By: Jean Ferdinandsen – Certified Arborist WI-0149A
A serious leaf blight disease on bur oaks has become a problem in the Midwest. Wet spring weather the last few years has especially increased the incidence of bur oak blight- BOB.
Caused by a fungus that overwinters on the leaf petioles (leaf stalk), BOB occurs mostly on naturally established trees. Bur oaks that produce smaller acorns and are on upland sites are more susceptible to the disease.
Leaf symptoms appear in late July or August. Affected leaves develop brown or purple lesions along the midrib and main veins. Large areas of the leaf die as the disease progresses, giving an overall wilted or scorched appearance. These symptoms may resemble and have been confused with oak wilt. A unique feature of BOB is that some of the killed leaves remain on the tree during the winter. Healthy bur oaks shed their leaves in the fall.
BOB symptoms often start in the lower branches and progress up the tree. The disease tends to intensify in individual trees from year to year; it is slower to move from tree to tree. Over time, severely affected trees may lose vigor and die. Such trees develop secondary problems such as armillaria root rot or two-lined chestnut borer that hasten decline and death.
Boosting tree vigor helps prolong the tree’s life and ward off secondary problems. This can be done through such things as appropriate watering, fertilization, mulching, and building up the trees root system. Removal of fallen leaves is not an effective management tool for BOB.
There is a large variation in the resistance of individual trees. Trees prone to infection may need to be injected with a fungicide in addition to the steps above. This treatment is done in late May or June prior to leaf symptoms. This can reduce symptoms for two to three years, and should be repeated the year following the re-appearance of moderate to severe symptoms.
Bur oaks have a high value, especially in our landscape, and are worth investing in. Have your Certified Arborist from Wachtel evaluate your bur oak and recommend treatments as needed.
It’s worth it.