Plant Pathology Infobytes
February 17, 1999
Focus on Oak Leaf Blister
There's still time to control one of the most prevalent diseases which attacks oak trees in the urban landscape. Oak leaf blister appears on species of oaks, and although this fungus disease doesn't cause tree death, affected trees appear unsightly.
Oak leaf blister was especially prevalent last season because of early spring rainy weather favorable for disease development. The fungus which causes the disease infects young leaves as they emerge, and the distinctive symptoms of oak leaf blister appear several weeks later.
Oak leaves affected by leaf blister tend to develop a distorted appearance. Consequently, homeowners often become concerned about the health of their oak trees and want to find out what's behind the problem.
Almost all species of oaks are subject to leaf blister disease. Water and willow oaks, two of our most common shade species, are more susceptible than such species as white and post oaks. Oak species such as pin oak and Shumard oak are resistant to leaf blister and are rarely affected.
The fungus which causes leaf blister attacks only the leaves, and does no damage to other parts of the tree. While landscape trees with affected foliage may appear unsightly, oak leaf blister causes little damage and will not cause any long term tree health problems.
Leaf blister can be identified by the presence of circular raised areas up to one-half inch in diameter on young foliage. The spots, or bulges, are scattered over the upper leaf surface, causing a depression of the same size on the lower surface of the leaf. Generally, spots become yellowish-white in contrast to the green color of healthy leaf tissue.
Although, oak leaf blister causes little damage to trees, some homeowners may wish to control the disease because of its unsightly appearance. Since infection of leaves by the blister fungus takes place only in early spring from fungus spores that overwinter in the bud scales of twigs, disease control steps have to begin before tree dormancy is broken in the spring.
Control of oak leaf blister requires a dormant application of a fungicide any time from January until late March or early April. The latter application timing probably gives better oak leaf blister control if the fungicide is sprayed onto trees just before new growth starts. Usually one fungicide application will give satisfactory control. Care should be taken to coat all buds and twigs.
It's unlikely that fungicide application will provide 100% control of oak leaf blister and the disease will appear on some of your tree's foliage. Infected foliage which falls from the tree should be collected and destroyed, or added to a compost bin (provided the leaves are composted properly).
For additional information on oak leaf blister and landscape disease problems, check with your county Extension office.
Infobytes newsletter was written by the late Dr. Frank Killebrew, Extension Specialist.