We have seen peach leaf curl in southern Illinois this year, so it is assumed that it will show on oaks as oak leaf blisters soon as well. Related species of the Taphrina fungus cause peach leaf curl and oak leaf blister. Look for distorted, thickened leaves and early leaf drop. Leaves turn downward and inward and may become red or purple. The disease may cause yield loss in edible peach but is not seriously harmful to ornamental species. Still, repeated yearly infection may weaken a tree and predispose it to other problems.
The causal fungi survive over winter in buds and twigs. They infect leaves and flowers in the cool, moist weather of early spring, from bud swell to bud opening (ideally, temperatures are 50° to 70°F). Evidently, such weather did exist at the time of infec-tion in parts of Illinois. It is too late now to spray a fungicide to help these trees for this growing season. Concentrate efforts on promoting tree health through pruning, watering, and fertilization.
Fungicides are not usually recommended for ornamental trees. For fruit growers, we recommend a single dormant fungicide spray applied in the fall after leaf drop or in the early spring before budbreak. This approach is common practice for most commercial growers. Home fruit growers in Illinois who use a dormant fungicide and are careful to provide full coverage of buds do not have problems with leaf curl. If you are having problems with curl and blisters, mark your calendar for a late-fall fungicide application so you don’t forget again.
For more on leaf curls and blisters, consult Report on Plant Disease no. 805, “Peach Leaf Curl and Plum Pockets,” or no. 663, “Oak Leaf Blister.” Both are available on the Vista Web site, http://www.ag. uiuc.edu/~vista/horticul.htm.