Branching Out

The next threat to our native species….BOB

BOB, or Bur Oak Blight is a serious threat to our bur oak trees. Oak trees are generally broken into two groups. In our area, the red oak group consists of Northern red oak, Northern pin oak, Black oak, Shingle oak, and Pin oak. Generally, leaves of the red oak group will have lobes that come to a sharp point. The white oak group is made up of White oak, Bur oak, Chinkapin oak, and Swamp white oak. The leaves of the white oak group will have rounded lobes. Bur oak leaves will have a narrowing in the middle of the leaf but will vary in shape. They also have ridged, corky twigs. They are native to our prairies, and a mainstay of our oak savannahs. You may have one in your yard. If you visit some of the older parks in Waukesha, you likely have seen some of these giants. In fact, in August 1868, Colonel Richard Dunbar famously drank his healing water under what is now the Dunbar (bur) Oak in Bethesda park.

Trees in the red oak group have been under pressure from a fungus called oak wilt since the mid 1900’s, a disease that quickly kills. Only in the last 10 years have we become concerned about trees in the white oak group. First described in Iowa on bur oak trees, BOB has been spreading throughout the Midwest ever since. Because it is so new, we are still learning about it. It is possible that this disease may not be specific to bur oaks, but other white oak trees as well. Unlike oak wilt, BOB weakens the tree over several years by interrupting photosynthesis. In their weakened state, it is usually other factors including insect feeding that will eventually cause a tree’s mortality.

Trees infected with BOB may look healthy until the middle of July, when signs of infection begin to show up on the leaves. As the growing season progresses, so does the visible damage. Infection usually begins low, and in the center of the crown. Diseased leaves will start showing signs of infection with purple spots. These spots will morph into brown wedges on the leaves. Leaves will continue their browning until the leaf is completely dead. These trees will appear to have a “thinner” crown as leaves shrivel. Another symptom of BOB is that some of the infected leaves will stay attached to the tree rather than shedding them which allows for disease transmission the following year. There are ways in which you can help protect these trees. Removing fallen leaves will reduce the number of fungal spores which could re-infect the tree the following year. Watering trees in times of drought is also another way to help reduce the stresses on an already weakened tree. The only way to stop the spread of BOB is with a fungicide application. The same fungicide used to prevent oak wilt in red oaks can be used to stop BOB from weakening these trees. While the fungicide can stop the disease, a bur oak declining because of the blight may need help fending off attacking insects and could benefit from further treatments. We will be monitoring the bur oaks in our parks and on our streets for BOB. While treating all our bur oaks would be unsustainable, we will evaluate the most valuable trees for potential treatments.

If you think you have a bur oak, and may be interested in protecting it, please contact an ISA Certified Arborist to have it evaluated. You can find a local Certified Arborist at www.waa-isa.org/find-certified-arborist/

Bur oak holding its leaves after fall leaf drop

Bur oak

Oak savanna at Dopp park