Although you might think we already have our fair share of forest health issues to deal with here, it’s time for Vermont to start thinking about yet another forest disease on the horizon – oak wilt. Oak wilt affects the vascular systems of oak trees and is caused by the fungus Bretziella fagacearum (formerly Ceratocystis fagacearum). Although there is still debate on whether this fungus occurs naturally here in North America or has been introduced from elsewhere, one thing not up for debate is that this fungus is a tree killer.
Many of us are already familiar with Dutch Elm Disease, and oak wilt affects trees in a similar manner. As the oak wilt fungus invades the xylem tissue of an infected tree, the tree struggles to transport water from the roots to the canopy. This causes leaves to wilt and be prematurely dropped from trees in spring or summer. Without adequate water or a large enough leaf surface area for photosynthesis, affected trees are essentially starved and killed.
Although some oak wilt symptoms are similar between groups, trees in the red oak group tend to be much more susceptible than those in the white oak group. Wilting leaves on trees in both groups usually become discolored from the outer edges inward (Fig. 1) and are usually dropped from the tree, although it is not uncommon for wilting trees to drop entirely green leaves as well. Symptoms usually progress from the farthest extremities, spreading from the top of the canopy downward and from the outermost parts of branches inwards. Again, the most important aspect of the leaf drop is the timing – rapid leaf drop in the middle of the growing season is cause for concern.
The disease progresses much faster in red oaks, often overtaking them within one growing season. The same anatomical features of white oak that make it the premier wood for building watertight barrels are the same features that help it defend against rapid invasion by oak wilt. Therefore, it can take several years for white oaks to succumb to the disease and some trees may only experience minimal loss each year.
Once an infection center is established, oak wilt can spread via either an underground or above ground pathway. Underground, the disease is transmitted through interconnected root grafts, a common phenomenon in oaks growing nearby one another. Above ground, sap beetles (Nitidulidae) are the ones responsible for transporting spores from infected trees. If they happen to feed on the sap issuing from wounds on other oak trees, spores can be inadvertently introduced into wounds and result in newly infected trees.
The fungus’s strategy for attracting beetles to carry spores is nothing short of fascinating. Soon after a red oak has succumbed to the disease, a fungal spore mat may form beneath the bark. Part of this consists of a pressure pad, a specialized structure that exerts enough pressure as it grows that it actually splits the bark of the tree from within, thus exposing the spore mat to the outside world. The spore mat itself emits a strong odor that rings the olfactory dinner bell for foraging sap beetles. Once sap beetles have crawled over the fungal mat and become spore-covered, they continue on their way in search of other wounded trees, some of which will inevitably be oaks. Fungal spore mats rarely is ever form on trees in the white oak group.
Although natural wounds on trees are hard to avoid, we can certainly avoid aiding the spread of oak wilt by carefully timing our pruning activities involving oak trees. By not pruning during sap beetle flight season (generally April through July), we can reduce the number of fresh wounds on oaks and therefore the chance of exposure to fungal spores. The alternative to prevention is the treatment of infection centers, which is only feasible in certain situations but invariably can become complicated and expensive. This involves removing obviously infected trees, removing additional healthy-looking trees to create a buffer zone around the known infected trees, and then using heavy machinery to create a trench around the entire area to disrupt the underground root grafts that would allow for continues spread of oak wilt underground. Needless to say, prevention is best.
Although oak wilt has caused a tremendous amount of damage in the Midwest, it has yet to take hold here in the Northeast. The nearest known locations for oak wilt right now are three locations in New York, the closest being Glenville, NY. This is only about 50 miles from the Vermont border near Bennington. Though we do not suspect oak wilt anywhere in Vermont currently, it is never too early to be vigilant. For more information, check out the oak wilt page here. To report a suspicious tree, visit the report it page.
Article Credit: Michael Parisio, VT Deptartment of Forests, Parks, and Recreation