Powdery Mildew is a fungal disease that can cause distorted growth of foliage and cause witches brooms on new growth in trees and shrubs. Powdery mildew infections are readily diagnosed by the white to gray powdery growth on leaves and shoots. Causes of powdery mildew are due to a combination of high humidity plus high temperatures. Powdery mildew is most likely to occur in the spring and or fall, when the weather is cool and humidity is high. The combination of hot days and cool nights and/or a change in season can accelerate on the onset of the disease. Most fungus move and disperse across long distances with the assistance of people, insects, animals or environmental factors such as wind or rain. Most fungal spores disseminate in air currents and can be carried over long distances. Once a tree is infected the disease can continue to spread from one tree to another, so treating diseased trees is imperative to the local control of the fungal pathogen.
Witches Brooms are weak twig growth, causing side shoots to elongate, forming a dense cluster or broom like mass of twigs. On an oak trees Powdery mildew causes the development of witch’s brooms. Witches broom when clustered together serve as a site for the accumulation of water and other pests and cause the tree to be more susceptible to mechanical damage. There is no cure or treatment aside from destroying the affected portions of the plant.