Light and Scanning Electron Microscopy Studies on the Penetration and Infection Processes of Alternaria Alternata, Causing Brown Spot on Minneola Tangelo in the West Mazandaran - Iran

Alternaria fungi causes two different diseases on citrus in Mazandaran province, North of Iran: Alternaria brown spot of tangerine hybrids and Alternaria black rot of the Navel orange fruit. A. alternata, conidium germination, Inoculation, penetration and colonization on the plant surfaces studied using light and electrone microscopies. The tissues cutted to (2×2 mm) pieces and were fixed over night at gluteraldehyde and phosphate buffer, post fixation was osmium tetroxide,after lyophilized,the specimens coated with gold and then studied using a ZEISS DSM_960A SEM. Multiple germ-tubes developed randomly from each conidium and grew in any direction across the leaf and fruit skin surfaces. Penetration in the plant surface, whether directly through the epidermis or via stomata occureed with or without the appressoria formation. The present study showed that occurrence of the infection was through stomata and direct penetration. Hyphal penetration continued through of the substomata cavity and then some of hyphal branches grew in the intercellular space of mesophyll tissue. Hyphal product, especially the toxin, caused cell and cell wall damages.