Oak Leaf Blister

Symptoms

• Spots on the leaves ¼ to ½ inch in diameter turn light green as young leaves expand.
• Cells in the spots multiply more than surrounding cells and result in a raised blister-like buckling of the leaf.
• As the spots age, their upper surfaces become covered with a buff-white coating of fungal growth that later turns brown.
• The leaves remain on the tree and do not fall prematurely.

Cool, moist conditions are required for this disease to occur. The fungus only infects young emerging leaf tissue in the spring, entering the stomates and growing between the leaf cells. The new spores of the fungus formed on the surface of the spots are blown to new buds and remain dormant there until the next spring. Thus, the fungus has only one infection period in the spring and does not continue to cause new spots to form later during the growing season.

Management in the Landscape

Fungicide application is not necessary because the leaves are seldom severely spotted and do not fall prematurely. Although infections may be extensive for some years, little damage actually results.

Management in the Nursery

To prevent spotting, fungicide must be applied prior to bud break, late in dormancy. Once bud break has occurred and symptoms are visible, it is too late to spray.

Prepared by Gary W. Moorman, Professor Emeritus of Plant Pathology