Oak wilt is an important disease to be on the lookout for in New England. This is especially true for Connecticut because it has been confirmed in three locations in our neighbor to the west and south, New York. The disease is important because it kills trees in the most susceptible red oak group (northern red, black and pin oaks in our area) within weeks or months of infection. White oaks are more moderately susceptible and are generally not killed for a few to several years. Early detection of this disease in any new location is critical to attempting to eradicate the problem before it becomes widespread. The causal fungus is Ceratocystis fagacearum.
Oak wilt was first confirmed in the U.S. in Wisconsin in 1944. Since that time it has become widespread in the upper Midwest and Texas. In the northeast, it has been confirmed in NY and western PA. Just this year, 2016, two new locations were confirmed in NY by the Cornell University Plant Disease Diagnostic Clinic: Central Islip on Long Island and Canandaigua in the Finger Lakes region. The origin of this pathogen is not known.
So what does oak wilt look like? In the most susceptible red oaks, symptoms include wilt, browning of the tips and edges of the leaves beginning in the upper part of the tree, twig and branch dieback, and browning of the outer sapwood. The fungus kills the tree by growing in the xylem vessels where water and nutrients are translocated from the roots to the crown. The fungal invasion results in the production of gummy blockages that prevent translocation.
Photo credit: Michigan State University
The disease is spread from one tree to another in two ways, via root grafts and sap beetles. A root graft is a ‘fusing’ of roots of neighboring trees that allows for movement between them of water, nutrients, and, unfortunately, the fungus. So trees growing in close proximity in forests, landscapes or along streets can share this disease readily. Sap beetles are attracted to fungal mats that form under the bark of dead and dying trees. Bark cracks form as the fungal mats enlarge. Spores of the fungus and an odor attractive to the beetles are both produced on the mats. Beetles come to feed there and sticky spores adhere to their bodies. The beetles are strongly attracted to fresh wounds on trees (ie pruning or other wounds) and when they move to those sites after picking up spores, the disease is spread to a new tree. The spore can only invade a tree via a wound. Long distance spread can occur when infected logs are moved to new areas.
If you’re not sure how to tell red oaks from white oaks, here’s the most visible difference: Oaks in the red oak group have leaves with pointed lobe tips and those in the white oak group have rounded tips as shown below.
Browning and wilt can also be caused by drought stress. If it’s oak wilt, remember that the browning will begin in the top of the canopy. Red oaks will die within months; not usually the case with drought stress or even other pest and disease problems.
What should be done if you suspect oak wilt on trees in CT? Contact your state’s diagnostic lab as soon as possible for information on sample collection and submission. You may send images via email for a quick look and to see if other causes of the symptoms can be ruled out. The UConn Plant Diagnostic Lab can be reached at 860-486-6740 or by sending an email to email@example.com. The diagnostic lab website is www.plant.lab.uconn.edu. Your vigilance will help protect oak trees in CT and throughout New England!