TIMING OF FUNGAL INVASION USING HOSTS RIPENING HORMONE AS A SIGNAL

In many postharvest fruit diseases, fungi remain latent until the frui t ripens. How the fungus times its infection at ripening of the host i s not known. We have fouled that the volatiles produced by the climact eric tomato, avocado, and banana fruits induce germination and appress orium for mation in Colletotrichum gloeosporioides and Colletotrichum musae. Exposure of the spores of these fungi to ethylene, the host's r ipening hormone, at less than or equal to 1 mu l/liter, caused germina tion, branching of the germ tube, and formation of up to six appressor ia from a single spore. Propylene, an ethylene analog, but not the hyd rocarbon gas methane was able to induce spore germination and multiple appressorium formation. The ethylene effect on the fungi appears to b e a plant like response as it was inhibited by silver ion and 2,5-norb ornadiene; the inhibition by the latter could be reversed by higher et hylene concentrations. Ethylene induced germination and appressorium f or mation in the Colletotrichum sp. penetrating climacteric fruit but not in other Colletotrichum strains. That the ethylene induction of mu ltiple appressorium formation could be relevant to postharvest infecti on was indicated by the observation that C. gloeosporioides spores for med multiple appressoria on normally ripening tomato that produces eth ylene, whereas on transgenic tomato and orange, fruits incapable of pr oducing ethylene, exogenous ethylene was required to induce multiple a ppressorium formation and lesion formation. These results strongly sug gest that these fungi must have coevolved to develop a mechanism to us e the host's ripening hormone as a signal to differentiate into multip le infection structure and thus time the infection process.