Infection of apple fruit by Sphaeropsis pyriputrescens in the orchard in relation to Sphaeropsis rot in storage

Authors: KIM, Y - Washington State University; Curry, Eric; Xiao, Chang-Lin

Submitted to: European Journal of Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/5/2014
Publication Date: 9/1/2014
Citation: Kim, Y.K., Curry, E.A., Xiao, C. 2014. Infection of apple fruit by Sphaeropsis pyriputrescens in the orchard in relation to Sphaeropsis rot in storage. European Journal of Plant Pathology. 140:133-143.

Interpretive Summary: Sphaeropsis rot, caused by the fungus Sphaeropsis pyriputrescens, is a major postharvest fruit rot disease of apple grown in Washington State, the primary region of apple production in the USA. The inoculum of the fungus responsible for infection of apple fruit is present in the orchard. In order to develop strategies for control of this disease, we conducted fruit inoculation studies in the field and laboratory to understand when and how the fungus invades apple fruit in the field and its relation to fruit rots in storage. We found that infection of apple fruit by the fungus initiated primarily at the stem and calyx of the fruit. Stem-end rot was prevalent on Golden Delicious, whereas calyx-end rot was prevalent on Fuji. Both stem- and calyx-end rots were common on Red Delicious. Field inoculation studies indicated that infection of apple fruit by the fungus occurred anytime from bloom to harvest but the fungus remained latent and decay symptoms developed during storage. Microscopy studies indicated that infection took place through natural openings of plant organs such as stomata on stem and sepal and lenticels on fruit skin as well as microcracks on the stem and sepal. The fungus also directly penetrated the stem and sepal tissues but most invasions were restricted between cuticle and epidermis. Our results indicate that wounding is not required for infection of apple fruit by S. pyriputrescens, though it may facilitate infections, and that control of Sphaeropsis rot should be focused on protecting the stem and calyx of fruit.

Technical Abstract: Sphaeropsis rot, caused by Sphaeropsis pyriputrescens, is a recently recognized postharvest fruit rot disease of apple in the United States. The objectives of this study were to determine the timing of apple fruit infection in the orchard in relation to development of Sphaeropsis rot in storage and to identify infection courts and mode of penetration by S. pyriputrescens on apple fruit. Red Delicious, Golden Delicious, and Fuji apples were inoculated in the orchard from 3 weeks after petal fall to 2 weeks before harvest at 5- to 6-week intervals in three consecutive seasons. All fruit were harvested and stored at 0ºC to monitor decay development. Light and scanning electron microscopy were used to examine the infection courts and mode of penetration of the fungus on/in the host tissues. At harvest, the fungus was re-isolated from the stem (pedicel), sepal, anther, or filament of the inoculated fruit, but no fruit rot symptoms developed on the fruit. Sphaeropsis rot developed on inoculated fruit during cold storage beginning 1 to 3 months after harvest. Infection initiated primarily at the stem and calyx of the fruit. Stem-end rot was prevalent on Golden Delicious, whereas calyx-end rot was prevalent on Fuji. Both stem- and calyx-end rots were common on Red Delicious. Infection also occurred at lenticels on fruit skin, particularly on Golden Delicious, but at low incidence. Relationships between the incidence of Sphaeropsis rot in stored apple fruit and the timing of infection (inoculation) in the orchard varied with cultivar and year. On Red Delicious apples, the incidence of Sphaeropsis rot generally increased as the timing of infection approached harvest. Fuji was least susceptible to infection among the three cultivars. Histological studies indicated that infection took place through natural openings of plant organs such as stomata on stem and sepal and lenticels on fruit skin. Fungal penetration also was observed at microcracks on the stem and sepal and at hair-holes where mechanical damage occurred in sepals. Direct penetration was observed on the stem and sepal of fruit, but most invasions were restricted between cuticle and epidermis. Our results indicate that wounding is not required for infection of apple fruit by S. pyriputrescens, though it may facilitate infections, and that control of Sphaeropsis rot should be focused on protecting the stem and calyx of fruit.