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Wright County News

Posted on: September 4, 2020

U of M Extension Explains Warning Signs of Bur Oak Blight

By Karen Johnson, UMN Extension Educator in McLeod & Meeker counties

Does your oak tree have brown leaves? If it is a Bur Oak tree, then it may be affected by Bur Oak Blight.

The Extension office has seen several pictures over the last couple of weeks of Bur Oak Blight. 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 top side 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. The Department of Natural Resources has a publication highlighting Bur Oak Blight: https://files.dnr.state.mn.us/assistance/backyard/treecare/forest_health/bob-one-pager.pdf

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 of your oak tree’s decline such as anthracnose, oak wilt, or even two-lined chestnut borer. 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.treesaregood.org. For more information on health issues with oak trees visit https://apps.extension.umn.edu/garden/diagnose/plant/deciduous/oak/

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