When oak trees drop their leaves in summer, it could be caused by the fungal disease anthracnose. Although the damage is found in summer, the problem begins in spring at bud break. Over-wintering spores are splashed onto newly-emerging leaves during periods of cool, wet spring weather. Infected leaves develop irregular brown, necrotic blotches. The necrotic areas are at first associated with veins, and then expand to include the interveinal area between the veins. Leaf distortion is common. Shoots may also be infected and drop at the same time. Anthracnose often appears on the lower, inner canopy first.
Anthracnose is mostly an aesthetic problem, only becoming a health issue if it has occurred over several consecutive growing seasons. Oak trees stressed from other causes such as poor soil conditions and irrigation practices are less tolerant of the disease and symptoms may be more severe.
To manage the disease, rake up diseased leaves and prune out dead or dying twigs. Fungicides are rarely needed.