Enhanced activity of abscission enzymes predisposes oranges to invasion by Diplodia natalensis during ethylene degreening

Stem-end rot (SER) caused by Diplodia natalensis is a major postharvest disease of fresh Florida citrus degreened with ethylene to enhance fruit appearance. Significant increase in disease incidence by the use of ethylene at concentrations above those needed for degreening may be related to the activity of the abscission enzymes, polygalacturonase (PG) and cellulase (CX). Quiescent mycelia of D. natalensis in necrotic tissue on the surface of the calyx and disk (button) at the stem-end of the fruit grow into the fruit upon separation of the button from the fruit during abscission. Activity of the abscission enzymes in oranges was enhanced by high ethylene (0.055 mi l(-1)), and a larger number of cells were degraded within the abscission layer by their activity than at lower (0.002 mi l(-1)) more typical degreening concentrations. Commercial enzymes or partially purified preparations of the abscission enzymes added to abscission areas of debuttoned oranges before inoculation with mycelia of D. natalensis caused a significant increase in SER. Fruit dipped in 2,4-dichlorophenoxyacetic acid (2,4-D) or silver thiosulfate (STS) before degreening were more resistant to SER when inoculated following degreening. Addition of these metabolic inhibitors or cycloheximide (CHI) to the fruit abscission zone after removal of the button during early stages of degreening (18 h) with high ethylene also caused a significant reduction in SER. The metabolic inhibitors had no affect on SER if applied at 60 h after terminating the ethylene treatment. Reductions in activity of PG and CX were obtained by applying 2,4-D or STS to fruit before degreening, and STS or CHI to fruit abscission zones at early stages of degreening (18 h). (C) 1998 Elsevier Science B.V. All rights reserved.