Jacqueline Mullen, Extension Plant Pathologist and Diagnostician , and Austin Hagan, Extension Plant Pathologist , Professor, Plant Pathology, both at Auburn University Plant Disease Notes Oak Leaf Blister Oak leaf blister is a fungal leaf spot disease caused by the fungus Taphrina deformans. It is a spring disease common in Alabama on all oak species. Severe leaf blister can result in widespread early defoliation. With well-established trees, early defoliation will not cause tree death, but it will reduce tree vigor. Gradually, the spots will 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. Persistence And Transmission. Microscopic spores are produced in leaf spots during midspring.