Incidence of Podosphaera clandestina on sweet cherries (Prunus avium) and the influence of postharvest handling practices on the survival of conidia on harvested fruit

Powdery mildew of sweet cherry in the Pacific Northwest (PNW) is caused by Podosphaera clandestina, which infects fruit and leaves. Disease symptoms are commonly observed near harvest and such infections are a major concern for offshore cherry markets due to the possibility of diseased cherries providing inoculum to establish the pathogen in a region where P. clandestina has not been reported. The present study was designed to identify periods at which fruit infections are most severe and to determine the efficacy of PNW’s industry postharvest handling practices on the survival of P. clandestina conidia on diseased fruit surfaces. Morphological characteristics and next-generation sequencing (NGS) platform was used to identify the causal agent of foliar and fruit powdery mildew found in the PNW. We combined qPCR and, the use of a viability dye, propidium monoazide (PMA), to differentiate between viable and dead (membrane-compromised) conidia. These tools helped to determine the viability of conidia in real-time. Conidia on the fruit surface naturally lost their viability within a few days of harvest. Postharvest treatments in the PNW include hydrocooling with chlorinated water and methyl bromide fumigation prior to export. These practices were extremely effective in deactivating all conidia on the fruit surface. The residual conidia recovered from the fruit surface after the postharvest treatments failed to establish consistent powdery mildew colonies on the susceptible host leaves; this confirms that diseased, treated cherries are unlikely to serve as a source of inoculum needed to establish the pathogen areas where it has not been documented. After postharvest treatments, regular sampling of conidia up to three weeks after harvest indicated that the conidia were non-viable. Results of this study indicated that the sweet cherry postharvest handling procedures in the PNW are effective for the elimination of the potential sources of P. clandestina inoculum associated with cherry fruit.