Chlorosis on the upper leaf surface of oak leaves due to powdery mildew caused by Microsphaera alni. Mycelia and cleistothecia (reproductive and survival structures) of the powdery mildew fungus on the underside of an oak leaf.

Bacterial leaf scorch of oak. Symptoms may include marginal "scorching" of the leaves (right side of tree) and extensive leaf drop in the spring followed by a flush of new growth (left side of tree). Marginal "scorching" of oak leaves caused by the xylem-inhabiting bacterium Xylella fastidiosa. The bacterium is transmitted by xylem-feeding insects, primarily sharpshooters. Once a tree is infected, nothing can be done to eradicate the bacterium from it.