Influence of preformed characteristics of lemon peel on susceptibility to Geotrichum candidum

Abstract

Hyphal growth and polygalacturonase liberation by Geotrichum candidum in wounds in the peel of resistant and susceptible lemons were similar for 10 h after inoculation; a stimulation of hyphal growth and polygalacturonase activity in susceptible fruit was detected after 15 h. The infectivity of the inoculum was increased by the addition of sterile macerating enzymes of the pathogen, but not by addition of nutrients. No evidence was found that preformed factors in resistant lemons limited fungal growth or liberation of polygalacturonase. Sterile macerating enzyme preparations injected into the albedo produced more maceration in yellow or turgid fruits, which are relatively susceptible to infection, than in the more resistant, light green or subturgid fruits. Albedo tissue strength (by penetrometer) of light green fruits was higher and remained higher during the maceration process than in yellow fruits, even though the rate of tissue softening was slightly faster in the former. The effect of macerating enzymes on potassium leakage from albedo cells paralleled their effect on tissue strength.