How to stop the spread of oak wilt disease this spring

Oak wilt symptoms on oak leaves
Oak leaves showing oak wilt symptoms. Photo by Monique Sakalidis, Michigan State University.

Earlier this month I attended a Woodland Wednesday program called “Leaves are for chumps: Winter Tree Identification,” hosted by Lake Soil and Water Conservation District at Lantern Court Estate at Red Oak Camp. The program was detailed, informative and interesting. I enjoyed the endless tree identification tricks, and I appreciated the conservation tips.

A cautionary tale about oak wilt disease that the arborist on site told stuck with me more than anything else. In Ohio, I’m not sure any tree seems grander than the oak. Maybe that’s why it was hard for me to imagine an entire stand being wiped out within a matter of weeks following the onset of oak wilt disease.

Spring never seemed like a dangerous season for trees with so many turning green and flowering. The season seems synonymous with the beginning of life rather than the onset of death. However, for oak trees in areas with oak wilt disease, it can be a dangerous time. The warm, wet conditions of spring provide optimal conditions for the fungus that causes oak wilt and the beetles that spread it to thrive.

Fortunately, there are steps you can take to identify, prevent and treat oak wilt disease to stop its spread and save some oak trees.

Oak wilt disease

Oak wilt is an aggressive disease caused by a fungus that moves from tree to tree underground through interconnected root systems and via insects. It is considered one of the most serious tree diseases in the eastern United States. It kills thousands of oak trees in forests, woodlots and home landscapes every year.

Red oaks, white oaks and live oaks are all affected by oak wilt to different degrees. Red oaks, which include northern pin oak, northern red oak and Texas red oak, are the most susceptible to oak wilt and die the quickest after they become infected. Red oaks can die within three weeks of being infected, and their recovery from oak wilt disease is rare. White oaks, which include bur oak and white oak trees, are a little more resistant. They can live for several years after being infected with oak wilt and are more responsive to treatments. Texas live oaks are moderately susceptible to oak wilt but are considered an important host because of their tendency to form large, root-connected clones.

Oak wilt disease spreads via interconnected roots and through insect interactions. 

Most new infestations occur in oak stands where the roots of an infected tree are grafted to the roots of healthy oaks. When the infected tree dies, the fungus spreads through the connected root systems, killing additional trees. Roots are not typically interconnected between different stands of oak species in the northern United States. However, roots can become interconnected in mixed stands of Texas live oaks and red oaks in the South.

Insects spread oak wilt disease by carrying fungus spores from an infected tree to a healthy tree with a fresh wound. This most frequently occurs in the spring because ideal temperature and moisture conditions allow oak wilt fungal mats to grow and produce spores. The fungal mats split the bark of infected trees and emit a fruity odor, attracting insects. The insects then spread the disease when they carry spores to a tree that has been pruned or damaged in some way.

Preventing oak wilt disease

Avoid planting monocultures. Instead of planting a bunch of the same type of trees in your landscape, choose a variety. Increasing the biodiversity in your landscape can help prevent the spread of disease.

Don’t prune, trim or cut down any oak trees this spring. The catchphrase that was used at the program I attended was: “April to November do not dismember.” Leaving open wounds on your trees when oak wilt fungus is producing spores leaves them vulnerable to infestation via insects.

Seal wounds with tree wound dressing. When you absolutely have to prune storm-damaged trees or remove trees for construction, make sure wounds and stumps are immediately sealed with tree wound dressing to avoid attracting sap beetles that may be carrying oak wilt fungus.

Recognize the symptoms of oak wilt early. Trees infected with oak wilt disease will typically start showing symptoms in June and July. Their leaves will start browning and falling off, typically around the crown of the tree first. By recognizing the oak wilt sooner, you can contact an arborist to confirm your suspicions and come up with a treatment plan to prevent further spread.

Treating oak wilt disease

If you suspect a tree or trees in your landscape, woodlot or forest have been infected with oak wilt disease, contact an arborist to confirm and formulate a treatment plan. The treatment plan will be dependent on the type of oak that’s infected, the number of oaks around it and how much the tree has been impacted.

Don’t make the mistake of thinking that simply removing the affected tree will keep it from spreading. If you remove an infected tree that’s interconnected with other oak trees via its root system, you could speed up the spread of the disease to the next tree. The connection between the grafted roots needs to be disconnected underground before the tree is removed to prevent continued spread. 

Additionally, the fungus can live on infected trees that have been cut down for a while after they’ve been cut down. Stumps should be sealed with tree wound dressing and any wood that is kept from a tree that was infected with oak wilt disease shouldn’t be moved from the site and needs to be tarped with the edges of the tarp completely buried to keep beetles from coming into contact with it. Diseased logs and branches will only produce spores once, so trees and logs seasoned for over a year are safe to uncover. Wood from diseased trees can also be burned, buried or chipped to prevent the spread of oak wilt disease.



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