Tips to Better Testing for Oak Wilt | |
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| July 3, 2006 | |
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In Illinois, the main concern with oaks that are suddenly stricken with dead limbs is a disease called oak wilt. That disease can kill a mature oak in a season or in a few years. Read details in Report on Plant Disease, no. 618, available on the University of Illinois Vista Web page at http://www.ag.uiuc.edu/%7Evista/horticul.htm. Of course, other problems can cause dead limbs. To efficiently manage the problem, a positive diagnosis may be necessary. The only way to positively identify oak wilt is to isolate the causal fungus from an infected limb. The University of Illinois Plant Clinic can test for the presence of the oak wilt fungus in a tree. Proper sampling is critical to accurate results, much like fasting may be necessary for some blood test results. And because the mailing costs are often higher than the testing cost, why not get the right sample in the first place? The oak wilt sample must come from live wood. Dead wood has always resulted in a negative diagnosis in our lab. In early stages of the disease, the fungus may not be present in all limbs in an infected tree, so the sample must come from a symptomatic area of the tree. Wood infected with the oak wilt fungus exhibits a brown stain, in stripes, in the vascular tissue. We call this vascular discoloration. Oak wilt samples must show vascular discoloration. The first image in this article shows vascular streaking when bark has been peeled back. The second image shows a cross section of an infected stem.   The streaking appears as brown blotches in a ring pattern within the stem. The discoloration of the center of the stem is not from oak wilt. Suspect samples will be flame-sterilized before bark is removed. We find that the ideal sample size for this process is thumb thickness and 8 to 10 inches long. If you are bringing a sample to the diagnostic lab, keep it in the cool part of your vehicle. If the sample is to be mailed, you will have to mail it on ice. Mail trucks or trunk temperatures are high enough to kill the oak wilt fungus. Accurate testing requires that the sample not become heated in transit. Samples that are sent with disposable or reusable ice packs seem to give best results. Stop by the clinic and ask for ice packs that have come in with samples. We recycle these at no charge. In summary, the ideal sample for oak wilt will be taken from the symptomatic but live area of the tree. The sample will show vascular streaking and will consist of several branch sections that are as thick as your thumb and 8 to 10 inches in length. The sample will be delivered to the Plant Clinic or sent on ice packs. I should also warn you that the oak wilt fungus grows slowly. A positive culture may take 10 to 14 days to develop in the lab. | |
| Author: | Nancy Pataky |
