Fungicide resistance in Alternaria alternata from blueberry in California and its impact on control of Alternaria rot

Authors: Wang, Fei; Saito, Seiya; MICHAILIDES, THEMIS - Kearney Agricultural Center; Xiao, Chang-Lin

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/3/2021
Publication Date: 4/6/2022
Citation: Wang, F., Saito, S., Michailides, T., Xiao, C. 2022. Fungicide resistance in Alternaria alternata from blueberry in California and its impact on control of Alternaria rot. Plant Disease. 106(5):1446-1453. https://doi.org/10.1094/PDIS-09-21-1971-RE.
DOI: https://doi.org/10.1094/PDIS-09-21-1971-RE

Interpretive Summary: Alternaria rot caused by Alternaria alternata is one of the major postharvest diseases affecting blueberries in California. Several fungicides with different modes of actions are available for use on various crops to control Alternaria diseases in California. However, fungicide resistance can result in the failure of chemical control. In this study, the sensitivity/resistance profiles of A. alternata from blueberry field to pyraclostrobin, boscalid, fluopyram, fludioxonil, cyprodinil and polyoxin D were examined. In total, 143 isolates were screened for resistance to these fungicides. Our results showed that all isolates tested were resistant to boscalid; 60 out of the 143 isolates were resistant to pyraclostrobin; 32, 69 and 42 were sensitive, low resistant, and resistant to fluopyram, respectively; and all were sensitive to fludioxonil and cyprodinil. Control tests on detached blueberry fruit inoculated with different Alternaria isolates showed that fludioxonil and cyprodinil significantly reduced disease incidence and severity; however, pyraclostrobin, boscalid, fluopyram and polyoxin D significantly reduced only disease severity. The obtained results will be helpful in making decisions on fungicide programs to control A. alternata isolates with resistance or reduced sensitivities to multiple fungicides.

Technical Abstract: Alternaria rot caused by Alternaria alternata is one of the major postharvest diseases affecting blueberries in California. The sensitivity profiles of A. alternata from blueberry field to quinone outside inhibitors (QoIs), boscalid, fluopyram, fludioxonil, cyprodinil and polyoxin D in California were examined in this study. EC50 values of 51 A. alternata isolates for boscalid varied greatly among the isolates, ranging from 0.265 to >100 µg/ml. EC50 values of 51 A. alternata isolates to fluopyram, fludioxonil, cyprodinil, and polyoxin D were 5.188 ± 7.118 µg/ml, 0.078 ± 0.021 µg/ml, 0.465 ± 0.302 µg/ml, and 6.238 ± 7.352 µg/ml, respectively. In total, 143 isolates were screened for resistance at 5 and 10 µg/ml for fludioxonil, cyprodinil, and fluopyram, 10 µg/ml for polyoxin D, and 10 and 50 µg/ml for boscalid. Based on the published discriminatory concentrations for phenotyping resistance, of the 143 isolates, all were considered resistant to boscalid; 32, 69 and 42 were sensitive, low resistant, and resistant to fluopyram, respectively; and all were sensitive to fludioxonil and cyprodinil. In a PCR-RFLP method for phenotyping, 60 out of the 143 isolates were classified as resistant to QoIs. Control tests on detached blueberry fruit inoculated with different Alternaria isolates showed that fludioxonil and cyprodinil significantly reduced disease incidence and severity; however, pyraclostrobin, boscalid, fluopyram and polyoxin D significantly reduced only disease severity. The obtained results will be helpful in making decisions on fungicide programs to control A. alternata isolates with resistance or reduced sensitivities to multiple fungicides.