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Core Rot Development in Red Delicious Apples Is Affected by Susceptibility of the Seed Locule to Alternaria alternata Colonization

First, second, third, and sixth authors: Department of Postharvest Science of Fresh Produce, Institute for Technology and Storage of Agricultural Products, ARO, the Volcani Center, Bet Dagan 50250, Israel; fourth author: Golan Research Institute, University of Haifa, P.O. Box 97, Katzrin 12900; and fifth author: Department of Horticulture, ARO, the Volcani Center, Bet Dagan 50250, Israel.

Alternaria alternata is the causal organism of core rot decay symptoms in susceptible cv. Red Delicious but not in resistant cv. Golden Delicious. The two cultivars did not differ in natural colonization of the style and ovary during the first week after full bloom; colonization of the ovary in the susceptible cultivar subsequently decreased with increasing distance from the calycine tube. By 30 days after full bloom, Alternaria recovery from ovary 1, adjacent to the end of the calycine tube, was 100 and 40% in the susceptible and resistant cultivars, respectively. In the susceptible cultivar, Alternaria recovery decreased from 75 to 20% in ovaries 2, 3, and 4, while there was only minor incidence in the resistant cultivar. Inoculation of the mesoderms of the two cultivars induced similar decay symptoms, but inoculated locules of Red Delicious were more susceptible than those of Golden Delicious. Increased inoculum concentration or isolate virulence enhanced the difference in locule susceptibility between the cultivars. Inoculation on isolated seed locules or on media amended with susceptible locule tissue as a carbon source induced greater transcript levels of several genes than the inoculation on resistant tissue. Endo- and exoglucanase activity levels were higher at pH 4.8 than at 4.2, conditions typical of the mesoderm adjacent to the seed locules of the susceptible and resistant cultivars, respectively. Current results suggest that susceptibility of Red Delicious apples to core rot decay is dependent on the sensitivity to locule colonization and on mesoderm pH, a factor that enhances fungal virulence.

Additional keywords:apple diseases, Malus sylvestris, moldy-core disease.