Abstract：The infection process and ultra-structure of Alternaria solani on potato（Solanum tuberosum L.）leaves were observed by microscope，scanning and transmission electron microscope．The results showed that conidia began to germinate，and germ tube start to form 2 h after inoculation．Appressorium formation in the hypha tip was exhibited 6 h after inoculation．The germ tubes directly penetrated the host epidermal cells junctions and formed infection hypha on leaf of susceptive variety‘Dongnong 303’8 h after inoculation．Hyphal invasion of host cells spread to the adjacent cell expansion，and cell inclusions and all organelles have been almost completely digested 24 h after inoculation in susceptible variety．The infectious process in susceptible variety was approximately the same as in resistant variety‘Kexin No.1’，but the time of occurrence in resistant variety was later than that of susceptible variety．There was more hypha in host cell in susceptive variety than that in resistant variety．The defense response of thicken cell wall was observed in resistant variety 24 h after inoculation，but no defense response was seen in susceptive variety．These results suggested that resistance was effective to invasion and spreading by A. solani in resistance variety.
