Influence of Ethylene on Increased Susceptibility of Oranges to Diplodia natalensis. Charles R. Barmore, Associate Professor, University of Florida, IFAS. G. Eldon Brown, Research Scientist III, Scientific Research Department, Florida Department of Citrus, Citrus Research and Education Center, 700 Experiment Station Road, Lake Alfred 33850. Plant Dis. 69:228-230. Accepted for publication 4 September 1984. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/PD-69-228.

Incidence of stem-end rot (SER) (caused by Diplodia natalensis) of Valencia oranges increased with exposure to increasing concentrations of ethylene from 0 to 50 μl/L of air. Although penetration of D. natalensis into the fruit depends on abscission of the floral calyx and disc (button), increases in SER at higher ethylene concentrations could not be related to early abscission. Germination of latent propagules of D. natalensis on the button was not enhanced by ethylene. Hyphal penetration from latent infections on the button into the area of abscission occurred more rapidly after the fruit was exposed to ethylene at 50 than at 1 or 10 μl/L. Resistance to infection was associated with the accumulation of red pigment in cells of the abscission area of the fruit within 2 days of inoculation. No differences in pectin demethylation or accumulation of starch and lipids within the separation layer were observed by histochemical methods after ethylene treatment or inoculation with D. natalensis. Histochemical techniques indicated that cells beneath the separation layer contained phenolics, ligninlike materials, and lignin that accumulated after abscission. Ethylene at 50 μl/L suppressed the staining intensity for ligninlike materials, whereas the presence of the fungus at low ethylene concentrations enhanced staining.

Keyword(s): Citrus sinensis.