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

Alternaria rot, caused by Alternaria alternata and A. arborescens, is one of the common postharvest diseases affecting mandarin fruit in California. Fungicide resistance profilings of A. alternata and A. arborescens to quinone outside inhibitors (QoIs), fludioxonil, pyrimethanil, imazalil, and propiconazole were examined in this study. Of the 100 isolates of A. alternata and A. arborescens, 40 were identified as resistant to QoI fungicides according to a PCR-restriction fragment length polymorphism based on a cytochrome b partial gene. Effective concentrations of fludioxonil that caused a 50% reduction in fungal growth relative to the control (EC50) were 0.089 +/- 0.020 and 0.101 +/- 0.032 mu g/ml for 43 A. alternata and 19 A. arborescens isolates, respectively. EC50 values of pyrimethanil, imazalil, and propiconazole for the 70 A. alternata isolates tested were 0.373 +/- 0.161, 0.492 +/- 0.133, and 1.135 +/- 0.407 mu g/ml, respectively. EC50 values of pyrimethanil, imazalil, and propiconazole for 30 A. arborescens isolates were 0.428 +/- 0.190, 0.327 +/- 0.180, and 0.669 +/- 0.452 mu g/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 Alternaria 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.

Automated summary

Alternaria rot, caused by Alternaria alternata and A. arborescens, is a common postharvest disease in California mandarin fruit. This study examined the resistance of A. alternata and A. arborescens to various fungicides. The results showed that fludioxonil, pyrimethanil, imazalil, and propiconazole effectively reduced disease incidence and severity in mandarin fruit.