Delayed incidence of stem-end rot and enhanced defences in Aureobasidium pullulans-treated avocado (Persea americana Mill.) fruit

Abstract

The stem-end rot of avocado, caused predominantly by Lasiodiplodia theobromae, can result in heavy postharvest losses. Application of a cell suspension of Aureobasidium pullulans at the stem-end of unripe fruit delayed (P 0.05) affect disease incidence. L. theobromae, when cultured together with A. pullulans, showed significantly (P < 0.05) reduced radial growth of colony towards A. pullulans at 48 and 72 h and shortened aerial mycelium compared to controls. Neither of the organisms overgrew on each other but their colony margins were in close proximity. Presence of A. pullulans significantly (P < 0.001) reduced germination of conidia in water, and the germ tubes were shorter and showed only emergence. Chitinase, β-1,3-glucanase and antifungal activity of the peel of control fruit declined during ripening. However, there was increased chitinase and β-1,3-glucanase activity in A. pullulans-treated fruits or fruits that were inoculated with L. theobromae after treatment with A. pullulans. β-1,3-glucanase activity increased only slightly in fruits that were inoculated with L. theobromae without treatment. Greater preformed antifungal activity was retained in A. pullulans-treated fruits during ripening. Enhanced activity of chitinase and β-1,3-glucanase and greater retention of preformed antifungal activity may have contributed to the delayed stem-end rot incidence in A. pullulans-treated avocados. Application of A. pullulans, 2 days prior to inoculation and retention of stalk at harvest, appears to have allowed better establishment of A. pullulans on the fruit surface.