Oak leaf blister
Damage, symptoms and biology
The fungus causes bulges and blisters to appear on the surface of leaves. Infection is initiated in the spring when the growing leaves are still immature. As with many other fungal diseases, cool, wet spring weather favours the development of the pathogen. The fungus grows in the leaves, creating greenish blisters on the surface. Around the middle of summer, the blisters become darker and turn brown, and the fungus begins producing spores. The development of the spores causes the epidermis of the leaf to break, allowing the spores to escape. They over winter in bud scales and cause new infections the following spring.
Spraying a biological fungicide before leaf-out in the spring generally eradicates the disease. For information on the products registered for controlling specific insects or diseases, please contact the Pest Management Information Service of the Pest Management Regulatory Agency (PMRA), at the following toll-free number: 1-800-267-6315.