If you’ve got a bur oak tree (Quercus macrocarpa) growing on your property you likely already know that you’re a lucky homeowner! Bur oaks are majestic, sprawling trees that are associated with the prairies of the Midwest. Because of their thick and somewhat fire-resistant bark and drought tolerance they have a natural ability to compete well with prairie grasses. An iconic Midwest image is a wide open prairie dotted with majestic oaks throughout.
Bur oaks are a member of the white oak group (including white, chinkapin, swamp white and post oaks). These oaks have rounded lobes on the leaves and acorns which mature in a single growing season and often sprout soon after they fall in the autumn. A mature bur oak can grow 50-75 feet tall and 40-70 feet wide and live from 200-300 years.
It’s also an adaptable tree due to its extensive root system. It can be found growing in moist areas like floodplains and next to streams and in drier soils on hills and uplands. Bur oaks also offer dense shade, can endure pollution and heat stress, and yield acorns that feed wildlife. Because of its tolerance to a wide variety of soil and other stressful conditions it’s often used in urban landscapes.
Unfortunately, over the last several decades bur oaks have become susceptible to a disease called bur oak blight. In this article you’ll learn all about bur oak blight – how to identify it, how it’s different than oak wilt, and treatment and prevention options.
What is Burr Oak Blight?
Bur oak blight is a fungal disease that started popping up in the Midwest in the 1990’s. It’s caused by a native fungal pathogen, Tubakia iowensis and causes the leaves of the tree to turn brown and eventually drop in late summer and early fall. Bur oak blight primarily affects the small-acorn variety of bur oak (Quercus macrocarpa var. oliviformis).
Luckily, many bur oaks can survive being attacked by this disease. Bur oaks can lose up to 50% of their leaves each year and still remain healthy. But, if the tree loses more than half of its leaves several years in a row it can become stressed. Weakened bur oaks are more susceptible to other diseases and insects, such as two-lined chestnut borer and Armillaria root rot.
How to Identify Bur Oak Blight
The symptoms of bur oak blight start revealing themselves in mid-July in most areas. The signs include dark veins on the undersides of leaves and brown areas between the leaf veins. The disease starts at the bottom of the tree canopy and slowly spreads upwards.
In late summer and early fall the petioles (the stalk that joins the leaf to the stem) develop black pimples which contain fungal spores that can be seen upon close inspection. It’s the presence of these black spots that will help you confirm your tree has bur oak blight.
Because bur oak leaves remain on the tree during fall and winter, the following spring the fungal spores are released and spread by early season rains and infect newly emerging leaves. There seems to be some correlation between the amount of moisture in spring and how severely the disease infects bur oaks that season.
Your tree won’t show any signs of infection until mid- to late summer when the disease starts to spread throughout the tree’s canopy. This cycle repeats again each year.
Bur Oak Blight vs. Oak Wilt
Because oak wilt has been around longer and it more familiar there is often some confusion among homeowners around whether their tree has bur oak blight or oak wilt.
All oaks are susceptible to oak wilt, however the red oak subgenus, including pin oak, scarlet oak and black oak are more affected. Trees in the red oak group generally die rapidly, usually within weeks or months after infection. Treatment will not save an infected red oak. Trees in the white oak subgenus, such as bur oak, can generally be treated for oak wilt and will recover within a year of treatment.
Oak wilt symptoms in red oaks will typically appear in the upper canopy and progress inward and downward within a few weeks. At first, there may only be a single branch that dies, showing leaves that often bronze, or turn tan or dull green, starting at the tips or outer margins. The leaves will turn brownish and prematurely drop mid-summer.
Oak wilt symptoms in white oaks are similar but progress more slowly. Generally, a single isolated branch will begin to “flag” and leaves will turn brown. If left untreated for several seasons the dead areas will spread out from the area around the initial infection.
This is in contrast to bur oak blight, which starts in the lower canopy and spreads upward and outward.
When a white oak is suspected of having oak wilt it’s imperative to make a positive diagnosis by sampling and submitting the specimen to a local University Extension tree lab for proper diagnosis since other factors or pathogens can cause similar dieback. Read more about oak wilt and its treatment.
How to prevent & Treat
Unfortunately, there aren’t any known preventions for bur oak blight at this point in time. Removing the leaves from under the tree isn’t an effective strategy since the fungal spores live on the petioles and leaf stems, which the tree often holds onto throughout the winter.
Many bur oaks may be able to survive for many years while infected with bur oak blight. But, if you have a particularly valuable tree in your landscape you might choose to proactively address the disease.
A fungicide injection of propiconazole in spring after the leaves have fully emerged has been shown to be the most effective treatment in reducing the effects of bur oak blight the following season. The University of Minnesota recommends only treating trees that have lost 40% or more of their leaves for more than two years and then not retreating again until the tree has lost 40% of its leaves for another two years in a row. They do not recommend treating for bur oak blight in the fall.
The bad news is that bur oak blight can infect a beloved tree in your landscape and cause damage from year to year. The good news is that it rarely kills the tree and there are options for treatment.
If suspect your tree has bur oak blight you should make an appointment with a certified arborist right away. And if you live in the Madison, WI area, contact ECO today for a free consultation to evaluate your bur oak and come up with a plan of action.
