||Oak leaf blister on pin oak leaf (Quercus palustris)
Oak leaf blister (oak leaf curl) is a fungal leaf disease caused by the fungus Taphrina caerulescens. Circular, raised areas ranging up to 2 inches in diameter are scattered over the upper leaf surface. During cool wet springs, almost all species of oak are subject to the leaf blister disease. Members of the red oak family are particularly susceptible to infection. The disease is closely related to Taprina deformans which causes peach leaf curl.
Symptoms and Diagnosis
Blister-like, circular, raised areas appear scattered on the upper leaf surface. This raised area causes a corresponding depression on the lower leaf surface at the same site and of the same size. The color of the upper convex area is yellowish white while the bottom concave area is yellowish brown.
Leaves with numerous spots may fall prematurely to the ground. If well-established trees defoliate before midsummer, they will sometimes leaf out later in the season. When defoliation occurs in late summer, leaf loss will have little impact on the overall health of the tree.
During mid-spring, microscopic spores are produced in leaf spots. These spores are carried by wind and splashing raindrops onto bud scales and twigs where they remain in a dormant stage until the following early spring. At this time, rain washes the spores onto young leaves where infection takes place. Depending on weather conditions, small circular spots begin to develop in 2 to 4 weeks. Spores produced on these spots will lodge in bud scales and again remain resting until the following spring. Cool wet weather is required for germination on young leaves, and if these conditions continue, severe infection can occur. If weather conditions are not favorable for spore germination shortly after bud break, only minor infection will occur. As the leaves mature, they become more resistant to infection.
Integrated Pest Management Strategies
1. Maintain plant vigor. Keep the tree well watered during drought conditions (approximately 1 inch of water per week) and well-drained during periods of heavy rain. Fertilize according to soil test recommendations. Apply nitrogen every 3 to 5 years for mature shade trees.
2. General health. Oak leaf blister does not seriously affect the overall health of the tree unless the tree is repeatedly defoliated in successive years. Even if this occurs, the second set of leaves should emerge at a time when conditions are not conducive to reinfection of leaves and newly formed bud scales.
3. Fungicides. A single application of a fungicide applied in the spring at the time of bud-swelling is usually adequate. Apply with a power sprayer and coat buds and twigs thoroughly for good control. chlorothalonil (Daconil) is currently registered for use in controlling oak leaf blister. Fungicides will not be effective if applied after bud break. As with the use of all chemicals, carefully read and follow the manufacturer’s directions.
Strategies 1 and 2 are strictly organic approaches.