INTERACTION OF 4 ANTAGONISTIC FUNGI WITH BOTRYTIS-ACLADA IN DEAD ONION LEAVES - A COMPARATIVE MICROSCOPIC AND ULTRASTRUCTURAL-STUDY

The colonization of dead onion leaves by Botrytis aclada and the funga l antagonists Aureobasidium pullulans, Chaetomium globosum, Gliocladiu m catenulatum, and Ulocladium atrum and the interactions between B. ac lada and each of the four antagonists were studied at the microscopic and ultrastructural level. This approach was used in an attempt to und erstand the colonization pattern of these fungi and the nature of the biocontrol activity of the antagonists that have shown a potential to suppress spore production of Botrytis spp, on necrotic plant tissues. When applied alone, B. aclada and U. atrum were found throughout the l eaf tissues in high densities after an incubation period of 6 days at 18 degrees C in a moist chamber. C. globosum and G. catenulatum coloni zed only the outer portions of the leaf, whereas A. pullulans appeared to be concentrated in the leaf stomata. When pathogen and antagonists were applied together, ultrastructural observations revealed that cel ls of B. aclada were plasmolyzed in the presence of G. catenulatum, su ggesting a reaction to antifungal molecules. Antibiosis also seemed to be involved, albeit to a lesser extent, in the antagonistic interacti ons between B. aclada and A. pullulans or C. globosum. No evidence of direct parasitism was recorded. On the other hand, U. atrum appeared t o completely exclude B. aclada from dead onion tissues when both fungi competed for the substrate. Ultrastructural observations of the in vi tro interaction between the two fungi did not reveal parasitism or ant ibiosis by either fungus. Based on previous records of its biocontrol potential and observations of its colonizing properties, it appears th at U. atrum can compete for and utilize necrotic tissues rapidly and e xtensively, thus, excluding competitors without any other antagonistic action.