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Activity of o-Diphenol Oxidase in Postharvest Apple Decay by Penicillium expansum and Physalospora obtusa. D. M. Wilson, Department of Botany, University of Vermont, Burlington 05401; G. J. Nuovo(2), and W. B. Darby(3). (2)(3)Department of Botany, University of Vermont, Burlington 05401. Phytopathology 63:1115-1118. Accepted for publication 10 March 1973. DOI: 10.1094/Phyto-63-1115.

The decayed areas of ‘McIntosh’ apples inoculated with Penicillium expansum after controlled atmosphere (CA) storage were darker than the same areas of apples inoculated in the identical manner after refrigerated storage. o-Diphenol oxidase activity was lower in sound apples after CA storage than in apples from refrigerated storage. There was little suppression of the o-diphenol oxidase system in extracts from CA-stored McIntosh apples when these were mixed with extracts from tissue decayed by P. expansum, but in refrigerated apples there was an 87% reduction in o-diphenol oxidase activity. o-Diphenol oxidase inhibitors were present in both cases, but the o-diphenol oxidase from CA-stored apples was not as sensitive to the inhibitors. When 3,4-dihydroxyphenylalanine (DOPA) and catechol were used as substrates, o-diphenol oxidase activity was altered in all areas of apples inoculated with either P. expansum or Physalospora obtusa. No o-diphenol oxidase activity was found in rotted areas in refrigerated and CA-stored apples rotted by P. expansum and in the early stages of apples infected by P. obtusa. As P. obtusa decay progressed, the skin turned from brown to black and the pulp darkened, at which time o-diphenol oxidase activity was found.