Understanding the Fungal Biology and Fungicide Control of Neofabraea Species Causing Bull's-Eye Rot of Apples Grown in the US Pacific Northwest

Neofabraea perennans and Neofabraea kienholzii are two of four fungal organisms causing bull's-eye rot of apples and other pome fruit grown in the US Pacific Northwest. Artificial wound inoculations conducted on 'Fuji' and 'Red Delicious' apple twigs using the aforementioned species demonstrated that both fungi are capable of inducing tree cankers that are similar in appearance. Cankers were largest following inoculations held in October compared to all other inoculation events evaluated. Additionally, artificial inoculations conducted on 'Fuji' and 'Red Delicious' apple fruit throughout the growing season revealed that fruit infections were more likely to be established during the final weeks approaching commercial harvest. Together, these studies demonstrate that these fungi are capable of surviving in the orchard as mycelium in tree cankers throughout the year, and under favorable conditions, can cause fruit infections throughout the fruit-growing season. These results highlight the importance of canker pruning as a means of reducing the pathogen inoculum load in the orchard.;Trials were also conducted to evaluate the efficacy of various pre-harvest and postharvest applied fungicides for the control of bull's-eye rot in 'Fuji' apples. Findings from these studies demonstrated that among the materials tested, thiophanate-methyl, thiabendazole and pyrimethanil were the most effective fungicides in suppressing incidence of bull's-eye rot and were capable of mitigating early, mid and late season fruit infections following a single application near/at the end of the apple-growing season. Although effective in controlling this important postharvest disease of apple, incorporation of these fungicides into spray programs should proceed with caution so as to minimize the risk of fungicide resistance in populations of Neofabraea spp. as well as other major pathogens in the Pacific Northwest.