Submitted to: Plant Disease
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/25/2014
Publication Date: 7/7/2014
Citation: Xiao, C., Kim, Y.K., Boal, R.J. 2014. Sources and availability of inoculum and seasonal survival of Sphaeropsis pyriputrescens in apple orchards. Plant Disease. 98:1043-1049. Interpretive Summary: The fungus Sphaeropsis pyriputrescens is the cause of Sphaeropsis rot, a recently reported postharvest fruit rot disease of apple. The fungus also causes a twig dieback and canker disease on apple and crabapple trees (crabapple planted as a source of pollen for apple production) in the orchard. Recent surveys indicated that Sphaeropsis rot is a major postharvest disease of apples grown in Washington State. In this study, we investigated the sources of inoculum of the fungus in the orchard and monitored the availability of viable inoculum of the fungus responsible for fruit infection during the fruit-growing season for three years. We found that diseased twigs, dead fruit spurs and bark, and mummified fruit on both apple and crabapple trees can serve as the sources of inoculum for fruit infection in the orchard, that viable inoculum (fruiting body) of the fungus is available during the apple growing season, and that S. pyriputrescens can survive in diseased twigs year-round in the region. Our results suggest that availability of viable S. pyriputrescens inoculum is unlikely a limiting factor for infection of apple fruit in the orchard leading to Sphaeropsis rot during storage.
Technical Abstract: Sphaeropsis pyriputrescens is the cause of Sphaeropsis rot, a recently reported postharvest fruit rot disease of apple. Infection of apple fruit by the fungus is believed to occur in the orchard, and symptoms develop during storage or in the market. S. pyriputrescens also is the cause of a twig dieback and canker disease of apple and crabapple trees. To determine sources of the pathogen’s inoculum in the orchard, twigs with dieback and canker symptoms, dead fruit spurs, dead bark, and fruit mummies on the trees were collected and examined for the presence of pycnidia of S. pyriputrescens. To monitor inoculum availability, dead fruit spurs or twigs from Fuji trees, and twigs with dieback from crabapple trees (as a source of pollen for apple production) in a Fuji orchard as well as dead fruit spurs and dead bark from Red Delicious trees in a Red Delicious orchard were sampled periodically and examined for the presence and viability of pycnidia of S. pyriputrescens. To determine seasonal survival and production of pycnidia of the fungus on twigs, apple twigs were inoculated in early December, sampled periodically for up to 12 months after inoculation, and examined for the presence of pycnidia; isolation of the fungus from diseased twigs was attempted. Pycnidia of S. pyriputrescens were observed on diseased twigs, dead fruit spurs and bark, and mummified fruit on both apple and crabapple trees, suggesting that these tissues were the sources of inoculum for fruit infection in the orchard. With the combined observations from the two orchards during the three growing seasons, viable pycnidia of the fungus were present at any sampling month and observed in 50-100% of the Fuji trees, >90% crabapple trees, and 0-50% of the Red Delicious trees. S. pyriputrescens was recovered from diseased tissues of inoculated twigs at all sampling times up to 12 months after inoculation. The results suggest that S. pyriputrescens can survive as mycelium in diseased twigs year round in the region and that availability of viable S. pyriputrescens pycnidia is unlikely a limiting factor for infection of apple fruit in the orchard leading to Sphaeropsis rot during storage.