Initial infection processes by Botrytis cinerea on nectarine and plum fruit and the development of decay

Infection of unwounded, detached plum and nectarine fruit by Botrytis cinerea was studied with light, fluorescence, and electron microscopy. Conidia in 25-μl drops of spore suspension germinated readily on the surface of green and mature fruit and penetrated the cuticle by means of a very thin infection peg formed from the inner appressorium wall. B. cinerea was unable to breach the cuticle of green fruit. The pathogen entered the substomatal cavity of green fruit, but further growth in the fruit tissue was not observed. Fluorescence microscopy studies indicated that hyphae confined to the stomata of green fruit may be localized by resistant reactions. In nectarines inoculated near picking-ripe stage, the majority of infection pegs had penetrated the cuticle. However, only a small number of attempted penetrations were successful on picking-ripe plum fruit. Successful penetration of both fruit types during the mature, susceptible phase was characterized by the presence of inter- and intracellular hyphae 24 h after inoculation. The fact that green fruit were resistant to decay and that no hyphae could be found in the epidermal cell layer of fruit during their resistant phase indicated that early field infections might be of lesser importance in postharvest B. cinerea rot