Oak Wilt Disease and How to Prevent It

By Gerry Donaldson

Learn more about oak wilt disease, its symptoms, and how to prevent its spread.

A close up of oak wilt staining
A close up of oak wilt staining

In the last two issues of the Conifer Quarterly, we have discussed some invasive species that are firmly established in wide-spread areas of the US, such as hemlock woolly adelgid, Adelges tsugae, and emerald ash borer, Agrilus planipennis; as well as some that are not well established, such as Asian longhorned beetle, Anoplophora glabripennis.

This time I would like to discuss an invasive species, oak wilt, which was first described in North America in the 1940’s.

An Infectious Disease in Red Oaks and White Oaks

Oak wilt (Ceratocystis fagacearum, also known as Bretziella fagacearum) was first reported in red oak in Wisconsin in 1944. It has now spread to at least 24 states. The red oak group is most severely affected by oak wilt, but oak wilt has been found in 16 native oak species.

Through inoculations, it has been learned that over 35 native and exotic oak species are susceptible. Other susceptible species include both American and European chestnuts, Castanea ssp., chinkapin, Chrysolepis spp., tanoaks, Lithocarpus ssp. and some cultivars of apple, Malus.

Members of the white oak group are susceptible to infection, but appear to be more resistant to the effects of oak wilt, although sporadic limb death can be common. There is also concern that white oaks can appear to recover yet remain infected and serve as a reservoir of the disease.

Dead oak leaves from oak wilt disease
Dead oak leaves from oak wilt disease

Symptoms of Oak Wilt Disease

Oak wilt fungus grows quickly within infected red oaks and plugs the xylem tissues, causing death of the tree in as little as two months. In some case, death can take up to a year or more. First symptoms in red oak are wilting leaves that turn dull green or brown and curl around the midrib, often mimicking drought stress.

Leaves will drop from the tree, usually starting with branch tips. Even what appear to be healthy green leaves can be shed. Dark streaking under the bark is often found in red oaks that have recently shown symptoms where the fungus has plugged the xylem tissues. If collecting samples for submission to a diagnostic lab, freshly cut twigs or small branches 15 to 20 centimeters long (approx. 6 to 8 inches) should be placed in zipper style plastic bags and kept cool until examined by a diagnostician.

Oak Wilt Disease Spread by Spores and Beetles

The oak wilt fungus, Ceratocystis fagacerum, can overwinter under the bark of living trees and as fungus mats under the bark on dead trees. These fungus mats can grow and cause the bark to split and emit an odor sometimes described as smelling like apple cider.

A variety of beetles feeds on the sap and/or fungus mats, picking up spores that then get spread to other trees during feeding or egg-laying. From early spring to mid-July, the fungal spores are spread by beetles from infected trees to other trees, especially trees that have been wounded or pruned.

Do not prune once spring temperatures reach 50 degrees Fahrenheit, as a few fifty degree days can get beetles moving and cause the fungus in infected trees to form fruiting structures, leading to the spread. Pruning can resume in mid to late July.

In the event a tree is wounded through pruning, equipment damage, another tree falling and damaging bark, or climbing with spurs, immediately paint the damaged area with tree-wound paint or latex paint. That will help keep beetles from feeding on the sap and introducing spores to the damaged tree.

The fungal mat of Ceratocystis fagacearum, a fungal pathogen of oak wilt disease
The fungal mat of Ceratocystis fagacearum, a fungal pathogen of oak wilt disease

Oak Wilt and Infected Wood

Oak wilt can also be spread with movement of firewood from oak-wilt killed trees to new areas. Don’t move infested wood. If oak wilt infected trees are cut and kept on site for firewood, the firewood should be completely covered with a tarp with no holes in order to prevent beetles from feeding on the wood and picking up spores.

Oak wilt can also be spread tree to tree through root grafting. The only currently recognized way to prevent the spread through root grafts is to root-prune the infected tree(s), severing all roots that can connect to surrounding oaks. Root pruning is typically done using a vibratory plow pulled behind a large tractor.

Root pruning should be done in the dormant season and requires a blade that penetrates the ground five feet deep. Once again, do not move firewood.

Click here to read more on how to prevent fungal diseases in conifers, and here to read more about other fungal diseases like cedar apple rust and white pine blister rust.

This article was originally published in the Summer 2018 issue of Conifer Quarterly. Join the American Conifer Society to access our extensive library of conifer-related articles and connect to a nationwide group of plant lovers! Become a member for only $40 a year and get discounts with our growing list of participating nurseries in our Nursery Discount Program.