Trott: Trouble with oak trees

Once again a number of people have recently expressed concerns about their oak trees. Oak trees are known to be majestic and long living, but some people are noticing that leaves are browning and even prematurely dropping leaves.

There are several issues oak trees can face that can cause your oak tree to decline. Anthracnose, bur oak blight, or oak wilt all can cause issues with oaks.

Oak anthracnose is caused by the fungus Discula quercina, and lives on infected twigs and leaves. The fungus spores are 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 prevent 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 more serious leaf blight disease but it only impacts bur oaks. It's important to truly identify the tree. 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.

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. For more information on health issues with oak trees visit apps.extension.umn.edu/garden/diagnose/plant/deciduous/oak .

Until next time, happy gardening!

Robin Trott is a horticulture educator with University of Minnesota Extension. Contact her at 320-762-3890, or at trot0053@umn.edu.