Postharvest use of natamycin to control Alternaria rot on blueberry fruit caused by Alternaria alternata and A. arborescens

Alternaria rot caused by Alternaria alternata and A. arborescens is a major postharvest disease affecting blueberries in California. The effectiveness of natamycin, a newly registered biofungicide in the United States, was evaluated for its potential as a postharvest treatment to control Alternaria rot. The baseline sensitivities to natamycin in A. alternata and A. arborescens were established. The effective concentrations of natamycin for 50 % reduction in growth relative to control (EC50) of 34 A. alternata isolates ranged from 0.640 to 1.032 mg L−1 with a mean of 0.778 ± 0.097 mg L−1 for conidial germination, and from 2.660 to 4.330 mg L−1 with a mean of 3.390 ± 0.435 mg L−1 for mycelial growth. EC50 values of natamycin for 34 A. arborescens isolates ranged from 0.540 to 1.335 mg L−1 with a mean of 0.950 ± 0.162 mg L−1 for conidial germination, and from 0.610 to 3.430 mg L−1 with a mean of 1.920 ± 0.637 mg L−1 for mycelial growth. For A. alternata isolates, the minimum inhibitory concentrations (MICs) for conidial germination were 5 mg L−1 for 27 isolates and 10 mg L−1 for 7 isolates, and the MICs for mycelial growth were 25 mg L−1 for all 34 isolates. For A. arborescens isolates, the MICs for conidial germination were 10 mg L−1 for 5 isolates and more than 10 mg L−1 for 29 isolates, and the MICs for mycelial growth were 10 mg L−1 for 6 isolates and 25 mg L−1 for 28 isolates. Control tests on blueberry fruit inoculated with four isolates of each Alternaria alternata and A. arborescens showed that even a quarter concentration of the label rate of natamycin significantly reduced disease incidence and severity of Alternaria rot, regardless of application method or incubation condition. Our results suggest that natamycin can be an effective tool for control of postharvest Alternaria rot in blueberry.