Ultrastructural characterization of infection and colonization of Colletotrichum gloeosporioides in onion

摘要

Anthracnose is the most destructive disease of onion in the Philippines. Infection by Colletotrichum gloeosporioides was studied by using light microscopy, transmission and scanning electron microscopy. At 6 hours after inoculation, a germ tube emerged from the conidium and a globular shaped appressorium formed from the end point of the germ tube. Between 24 to 48 hours after inoculation, the appressoria matured and an infection hypha emerged through a pore at the base of the appressorium. After 48 hours it penetrated the host cuticle directly and formed a primary hypha. Simultaneously, formation of papilla and penetration of the host through stomata also occurred although no evidence of an infection vesicle was observed. From 48 to 72 hours, primary hypha started to branch out to form secondary hyphae within the epidermal cells followed by a massive growth of both intra- and intercellular hyphae leading to the development of small whitish and water soaked lesions. At 72 to 96 hours after inoculation, intra- and intercellular hyphae radiated from cell to cell resulting in the formation of acervuli. At 96 hours after inoculation, typical onion anthracnose symptoms with salmon coloured mucilaginous spore matrix was observed on the infected leaf surface.