Fungicide Resistance in <i>Alternaria alternata</i> from Blueberry in California and Its Impact on Control of Alternaria Rot

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. EC 50 values of 51 A. alternata isolates for boscalid varied greatly among the isolates, ranging from 0.265 to &gt;100 μg/ml. EC 50 values of 51 A. alternata isolates to fluopyram, fludioxonil, cyprodinil, and polyoxin D were 5.188 ± 7.118, 0.078 ± 0.021, 0.465 ± 0.302, 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-restriction fragment length polymorphism 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.