Powdery Mildew (fungi – Microsphaera alni and Phyllactinia guttata): Powdery mildew occurs on all groups of oaks. Infected leaves have a faint indistinct spot on the upper leaf surface and a white to off-white powdery growth on the lower side. The fungus will most often be found along the veins and midribs of the leaf. It is normally observed in late fall. Infected leaves will be slightly disfigured in severe cases. Powdery mildew is seldom a problem on large individual trees. Damage can occur in nurseries and on low sucker limbs but rarely is disease sufficient to require spraying. Limb removal improves air circulation and will help reduce the occurrence of this disease.