Bur oak blight is a fungal leaf disease that gained attention in Minnesota and Iowa in the mid- to late-2000s. This blight causes leaf browning and leaf loss in late summer and early fall, and affects only the small-acorn variety of bur oak (Quercus macrocarpa var. oliviformis). Unfortunately, that’s the one variety we have in Minnesota. Bur oak blight is caused by a native fungal pathogen, Tubakia iowensis, which likely has been intensified by above-average spring rainfall since the 1990s.
While the disease can cause severe symptoms on individual trees, it does not affect all bur oaks. Bur oaks can lose about 50% of their canopies every year and still remain relatively healthy. However, when a bur oak loses more than half of its leaves for several years in a row, it may become stressed and susceptible to other problems such as two lined chestnut borer and Armillaria root disease.
Bur oak blight’s early symptoms appear in midsummer, but the most obvious damage occurs in late summer. Leaf symptoms include dark veins on the undersides of leaves and brown, wedge-shaped segments between leaf veins. The disease starts in the lower canopy and progresses up the tree. In severe cases, all but the outermost leaves around the canopy will die. Bur oak blight might cause minor dieback (death of branches starting at the tip), but it will not kill major limbs.
The best time to evaluate bur oak health is in June. If the tree does not have branch dieback or epicormic sprouts (small, young branches growing out of the trunk and big limbs), it is probably not stressed. If your bur oak has significant problems and you choose to cut it down, do not do so in April, May, June, or July. These are prime months for spreading oak wilt infection.
Unstressed bur oaks that get bur oak blight may be able to survive without any treatments. However, for particularly valuable yard trees, you may choose to do preventative fungicide injections. This fungicide, when injected in late spring (as soon as leaves have formed), can reduce bur oak blight in some healthy bur oaks for at least three years. Only treat trees that do not have any dieback or epicormic sprouts and have had two consecutive years with more than 40% leaf loss. After treatment, don’t treat again until the tree has lost roughly 40 percent of its leaves to bur oak blight for two years in a row.
If you have a bur oak that doesn’t look quite right, submit a sample to the Plant Disease Clinic. The University of Minnesota Plant Disease Clinic is a diagnostic laboratory that provides testing for fungal, bacterial, viral, and other plant health conditions for commercial growers and the general public. The goal of the Plant Disease Clinic is to provide clients with an accurate, unbiased diagnosis. Directions on how to collect and submit a sample are on the website at https://pdc.umn.edu/.
Until next time, happy gardening!
“An oak tree is a daily reminder that great things often have small beginnings.”
― Matshona Dhliwayo
Robin Trott is a horticulture educator with University of Minnesota Extension. Contact her at 320-762-3890, or at trot0053@umn.