Most fungal organisms infect tree and plant leaves in the spring, when temperatures are mild and conditions are moist. Depending on the type of fungus and the disease it causes, some are simply cosmetic problems while others do more serious damage. Oftentimes the symptoms become apparent weeks after the infection starts. A few leaf diseases also infect deciduous oak trees (Quercus spp.) planted in residential and commercial areas alike.
Anthracnose (Leaf Blight)
Commonly called leaf blight or leaf spot disease, anthracnose infects the foliage of all varieties of oaks. The fungus Gnomonia quercina causes leaf tissues along the veins and margins to turn brown, accompanied by leaf drop in extreme conditions. The disease sometimes engulfs the entire leaf, causing it to appear burnt or scorched before it spreads to shoots and smaller twigs. Fortunately, established oaks withstand the disease as the summer progresses because the fungus needs cool, wet temperatures to survive, which are rare in the warm, dry summer months. Rake and remove infected leaves in the fall, thin the tree's crown to allow sunlight to penetrate the canopy and prune and burn infected twigs.
Oak Leaf Blister
Oak leaf blister infects all varieties of oak trees, although water oaks appear the most susceptible to damage. The fungal disease causes inflamed spots or blisters on the upper surfaces of infected leaves, shape distortion and premature drop in extreme cases. Lower leaf surfaces turn grey as the fungus spreads. Although leaf drop stresses infected oaks and mars the appearance, the disease alone does not kill the tree.
The fungus Taphrinia caerulescens is responsible for oak leaf blister disease that occurs in the cool and wet spring season. Fungal spores exist between scales of buds and cuts or crevices in the bark and infect buds as soon as they open.
Cornell University Extension recommends preventing the disease with a single application of pesticide in spring, just before the buds swell. Fungicides applied after bud break are ineffective.
Powdery mildew is common in oak trees planted along the coastline because the cool and humid temperatures favour its growth, development and spread. It is caused by the fungi Cystotheca spp., Phyllactinia spp. and Microsphaera spp. The disease infects new growth as opposed to older or mature foliage. A white or grey powder-like substance covers infected leaves, causing foliage to distort in shape and appear cupped or needle-like.
Prevent oak powdery mildew by planting the trees in sunnier spots, providing adequate spacing for air circulation and pruning the crown of the tree. Treat infected foliage with neem oil or registered fungicides.