Emerging Oak Disease in Minnesota; Bur Oak Blight

Bur oak blight is a fungal leaf disease that gained attention in Minnesota and Iowa in the mid-to-late 2000s. Bur oak blight causes leaf browning and leaf loss in late summer and early fall. It 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.

Identification of Bur Oak Blight

Bur Oak Blight symptoms start to appear in June, generally with early leaf drop. Later in summer, foliar symptoms become more noticeable. These include dark veins on the leaves and browning, wedge-shaped areas between leaf veins. Similar to other fungal diseases, BOB shows up first in the lower canopy and then moves into the upper crown. For severe infections, all large percentage of the leaves will exhibit symptoms with potential dieback of smaller limbs.

Bur Oak Blight in Minnesota

Early symptoms of Bur Oak Blight in Minnesota including black pycnidia along the veins.

Bur Oak Blight Background Information

Since Bur Oak Blight (BOB) is a relatively newer concern, its long-term impact is not yet known for Minnesota. BOB can have a severe impact on individual bur oaks, but a large percentage are not impacted. Similar to other foliar fungal diseases, symptoms from BOB are worst during years with a wet spring.

Bur oaks can have a large percentage of its canopy impacted but still remain relatively healthy. However, when a bur oak loses a large percentage of its leaves for several consecutive years, it is likely to become stressed and potentially decline. When a bur oak is stressed from BOB, it can become susceptible to other problems such as Two-lined Chestnut borer, drought conditions or other insect pests.

It can be difficult to diagnose oak trees with health concerns, because other issues can be present. Oak wilt, Two-lined Chestnut borer and drought conditions can all show issues during mid-summer. June and July are a good time to assess BOB versus other oak issues.

Bur Oak Blight in Minnesota

Overall symptoms and dieback from Bur Oak Blight in mid-summer.

Bur Oak Blight Treatments and Recommendations

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 injections of the fungicide propiconazole. This fungicide, when injected at half the maximum label rate in late spring (as soon as leaves have formed), can reduce bur oak blight in some healthy bur oaks for at least three years. Propiconazole can burn bur oak leaves, but healthy trees can overcome this temporary

Fallen bur oak leaf in June in Minnesota from Bur Oak Blight.