Squirrels and humans alike love the acorn nut, the fruit of the oak tree (Quercus). What lacks in popularity are oak trees with leaves that are covered with unsightly blisters and fuzzy white or grey patches. This is the sign of a fungal disease that, although unseemly, rarely causes significant harm to the tree.
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Leaf blister, a disease that infects most species of oak, but especially members of the red and black oak families, is caused by the fungus Taphrina carelessness. It results in the formation of a white, fuzzy substance on the leaves of an oak tree that appears in early summer. The initial signs of this disease are bulges on the upper surface of the leaf that that start off as light green or yellow but turn grey with age.
The fuzzy patches are fruiting bodies of the fungus, spores of the disease that take on a white colour early in the growing season. A large number of these fuzzy white things will force the leaves to become twisted and deformed. In most cases, oak leaf blister is only an aesthetic problem and results in little long-term injury to the tree. In cases of severe infestation, or if the disorder continues unabated for several years, the overall health and vigour of the oak can decline.
The fungus overwinters on leaves, either on the tree or on the ground, and comes to life in the spring. Cool and wet weather in late winter and early spring provides favourable conditions for the spread of oak leaf blister, and the disease typically attacks new foliage at bud break. More mature leaves often resist the fungus with success. The lesions produced by this fungus vary in size from 1/8 to 1/2 of an inch in diameter.
Application of insecticide on large, landscape oaks is normally not effective due to their size. If the disease persists, fungicides will only be effective if they are applied prior to bud break. Large trees in the landscape will require the services of a professional applicator. Since the disease overwinters on litter, the disposal of twigs and fallen leaves in autumn will help to stem the transmission of oak leaf blister, as will proper maintenance of a healthy tree through correct watering and fertilisation.
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