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Late summer bur oak blight

August 28, 2020
Large mature oak tree with no leaves.
A bur oak infected with blight. The edges of the tree have more leaves than the inner canopy.

A recent increase in questions from homeowners about sick oak trees have revealed their concerns about oak wilt. But another major disease that shows symptoms in August is bur oak blight or BOB.

BOB is specific to bur oaks, but not all bur oak trees get sick. There may be genetic resistance or tolerance in some bur oak populations but not in others. As a result, you may have a healthy oak right next to a severely diseased one.

If you are not sure if you have a bur oak, check out the Minnesota Department of Natural Resources profile for identification.

How to tell bur oak blight and oak wilt apart

Oak leaf with wedge-shaped brown areas.
Bur oak blight affected leaf. Note the wedge-shaped lesions.

Like oak wilt, leaves have dead, brown areas, and many prematurely fall. However, with BOB, the splotches on the leaves have better defined edges, and often are shaped like a wedge.

On the underside of the leaf, BOB also can have black spotting on the veins, especially early on when they are just starting to show symptoms. Another sign it is BOB rather than oak wilt is seeing more leaf loss on the inside and lower parts of the canopy rather than the edges.

Still, it can be tricky to separate oak wilt from BOB, and it is possible that a tree can have both diseases at the same time.

In 2015, the UMN Sustainable Forests Education Cooperative recorded an excellent presentation that can walk you through both of these diseases and offer some clues on how to tell them apart.

Thankfully, having BOB does not mean your tree will die. Oaks can handle a large amount of leaf loss for a year or two and be fine. However, if the oak keeps getting BOB year after year, it increases stress, and that can open the door to more damaging problems.

Depending on disease severity, the number of years infected, and its value to you, there may be fungicide treatments available. We urge homeowners to work with certified arborists and have a lab-confirmed diagnosis of BOB before any chemicals are injected into the tree.

The University of Minnesota’s Plant Disease Clinic can also examine samples for both oak wilt and BOB if you need help.

Shane Bugeja, Extension educator, Blue Earth and Le Sueur counties

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