Prevalence and management of two emerging postharvest diseases of apple in Washington State

Abstract: Lambertella corni-maris and Phacidiopycnis washingtonensis, the causal agents of yellow rot and speck rot, respectively, are two postharvest pathogens of apple reported recently in Washington State. This study was aimed to assess their prevalence and distribution in packinghouses and evaluate the efficacy of existing fungicides for their control. A statewide survey was conducted among 160 grower lots (50 fruit per lot) distributed throughout central Washington between February and June of 2016. Pure cultures of all pathogens were made on Petri plates containing potato dextrose agar (PDA). Purified cultures of all isolates were made on PDA, and pathogens were identified using key morphological and molecular traits. Yellow rot was found in 50 lots (31.3%) of the 160 surveyed, with frequencies ranging from 2 to 38% of total decay. Speck rot was found in 44 lots (27.5%), at frequencies ranging from 1.7 to 68%. Isolates of L. corni-maris and P. washingtonensis were tested for sensitivity to three postharvest fungicides including thiabendazole, pyrimethanil and fludioxonil. Effective concentrations necessary to inhibit 50% growth (EC50 values) were determined in vitro and were lower than 1 μg/ml for fludioxonil and pyrimethanil for P. washingtonensis and for fludioxonil in L. corni-maris. Mean EC50 values were the highest for thiabendazole for both pathogens, wherease the highest variability in sensitivity within the populations was seen for pyrimenthanil.