.407 µg/ml, respectively. EC values of pyrimethanil, imazalil, and propiconazole for 30 isolates were 0.428 ± 0.190, 0.327 ± 0.180, and 0.669 ± 0.452 µg/ml, respectively. Control tests on mandarin fruit inoculated with representative isolates of both species showed that fludioxonil, pyrimethanil, imazalil and propiconazole significantly reduced disease incidence and severity. Azoxystrobin significantly reduced Alternaria rot severity and incidence on mandarin fruit inoculated with sensitive isolates but not with resistant isolates, regardless of spp. There were no significant differences in Alternaria rot control effectiveness if treatment with any of the fungicides tested was delayed after inoculation by 6 or 12 h. These results could help in the development of postharvest fungicide programs to control Alternaria rot on mandarin fruit during storage.