Oak leaf blister is a fungal leaf spot disease caused by the fungus Taphrina deformans. It is a spring disease common on all oak species. Severe leaf blister can result in widespread early defoliation. On well-established trees, early defoliation will not cause tree death, but will reduce tree vigor.

Symptoms:

Early in the spring, small, rough (concave-convex) spots appear as the oak leaves expand. The spots become pale green in color and somewhat thickened. Older spots are brown or greenish brown. Leaves with numerous spots will 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 the late summer, leaf loss will have little impact on the tree's health.

Photo courtesy of Robert L. Anderson, USDA Forest Service

Persistence And Transmission:

Microscopic spores are produced in leaf spots during midspring. These spores are carried by wind and splashing rain drops to bud scales and twigs, where they remain in a resting stage until the following spring. At this time, rain washes the spores onto young leaves, where infection takes place. After two to four weeks, depending on weather conditions, small circular depressions (spots) will begin to develop. Spores produced on these spots will lodge in bud scales and, again, remain dormant until the following spring.

Control:

Because oak leaf blister does not seriously affect the overall health of the tree, chemical control measures are usually not recommended. Cultural controls tend to be ineffective because of the nature of the fungus and its method of infection and transmission. On small, newly established or especially valuable specimen oak trees previously damaged by leaf blister, apply a protective fungicide at budswell.

Adapted from Jacqueline Mullen and Austin Hagan, Alabama Cooperative Extension Service, 2000