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Extension > Local Extension Offices > Stearns > County Horticulture Educator > Articles > Declining Oak Trees

Declining Oak Trees

University of Minnesota Extension, Stearns County News
August 5, 2015

Source:  Beth Berlin, Extension Educator-Horticulture
University of Minnesota Extension
Stearns, Benton & Morrison Counties


Declining Oak Trees
By Beth Berlin, University of Minnesota Extension

ST. CLOUD, Minn. (8/5/15) — Does your grand oak look a little rough? Is your oak tree dying? There isn’t a clear answer for every oak. Several culprits could be the cause of your oak trees decline.  Anthracnose, bur oak blight, oak wilt, or even two-lined chestnut borer are just a few causes of oak decline.

Oak anthracnose is caused by the fungus Discula quercina, and lives on infected twigs and leaves.  Jill Pokorney from the University of Minnesota’s Plant Disease Clinic has reported extreme cases of oak anthracnose this year.  This is due to the wet spring and summer we have had. The fungus spores were able to move around readily throughout the tree with splashing water and cause infection more broadly throughout the entire tree. In addition, high humidity creates ideal conditions for the spores to multiply and spread.  Symptoms can include misshaped leaves with black spots or blotches that can expand, turn brown, and dry out. Anthracnose can also cause leaf and shoot dieback which result in clusters of dead leaves clinging to the tree. Premature dropping of leaves is another symptom of anthracnose. 

Typically anthracnose will not cause severe damage and the tree will be able to survive. However, due to other stresses, such as drought, the anthracnose may prevent enough photosynthesis for the tree 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 in a refuse.

Bur Oak Blight is a serious leaf blight disease but it only impacts bur oaks.  True identification of the tree should be done. 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. Over time, 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 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 of oak wilt 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 over land by insects can best be accomplished by not pruning oaks during the months of April, May, and June. Risk level drops to low from July through October, but the best time to prune oaks is November through March since the fungal pathogen and the insects are not active.

The two-lined chestnut borer, Agrilus bilineatus is a beetle 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. Pesticides are available on the market for high-value trees; products containing carbaryl, bifenthrin, or chlorpyrifos can be used. Be sure to read and follow all safety and application instructions.

To help ensure the health and vigor of all trees, watering, mulching, and proper pruning are all important steps a landowner can do. Trees need approximately one-gallon of water per every inch of DBH (diameter at breast height) per week. Mulching helps maintain a more consistent, cooler soil temperature and even moisture. Proper pruning helps remove dead, damaged, and diseased branches as well as allows for more air movement in a tree canopy.

There can be many other causes to your oak tree’s decline. Do some research and investigate your trees closely. Homeowners can send in a tree leaf sample to the University of Minnesota Plant Disease Clinic to determine the cause before jumping to any conclusion. To find a certified tree care professional, visit www.isa-arbor.org.   For more information on health issues with oak trees visit http://www.extension.umn.edu/garden/diagnose/plant/deciduous/oak/.

Contacts

Beth Berlin
Extension Educator, Horticulture
(320) 255-6169
adam0062@umn.edu
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