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.