Bur Oak Blight in Waukesha: How to Spot It and Protect Your Oak
Brown wedges on your bur oak's leaves in late summer? Here's how to recognize bur oak blight, what you can do about it, and when to call an arborist.
Big bur oaks are some of the oldest and most valuable trees in Waukesha yards. They’re also the main target of bur oak blight (BOB), a fungal leaf disease that the City of Waukesha’s Forestry Division now monitors in local parks and along streets. The disease rarely kills a tree outright, but years of repeat infection can weaken an oak enough to invite other problems.
Which oaks get it
Bur oak blight affects the white oak group: bur oak, white oak, swamp white oak, and chinkapin oak. Bur oak is hit hardest.
Not sure which group your tree is in? Look at the leaves:
- White oak group: rounded lobes
- Red oak group: pointed, bristle-tipped lobes
Red oaks have their own serious threat (oak wilt), so it’s worth knowing which kind you have.
What bur oak blight looks like
Symptoms usually show up around mid-July:
- Small purple-brown spots along the leaf veins on the underside of leaves
- Spots that grow into brown, wedge-shaped dead areas
- A canopy that thins out as infected leaves wilt and drop, often starting in the lower branches
- Dead leaves that cling to the tree through winter into the next year. Those leaves carry the fungus that reinfects the tree in spring.
If you’re seeing dead, brown leaves still hanging on your oak right now, that’s a strong reason to have it checked.
What you can do yourself
- Rake and remove fallen leaves. This cuts down on the fungal spores waiting to reinfect the tree.
- Water during dry spells. Drought stress makes a blighted tree weaker. Set a sprinkler under the canopy and put an empty can nearby; turn it off when the can holds about an inch of water. Once a week during a dry stretch is plenty.
- Mulch under the canopy (not piled against the trunk) to hold moisture in the soil.
- Protect the roots during construction. Driveway, patio, or utility work near a mature oak adds stress on top of the disease.
Treatment
A fungicide injection is the only treatment that stops bur oak blight from spreading within a tree. It’s the same type of product used to protect red oaks from oak wilt. Treatment is an ongoing cost, which is why the city only treats its most valuable trees. For homeowners the math is similar: it makes the most sense for a large, healthy, well-placed oak you want to keep for decades.
The city recommends having your bur oak evaluated by an ISA Certified Arborist before deciding. The Wisconsin Arborist Association keeps a directory of certified arborists.
When the tree is past saving
After many years of blight, an oak can develop dead branches in the upper canopy. Those are worth removing before they come down on a roof, car, or person. And if the tree is in steep decline, removal may be the safer choice.
The local tree service featured below works in Waukesha and New Berlin. Call them to have your oak looked at, deadwood pruned out, or a failing tree taken down safely.
Source: City of Waukesha Forestry Division.