Oak Wilt Disease – Fall 2020

I promise that if I ever find anything positive to report on I will make every effort to send along asap. This, however, is not that. We have a new disease to worry about, -at least new to us. Several club members with Master Gardener certificates have discussed this and think it is both appropriate and timely to sound the alarm.

The disease is called “Oak Wilt Disease”. For the horticultural wonks it is caused by a fungus known as Bretziella fagacearum. This fungus proliferates in the xylem of the tree, the layer of specialized cells just below the bark which facilitates the movement of water both up from the roots and down from the leaves of the tree. Once afflicted by the disease, the tree cannot hydrate sufficiently and dies -sometimes very quickly.

Oak Wilt is not new. It was first discovered in Wisconsin in 1944. What is new is that it is now being found in New York area, especially Long Island and it is anticipated to be just a matter of time before it visits us.

The fungus which causes this disease is carried by Nitidulid beetles (both black and brown, as shown here) which pick up the fungus from an affected tree and deposit it on an unaffected tree.  Like all such creatures they have the ability to fly from one tree to the next so infestation in nearby Oaks is just a matter of time.

Oak Wilt affects all oak trees but it seems to prefer Red Oaks (Quercus rubra), the ones with the “pointed” lobes on the leaves. When the tree is infected you will see rapid leaf discoloration beginning in the early summer with browning along the tips and margins of the leaf.

The disease progresses from the top of the tree downward so that during the course of the disease the leaves at the top of the tree will turn brown and fall off while the lower leaves remain green -at least until the disease affects that portion of the tree. Oak Wilt often produces complete defoliation within 3 to 4 weeks of the onset of symptoms. Note: other diseases can also progress in this manner.

IMPORTANTthe fungus introduced to the new tree infects it through “wounds” on the tree. This means that if the uninfected oak does not have any wounds (i.e. openings in the bark) it may in fact be spared from infestation. Please also keep in mind that when you prune a tree you create a wound. So if you prune the tree in the spring and early summer when these beetles you will have exposed that tree to infestation. Because of this the window for pruning Oaks, especially Red Oaks has been reduced to winter months (December through February).  If for any reason you must prune outside that window, be sure to paint over any breach of the bark with horticultural paint or even latex house paint in an effort to seal the Xylem from the fungus. [see resource 2]

When afflicted with Oak Wilt, the tree will ultimately die, sometimes as quickly as 3 weeks from onset. There is no cure for this disease that would salvage an afflicted tree. The tree must be taken down under extreme caution to prevent the fungus from spreading to nearby trees. As if that weren’t onerous enough, after removing the tree it is also necessary to surround the trunk with a 6’ trench. Many trees, including Oaks form “root grafts”, which are a natural anastomoses or joining of the roots of two compatible plants. Because of this phenomenon, it is necessary to trench around the tree in order to mechanically interdict the spread of the fungus through the root system.

If you see an Oak with possible symptoms, don’t touch it. The disease is a fungus and can be spread easily by contact with your gardening tools. Instead, contact:

Robert E. Marra

Department of Plant Pathology & Ecology

The Connecticut Agricultural Experiment Station

123 Huntington Street

P.O. Box 1106

New Haven, CT 06504-1106

Voice: (203) 974-8508 Fax: (203) 974-8502

E-mail: Robert.Marra@ct.gov


Do not try to send any cuttings or other samples. Doing so will only spread the disease. Instead,  he will have a crew come to you to obtain samples according to state guidelines.

Oak trees are a favorite source of food and habitat for our local fauna. Losing them would create yet another headache for our environment. Please be on the look-out for this disease and report it as soon as you see it. There’s no penalty for being wrong about whether the symptoms you see are Oak Wilt; there is a significant penalty, however, for being complacent.

Kevin M. Tepas
August 20, 2020