Growing Green

By Robin Trott, Extension Educator

I have received several calls in the last few weeks about oak trees that seem to be prematurely browning and losing their leaves.  The odd weather we have had this summer could be the culprit, or your tree might be showing symptoms of Bur Oak Blight (BOB). BOB is a leaf disease that gained attention in Minnesota and Iowa in the mid-to-late 2000s. It causes leaf browning and leaf loss in late summer and early fall, and affects only the small-acorn variety of bur oak, which is the one variety we have in Minnesota. 

While the disease can cause severe symptoms on individual trees, it does not affect all bur oaks. Bur oaks can lose about 50 percent of their canopies every year and still remain relatively healthy. However, when a bur oak loses more than half of its leaves for several years in a row, it may become stressed and susceptible to other problems such as two-lined chestnut borer and Armillaria root disease.

Leaf symptoms include dark veins on the undersides of leaves and brown, wedge-shaped segments between leaf veins. The disease starts in the lower canopy and progresses up the tree. In severe cases, all but the outermost leaves around the canopy will die. BOB might cause minor dieback (death of branches starting at the tip), but it will not kill major limbs. (Armillaria root disease and two-lined chestnut borer will kill large branches.)

Just because a bur oak has BOB does not mean you should cut it down. In most cases, the tree will leaf out just fine the following year. The best time to evaluate bur oak health is in June. If the tree does not have branch dieback or epicormic sprouts (small, young branches growing out of the trunk and big limbs), it is probably not stressed. 

The following are recommendations from the Minnesota Department of Natural Resources about management of BOB in your yard and garden:

Unstressed bur oaks that get BOB may be able to survive without any treatments. However, for particularly valuable yard trees, you may choose preventative injections of the fungicide propiconazole. This fungicide, when injected at half the maximum label rate in late spring (as soon as leaves have formed), can reduce BOB in some healthy bur oaks for at least three years. Propiconazole can burn bur oak leaves, but healthy trees can overcome this temporary stress. 

Only treat trees that: 

• Do not have any dieback or epicormic sprouts, or 

• Have had two consecutive years with more than 40 percent leaf loss. 

After treatment, don’t treat again until the tree has lost roughly 40 percent of its leaves to BOB for two years in a row.

Until next time, happy gardening!

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“Today’s mighty oak is just yesterday’s nut that held its ground.” ~David Icke