As a result of our damp spring, this fungal disease has been appearing on our landscapes more frequently than normal this summer. Members of the red oak group, the ones with the sharp points on their leaves, are particularly susceptible. Less so are the white oaks, identifiable by their leaves with rounded lobes.
The only tree damage from this disease is to the leaves. Symptoms are yellow, blister-like, raised areas, 1/16 to ½ inch in diameter, scattered over the upper leaf surface, with corresponding grey depressions on the lower surface. They will turn from yellow to reddish brown with age. Several blister areas may merge and cause the entire leaf to curl.
The fungus reproduces from spores that lodge in and overwinter in the buds.
If the tree is in good health, little harm is done. No control is really needed. However, in a landscape situation where “near perfection” is the standard, a dormant application of a fungicide containing chlorothalonil just before new growth begins in the spring, can be helpful. One application should suffice. To be effective, fungicide must cover all the buds and twigs on the tree. Generally, this is a job for the tree professional.
Article written by Glenn Palmer, Extension Master Gardener Volunteer.