Powdery Mildew on Oaks
A fungus can cause odd (abnormal) branch tip growth and velvet-like growth on the leaves. The fungus is a species of powdery mildew. In the case of the velvet leaves, the fungus invades the surface cells on the maturing leaf, and the interaction of host and fungus cause the velvet growth, and may also cause a bronzing or reddening of the leaf tissues. The abnormal branch tip growth occurs when the powdery mildew fungus invades the terminal bud of the branch. The fungus becomes systemic in this bud, and the subsequent growth is multi-branched with stunted, deformed, grayish-white leaf and stem tissues.
Control of powdery mildew in oaks, both the coastal and valley species, centers on water management. All coastal and valley oaks evolved in a Mediterranean climate, i.e., winter-spring rains and summer-fall drought. Watering these trees in the summer causes them to continue to grow. This tender summer growth is extremely susceptible to attack by oak powdery mildew fungi. By the way, there are actually several species of powdery mildew fungi that can infect oaks. Therefore, reduce or eliminate summer watering of oaks, if possible. Let the spring growth harden off. Mature, fully expanded leaves are very resistant to powdery mildew attack. In trees that have developed “witches brooms,” that is what the abnormal tip growth is called, wait until fall or winter and then prune all the “witches brooms” out of the trees. As for the leaf infections, a spray when trees are dormant in December or January will help reduce the incidence of powdery mildew in coastal and valley oaks, but probably will not eliminate the problem. Remember, summer drought is the best control.