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Oak Wilt or Something Else?

University of Minnesota Extension, Stearns County News
August 6, 2014       
Source:  Beth Berlin, Extension Educator-Horticulture
University of Minnesota Extension
Stearns & Benton Counties


Oak Wilt or Something Else?
By Beth Berlin, University of Minnesota Extension

ST. CLOUD, Minn. (08/06/14) —Oak trees are known for their majestic presence in the landscape. Unfortunately, they too can suffer from different diseases or infestation and their grand presence can come to an end.  Many have heard of oak wilt; however it shouldn’t be the only thing blamed when an oak tree’s health takes a turn for the worse. Oak trees can also suffer from Bur Oak Blight, Anthracnose, and insect damage.

Oak wilt is caused by a nonnative fungus Certocystis fagecearum. Oak wilt spreads most commonly through root grafts of similar species, but can also spread above ground by oak sap beetles that carry the fungus from tree to tree. The red oak group including the Northern Pin, Red, Black, and Scarlet, are highly susceptible to infection. Oak wilt in red oaks can be identified by rapid wilting from the top of the tree down. It infects a few branches at a time, and leaves will begin to drop rapidly. Leaves that drop may be brown, green, or a combination. Once symptoms first appear, the red oak will generally wilt completely in two to six weeks.  White oak group, including bur, white, and bicolor, are also susceptible at a lower level.  This group of oaks can sometimes live with the disease for a long time before dying. This gives the tree owner the opportunity to have the tree treated by a tree care professional.

Preventing the spread by root grafts is difficult; stopping the spread of fungus through the roots requires the use of a mechanical vibratory plow with a five-foot blade. Minimizing the spread overland by insects can best be accomplished by not pruning oaks during the months of April, May, and June. Risk of insects spreading the Oak wilt disease decreases but is still possible from July through October, so the best time to prune oaks is November through March since the fungal pathogen and the insects are not active.

Bur oak blight is a severe leaf blight disease and only occurs in bur oaks. Symptoms of bur oak blight appear in late July or August. Infected leaves have purple-brown lesions along the mid-vein and large lateral veins on the underside of the leaf, which later become visible on the topside of the leaf. This leads to large, wedge-shaped areas of chlorosis and necrosis, yellowing and death of tissue. The infection continues and causes large areas of the leaf to die, eventually giving a wilted or scorched appearance. Overtime the infected trees may die because the tree is stressed and now is susceptible to secondary invaders such as insects. Management may include injections of the fungicide propiconazole in late May or early June. However confirmation of bur oak blight should be done through a laboratory test before any treatment is done.

Oak trees can also be impacted by anthracnose, which is caused by the fungus Discula quercina. Anthracnose lives in the twigs and leaves of the tree, and when cool, wet springs occur the spores travel onto young, new growth and cause further infection. Symptoms of anthracnose can include irregular shaped black blotches that dry out and turn brown or tan. The leaves may also become cupped, curled, or distorted, and cause premature leaf drop and defoliation.  Symptoms of anthracnose on an oak will appear in the lower and inner branches.

Typically anthracnose will not cause severe damage and the tree will be able to survive. To minimize the chance of reoccurrence and repeated stress to the tree, dispose of the leaves once they drop by burning them or disposing of them with refuse.

Finally various different insects can cause similar symptoms. The two-lined chestnut borer, Agrilus bilineatus and the twig girdler, Oncideres cingulata are two insects that impact oaks.  The two-lined chestnut borer is bluish black in color with two light stripes running down the wing covers. Larvae are about one inch long, white in color, and have two spines at the tip of the abdomen. If a tree is infested with this bug it will exhibit wilting in the upper canopy’s leaves that turn brown but may remain attached to the branch for several weeks.  The exit holes of the insect found on the trunk are about 1/8-inch and “D” shaped, and the larvae will create “S” shaped galleries underneath the bark. Damage is usually visible mid to late summer. Since infestation of the two-lined chestnut borer is typical in stressed oaks encouraging good tree health and using good management practices is the best prevention. Management includes thinning overstocked stands to decrease competition and increase the vigor of the remaining trees. In residential areas homeowners can increase tree vigor by mulching, watering, and fertilizing. Pesticides are available on the market for high-value trees; product containing carbaryl, bifenthrin, or chlorpyrifos can be used.

The twig girdler adults are about 9/16-inch long, grayish-brown with scattered yellow spots, and antenna as long as it’s body length. They will cause dead twigs scattered throughout the canopy, and small branches will fall to the ground, however they may remain on the tree until a sufficient wind causes them to fall. Look for the outer half of the branch being smooth while the center is rough. Damage from twig girdlers can be seen in August and September. Control by chemical treatment is impractical, and the best recommendation would be to collect and dispose of fallen twigs to remove the larvae for future year infestation.

There can be many other causes to your oak tree wilting so complete your investigation and do your research or send in a sample to the University of Minnesota Plant Disease Clinic to determine the cause before jumping to the conclusion that it is oak wilt. For more information on health issues with oak trees visit



Beth Berlin
Extension Educator, Horticulture
(320) 255-6169
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