They are often lighter green than the surrounding tissue and later turn brown. The blister-like patches result from overgrowth of infected leaf tissue, which is caused by substances secreted by the fungus. The fungus survives the winter as spores in buds. The spores germinate in the spring to infect the leaves. Infected leaf tissue grows much faster than uninfected tissue, resulting in the distorted blisters.
Spore dispersal occurs in the fall.
Leaf blister is rarely severe enough to require control measures. Once infection has occurred, fungicide treatments usually are ineffective. If infection is very heavy and if the tree is small enough to obtain good coverage, a fungicide could be applied in the spring just before bud break. This disease is caused by the bacterium Xylella fastidiosa and is characterized by a disruption of water movement, decline of vigor, marginal reddening or yellowing followed by browning of leaves, dieback, and eventual death. The symptoms usually first appear in early fall, following summer drought.