Issue 10, June 25, 2010

Watch for Oak Wilt

As the season progresses, we see many oaks that look stressed. How do we know the cause of the stress? Some possible causes include Armillaria root rot, root compaction or injury, severe oak anthracnose, bacterial leaf scorch, and oak wilt. If the oak in question is in the white oak group, symptoms for all of these problems might appear for several years. If the tree is in the red oak group, oak wilt will kill it in about one year. That distinction, at least, is fairly distinct.

As I have written in many newsletters, oak wilt begins as scorching of the foliage, often in sections of the tree rather than uniformly over the tree. The scorched foliage has a half leaf symptom, with scorching starting at the tip of the leaf and moving toward the base of the leaf. Sometimes the scorching is severe enough that only the base of the leaf is green. The most diagnostic symptom is vascular streaking. Both the scorch symptom and vascular streaking are shown in the images. Vascular streaking will extend the length of the stem and into the larger branches that are showing symptoms. Injuries and secondary canker fungi can also stain wood, but that stain will not extend more than about 12" on either side of the canker. If you have seen lots of oak wilt and have cut into many stems, you can probably diagnose this disease based on symptoms. Still, the only way to make a positive diagnosis is to sample live, symptomatic wood showing vascular streaking and have a lab prepare cultures to isolate the oak wilt fungus. I am asked this question often by oak owners who have been told they must remove a tree infected with oak wilt. They want to be certain they are doing the right thing by removing the tree. Culturing is the only way to be sure. The oak wilt fungus is a bit slow to develop spores in culture. Expect a positive diagnosis to take 10-14 days in the lab.

We have had a few positive cases of oak wilt at the Plant Clinic already this year. It is not too early to watch for this disease. Oak wilt is a disease you want to diagnose as early as possible to prevent further spread. Information on oak wilt can be found in the University of Illinois Report on Plant Disease #618.--Nancy Pataky

Nancy Pataky

Return to table of contents