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Postharvest Pathology and Mycotoxins

Interactions of Ethylene with Citrus Stem-End Rot Caused by Diplodia natalensis. G. Eldon Brown, Scientific Research Department, Florida Department of Citrus, Citrus Research and Education Center, Lake Alfred, FL 33850; Hyoung S. Lee, Scientific Research Department, Florida Department of Citrus, Citrus Research and Education Center, Lake Alfred, FL 33850. Phytopathology 83:1204-1208. Accepted for publication 9 July 1993. Copyright 1993 The American Phytopathological Society. DOI: 10.1094/Phyto-83-1204.

Ethylene treatment of citrus fruit to improve rind color (degreening) significantly enhances the incidence of stem-end rot caused by Diplodia natalensis and stimulates disease development. In an effort to identify the role of excessive exogenous ethylene in pathogenesis, tissue exposed to different ethylene and air atmospheres was taken from the base of citrus fruit adjacent to the abscission area at the point of fungal ingress and examined for the presence of fungal inhibitors and lignin. The major inhibitor, identified as scoparone (6,7-dimethoxycoumarin), accumulated most extensively in tissue taken from asymptomatic fruit receiving either high ethylene treatments (55 ?l L1) followed by low inoculum levels, or low ethylene treatments (2 ?l L1) followed by high inoculum levels. Accumulation of lignin, extracted from the same tissue and measured as lignothioglycolic acid, followed a response pattern similar to that observed for scoparone. Evidence for some role of scoparone and lignin in fruit resistance was indicated by the correlation between enhanced accumulation of these inhibitors and the lack of decay. However, other factors affected by high ethylene play a role in disease development since levels of inoculum that induced little disease at low ethylene were capable of causing decay at high ethylene. Sensitivity of D. natalensis to scoparone was reduced and its growth rate was increased when the fungus was grown in high-ethylene atmospheres. One role of such ethylene atmospheres in inducing extensive Diplodia stem-end rot could be that of stimulating more rapid growth of the fungus and invasion of tissue in spite of the presence of threshold inhibitory levels of scoparone.

Additional keywords: Postharvest pathology, scoparone, lignin.