Based on the pathological anatomy of infected phyllodes of Acacia cnariculiformis,it was demonstrated that the pathogens,Oidium sp. ,could directly invade the tissues via. the epidermal cells,intercellular space or stomata, and assimilate nutrient by their haustoria. After inoculation tests the highly susceptible phyllodes showed death and deformity of epidermal cells and decrease and partial degradation of chloroplasts in the parenchyma of the leaves whereas highly resistant phyllodes showed only allergic necrosis in their epidermal cells. There was a correlation berween the thickness of the epidermal cell wall and its cuticule in the phyllode and disease resistance.