A quantitative study was made of the infection processes of Microsphaera alphitoides, powdery mildew, on seedlings of three oak species, Quercus robur (leaves highly susceptible until mature), Q. cerris (moderately resistant) and Q. borealis (highly resistant), to determine the mechanisms underlying their different resistances to disease. Some inhibition of spore germination occurred on mature leaves of Q. robur, and on leaves of Q. cerris and Q. borealis, but the number of appressoria formed from germinated spores was not affected either by oak species or leaf age. Formation of elongating secondary hyphae (ESH) from appressoria was strongly inhibited on mature leaves of Q. robur and on leaves of Q. cerris, and was completely inhibited on leaves of Q. borealis. At this stage of infection, host resistance was expressed most effectively.

Challenged cells and the host cell wall around the fungal penetration point were examined by light and scanning electron microscopy (SEM). SEM studies of the leaf surface around the penetration point showed no difference between susceptible and resistant leaf surfaces and, in neither case, was there any evidence of chemical dissolution of cuticle or epicuticular wax. In light microscope investigations, limitation of ESH formation was associated with two factors. First, death of the challenged host cells occurred within 12 h of infection. This was a normal component of the resistance of Q. cerris but was uncommon in Q. robur and Q. borealis. Second, material was deposited in the form of a papilla on the inside of the epidermal cell wall, opposite the penetration point of the fungus. Large dome-shaped papillae, which histochemical tests showed were typically lignified, were not penetrated by the fungus, while smaller, cone-shaped, non-lignified papillae failed to inhibit fungal development. Both types of papillae could occur in the same leaf but their frequency reflected the resistance of the host species. Lignified papillae were most common in resistant leaves.