Extension Educator, Horticulture
Few things change the look of a landscape as much as the loss of a tree. Maybe it's a reawakening of our aboriginal roots that make us feel naked and exposed once our lofty friends are gone. The outstretched arms of a mature oak wrap us in the comfort of permanence and reliability. Rightfully so since oaks are one of our most long-lived species often with trees reaching 200 plus years old.
Unfortunately even oaks have enemies. Besides caterpillar bull dozers and caterpillar leaf browsers, oaks have a fungal enemy.
Oak wilt is a serious disease of many oaks and has also been found in Chinese chestnuts. Much like Dutch elm disease and chestnut blight, oak wilt is a fungus. It kills oaks by clogging the water conducting vessels of the tree.
The oak wilt fungus mainly attacks oaks in the red oak group which include oaks with pointed tipped leaves such as pin, shingle, red and black oaks. Oaks in the red oak group are very susceptible to the fungus and often die quickly in just one season.
Oaks in the white oak group which have round tipped leaves such as swamp white, bur and white oaks are more resistant, but can be infected with the fungus. In the white oak group the disease is often localized in one area of the tree; therefore, the tree may live with the disease for many years.
Once a red oak is infected with the fungus, the tree usually starts to die from the top down in late spring and early summer. Leaves may first turn a dull green, bronze, or tan starting at the margins. Immature leaves may also droop and roll lengthwise. Mature leaves usually remain stiff during the different stages of the disease and for some time after the tree dies.
Typically scorching of the leaves is noticed next, often in sections of the tree rather than uniformly throughout the tree. The scorched foliage has a half leaf symptom, with scorching starting at the tip of the leaf and moving toward the base of the leaf. Symptoms progress downward and inward until all the foliage is affected. Defoliation may occur any time after the symptoms appear.
Another diagnostic characteristic of oak wilt is a brown or black streaking which develops in the current-season sapwood of wilting branches. The discoloration appears as longitudinal streaks once the outer branch layer is removed. In cross-section, a brown ring or broken circle of dark-colored tissue may be seen. Discoloration in the center of the stem is not associated with oak wilt.
Oak wilt is spread via root grafts, animals such as squirrels, tools and insects. Avoid pruning oaks during the growing season when sap beetles that can carry the fungus are present. Root grafts can form when two or more oaks of related species are growing close to each other. If one of the trees becomes infected with oak wilt, the disease can spread from one tree to another through root grafts. Oak wilt is the primary suspect when clusters of red and black oaks start dying.
Keep in mind oaks can suffer from environmental issues as well as a foliar fungal disease called anthracnose but it is mainly a leaf disease causing brown edges on leaves in the lower branches. Conversely oak wilt appears first in the upper branches.
For more information on oak wilt and how to properly sample for the disease http://web.aces.uiuc.edu/vista/pdf_pubs/618.pdf or contact your local UI Extension office for a copy. The only way to make a positive diagnosis is to sample and have a lab such as the UI Plant Clinic http://web.extension.illinois.edu/plantclinic/ isolate the oak wilt fungus. Oak wilt is a disease you want to diagnose as early as possible to prevent further spread.d