On this blog and through a variety of media, The Pittsburgh Parks Conservancy has had an open discourse about the health of our parks and the battles to keep threats such as Emerald Ash Borer and oak wilt at bay. Earlier this year, these conversations with the community really paid off for our parks. Thanks to an observant park visitor, we were able to act quickly to curb the damaging effects of what we were holding our breaths not to discover: oak wilt in Schenley Park.
After oak wilt was discovered in Pittsburgh city parks in 2009, it didn't take long for it to make its way to three of the four major parks: Frick, Riverview, and Highland. Now, only four years later, we are seeing that this devastating fungus has finally put down roots in Schenley Park.
The oak wilt site in Schenley Park was called in this summer by a park visitor who became suspicious of five or six oaks that seemed to be losing their leaves and generally looking unhealthy. Parks Conservancy staff immediately visited the site, where five to seven oak trees were showing signs of active oak wilt infection. Added to the toll were 25 nearby oaks that had already succumbed to oak wilt and died. In all, we believe 50-55 red oaks will be affected on this site, covering an area of about 1.5 acres. City of Pittsburgh and Parks Conservancy staff have since been fighting to arrest the spread of oak wilt on this site.
As in the other major parks, the fungus will force the removal of infected trees in Schenley Park. The loss of so many trees is unfortunate and will be a noticeable change to the landscape. But there is good news; oak wilt can be managed and stopped (unlike the Emerald Ash Borer). There are also positive outcomes, such as the restoration of a more bio-diverse tree canopy. The site at Schenley was dominated by just two types of trees: Norway maple and red oak. Plans are already being made to replant the site with an enhanced, diverse assemblage of native tree species. These new species will not only be visually appealing, but will also provide renewed habitat for wildlife, help capture storm water, and reduce erosion potential.
Oak wilt will continue to be found from time to time throughout our city parks. But with careful management and mitigation the City of Pittsburgh and the Pittsburgh Parks Conservancy will minimize the impact it has on our parks and, when possible, make our parks even stronger.
You can help prevent oak wilt in your own trees by avoiding trimming between April and October. Wounding trees during the growing season makes it easier for the fungus to spread. If you have any questions about oak wilt, call us at 412-682-7275 or the City Forester at 412-422-6655.
Bryan Dolney, Parks Conservancy Field Ecologist
Notes about Oak Wilt
Oak wilt disease is a fungal vascular disease. Aided by its biology, it readily spreads; once infected, oak trees will begin to lose leaves in just a few weeks. Some trees can be killed within a month of first infection. Oak wilt is so rapid because of how efficiently the fungus fills the water conducting vessels (xylem) of the tree. This not only spreads the fungus, but also blocks the tree from attaining water.
Oak wilt spreads underground by root grafts, which is noticeable while viewing stands of oak trees growing in groups close together. Red oaks in particular form these root grafts, making them the primary target for oak wilt. Roots of adjacent trees grow together as they share nutrients, water, and sometimes oak wilt. When one tree becomes infected and spreads, infecting adjacent oaks, we call this an infection center. This is how oak wilt is spread from one tree to the next, eventually infecting the entire stand.
Infection centers can move long distances via picnic beetles from mats of fungus called pressure pads. Once a tree has been killed by oak wilt, these pressure pads form under the bark and crack open. This often happens the spring after a tree has been killed by oak wilt (about 9-10 months after the tree has died). These pressure pads produce a sweet odor that attracts picnic beetles and sap feeding beetles that feed on it. Once the beetles crawl on these pressure pads they pick up the oak wilt fungal spores and carry it to healthy oak trees.
Signs of infection are often noted in July. A tree infected with oak wilt is characterized by:
- Leaves at top of tree wilt and turn brown along tips and margins.
- Leaves begin to fall while they are still green. This can be rather conspicuous in the summer when tree should have full canopy of leaves, not a Fall canopy.
- On the forest floor one can see a mixture of brown, green, and partially green leaves amassed at base of tree.