Oak Wilt D.L. Clement Regional Specialist University of Maryland Extension

Oak Wilt Oak wilt was first described in Wisconsin in 1944 where trees were found dying in localized areas. This disease kills young and mature oak trees and has been found in 21 states. The predominant damage has occurred in the upper Midwest although the range extends from New York to Texas.

Distribution of Oak Wilt 2005

Oak Wilt

Biology: The fungus can overwinter as mycelial mats under the bark. As the fungal mats enlarge, they cause the bark to split and the emitted odor, which smells like “apple cider”, attracts bark and sap beetles that feed on the fungal mats. The beetles move the fungal disease from diseased trees to healthy trees beginning in the spring through feeding and reproduction activities. Trees can also become infected through root grafts between infected and adjacent healthy trees. Once inside the trees the fungus spreads rapidly inside the water conducting tissues of the xylem causing them to become plugged and nonfunctional.

Oak Wilt Things to look for: Red oak symptoms can occur as early as May with leaves turning dull green or bronze before wilting and becoming yellow or brown. These symptoms typically affect the leaf tip and margins first as they move inwards towards the midrib and leaf base. Wilted leaves will curl around the leaf midrib. Dying red oak leaf symptoms often intensify within a few weeks throughout the canopy and the leaves at the ends of the branches show heavy defoliation. Leaves will fall in all stages of symptom development. Even apparently healthy green leaves will be shed as trees begin to decline. Severely diseased red oaks can die within 2 months and most die within a year of visible symptoms. White oaks often have less severe symptoms and may only lose one or two branches a year. Infected white oaks often have discolored annual rings, but seldom develop fungus mats under the bark. Some white oaks may even recover and serve as symptomless reservoirs for this disease.

Oak Wilt Host Trees: This disease attacks all oak species and has been found in 16 native oak species. In general, red oaks are more frequently infected compared to white oaks. Additional inoculations have demonstrated over 35 native and exotic oak species are susceptible as well as American and European Chestnuts, chinkapin and tan oaks and several cultivars of apple.

Oak Wilt

Dark streaks under the bark of infected branches.