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.
Wedge-shaped leaf death (tip of leaf) and leaf vein browning (bottom right of leaf) are telltale signs of bur oak blight.
Bur oak showing extreme sign of bur oak blight.
Since bur oak blight is a relatively new phenomenon, we don't know its long-term impact to Minnesota. Forest health staff has surveyed hundreds of bur oaks in central and southern Minnesota. From 2017 to 2020, the average annual number of bur oaks surveyed with severe defoliation from bur oak blight ranged from 0 to 5 percent. Our surveys show that when May precipitation is abundant, bur oak blight is more common. We have also found that some bur oaks that have incurred root damage from past home remodeling show much more severe bur oak blight.
Bur oak blight can have a severe impact on individual bur oaks, but most bur oaks in Minnesota are not severely affected by it.
It's important to remember that bur oaks can lose up to 50 percent of their canopies every year but still remain relatively healthy. However, when a bur oak loses more than half its leaves for several consecutive years, it may become stressed and susceptible to other problems, such as two-lined chestnut borer and Armillaria root disease
The same bur oak in Zimmerman, Minnesota, photographed in September 2015, 2016, and 2017, from left to right
Even when a bur oak has had severe bur oak blight, it may be relatively healthy. The best time to evaluate bur oak health is in June: if the tree does not have branch dieback or epicormic shoots (small, young branches growing out of the trunk and big limbs), it is probably not stressed.
Bur oak blight can be found wherever bur oak grows in Minnesota.