Fungicide resistance of Alternaria alternata and A. arborescens isolates from mandarin fruit and its influence on control of postharvest Alternaria rot

Authors: Wang, Fei; Saito, Seiya; Xiao, Chang-Lin

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/4/2022
Publication Date: 5/11/2023
Citation: Wang, F., Saito, S., Xiao, C. 2023. Fungicide resistance of Alternaria alternata and A. arborescens isolates from mandarin fruit and its influence on control of postharvest Alternaria rot. Plant Disease. 107(5):1538-1543. https://doi.org/10.1094/PDIS-09-22-2157-RE.
DOI: https://doi.org/10.1094/PDIS-09-22-2157-RE

Interpretive Summary: Alternaria rot caused by Alternaria alternata and A. arborescens is one of the common postharvest diseases of mandarin fruit in California. Control of citrus postharvest diseases has largely relied on the use of postharvest fungicides, but fungicide resistance in the pathogens can result in the failure of disease control. In this study, we examined resistance of Alternaria alternata and A. arborescens to the postharvest fungicides that are commonly used on citrus fruit. We found that 40% of the Alternaria spp. isolates tested were resistant to azoxystrobin and the fungicide failed to control Alternaria rot on the fruit inoculated with the fungicide-resistant isolates. All isolates tested were sensitive to fludioxonil, pyrimethanil, imazalil and propiconazole, and these four fungicides were effective to control Alternaria rot on the fruit inoculated with the pathogens. These findings could help packers develop postharvest fungicide programs to control Alternaria rot on mandarin fruit during storage.

Technical Abstract: 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 fungicide according to a PCR-RFLP based on cytochrome b partial gene. Effective concentrations of fludioxonil resulting in 50% inhibition in fungal growth relative to the control (EC50) were 0.089 ± 0.020 µg/ml and 0.101 ± 0.032 µg/ml for the 43 A. alternata and 19 A. arborescens isolates tested, respectively. EC50 values of pyrimethanil, imazalil, and propiconazole for the 70 A. alternata isolates tested were 0.373 ± 0.161 µg/ml, 0.492 ± 0.133 µg/ml, and 1.135 ± 0.407 µg/ml, respectively. EC50 values of pyrimethanil, imazalil, and propiconazole for the 30 A. arborescens isolates tested were 0.428 ± 0.190 µg/ml, 0.327 ± 0.180 µg/ml, and 0.669 ± 0.452 µg/ml, respectively. Control tests on mandarin fruit inoculated with representative isolates from each species showed that fludioxonil, pyrimethanil, imazalil and propiconazole significantly reduced disease incidence and severity. Azoxystrobin significantly reduced disease severity and incidence on the mandarin fruit inoculated with sensitive isolates, but not with resistant isolates regardless of Alternaria species. There were no significant differences after 6 and 12 h delayed treatments of all fungicides tested. The results presented in this study could help develop postharvest fungicide programs to control Alternaria rot on mandarin fruit during storage.