Oak wilt is a serious disease that can infect many oak species. It is caused by the fungus, Ceratocystis fagacearum. Red oaks are very susceptible to the oak wilt fungus and can die within 4-6 weeks. White and bur oaks are moderately resistant to the disease. Trees can be infected by the fungus through root grafts or by beetle vectors that carry spores to newly wounded trees. When a tree is infected it tree tries to protect itself by producing gummy material called tyloses which can clog the water conducting vessels. Water is prevented from moving to the canopy and leaves begin to wilt. Leaves of infected oaks can wilt, turn brown at the edges, and fall off. The outermost ring of sapwood sometimes turns brown and appears as streaks when the bark is peeled; or as a ring when the branch is cut in cross-section. Because oak wilt is often confused with other disorders, positive identification requires recovery of the causal fungus from the tree.
Avoid wounding or pruning oaks from April through July since sap attracts the beetles that carry the oak wilt fungus. Severing root grafts connecting infected and healthy trees up to 50 feet apart can be used to prevent spread. Fungicide injections are now available to protect healthy trees from the disease. Infected trees can also be treated, but a tree with more than 20% crown loss has little chance for survival.
To test for oak wilt in the clinic, we need a special type of sample. Resources from ISUPIDC as well as the forest service mention the samples need to be refrigerated; otherwise our chances of recovering the pathogen reduce greatly. Pictures would help us greatly to diagnosing trees. Particularly for oak wilt, the look of the leaves give us good clues. Keep in mind your oak may be suffering from other diseases so including some branches with leaves will be helpful to rule out other diseases. Send us digital pictures of the tree, please follow the guidelines to take and send us pictures.
Oak wilt sampling checklist
Click here to download this printable checklist.
Oak wilt symptoms occur early in June and include wilting (droopy looking leaves and limbs), leaf yellowing and/or bronzing, followed by defoliation. When suspecting oak wilt, is important to confirm the diagnosis to take the adequate management measures and protect other healthy oak trees in the area. Collecting the proper sample is key. Oak wilt samples have to be handled with care and mail refrigerated and as quickly as possible as the pathogen responsible for this disease cannot survive hot weather.Follow this checklist when submitting samples for oak wilt testing:
· Collect the sample only when you can either deliver it directly to the Clinic or to send it overnight (never send on a Friday!).
· Look for the symptoms (wilting, yellowing and/or bronzing) and select 3 to 6 living branches from the symptomatic area in a tree.
· Select some leaves attached to twigs, sending symptomatic leaves (bronzing) is a bonus, it gives important diagnostic information. Pictures of the whole tree are appreciated. Follow these guidelines on how to take better pictures http://bit.ly/PIDC-DP.
· Select branches at least 1 in diameter and 6 to 8 inches in length (we need branches, not twigs). With a knife peel the bark and inspect for vascular discoloration. Collect additional branches from that symptomatic limb.
· Refrigerate the sample, have a styrofoam chest with cold packs or ice bags. The pathogen that causes oak wilt cannot survive in warm or hot weather. If the samples are left in the truck or on the house porch for as few as a couple of hours, the chances of recovering the pathogen significantly decrease.
· Place your completed submission form in a bag to prevent it from getting wet.
· Ship the package early in the week via overnight delivery. Do not send on a Friday. Walk-ins are welcome!
· For more information about oak wilt visit this bulletin.