Submitted to: Plant Disease
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/28/2014
Publication Date: 11/1/2014
Citation: Silkdar, P., Mazzola, M., Xiao, C.L. 2014. Infection courts and timing of infection of apple fruit by Phacidiopycnis washingtonensis in the orchard in relation to speck rot during storage. Plant Disease. 98:1467-1475. Interpretive Summary: Speck rot caused by the fungus Phacidiopycnis washingtonensis is a recently reported postharvest fruit rot disease of apple. The disease can cause significant economic losses if it is left uncontrolled. The pathogen is believed to incite infections in the orchard, and fruit rot symptoms become evident only during storage or in the market. In this study, we inoculated ‘Red Delicious’ and ‘Fuji’ apple fruit with the fungus at different times during the fruit growing season in the orchard to determine the timing of apple fruit infection in the orchard in relation to development of speck rot in storage. We found that the amount of speck rot in storage increased as the fruit infection time approached harvest. Microscopy studies indicated that after invasion, the fungus was primarily restricted to reside in the surface layer of the stems and floral parts of apple fruit and did not penetrate further into deeper layers beyond the epidermis. Our findings suggest that although infections of apple fruit by P. washingtonensis may occur anytime during the fruit growing season, application of fungicides near harvest or immediately postharvest prior to storage may help control speck rot.
Technical Abstract: Phacidiopycnis washingtonensis is the cause of speck rot, a recently reported postharvest fruit rot disease of apple. The pathogen is believed to incite infections in the field, and disease symptoms become evident only during storage. To determine the timing of apple fruit infection in the orchard in relation to development of speck rot in storage, ‘Red Delicious’ and ‘Fuji’ apple fruit were inoculated with P. washingtonensis at different times during the growing season in the orchard, harvested, and monitored for decay development during storage at 0°C. Fruit inoculated in both field and laboratory also were used to identify the infection courts and mode of apple fruit penetration by P. washingtonensis. In all 3 years, stem-end speck rot and calyx-end speck rot developed during cold storage on fruit inoculated during the growing season, regardless of inoculation time; and the incidence of total speck rot in storage increased as the fruit inoculation time approached harvest. On fruit floral parts, the pathogen colonized sepals at higher rates than stamens. Availability of naturally occurring necrotic tissues favored the colonization of the fungus on sepals. Histological studies indicated that infection occurred through micro-cracks on the surfaces of pedicels and sepals of the fruit, and invasion was restricted between the cuticle and epidermis. Findings of this study will assist in the development of effective control strategies for speck rot.