Oak Wilt Pruning Precautions

Oak wilt disease (OWD) is caused by a fungus that plugs up a tree’s "plumbing" (a tissue called xylem) and keeps it from moving water from the roots to the top of the tree. This causes the tree to die back rapidly from the crown downwards. It has been a problem primarily in western Pennsylvania for many years, but foresters are now reporting that they are seeing more cases in the eastern part of the state. All species of oaks can be infected with OWD (although it will kill some species faster than others), and once a tree is infected, there is no treatment.

Sealing a fresh cut on a tree with pruning paint is usually not recommended, as it interferes with the natural healing process. However, if an oak tree must be pruned during the warmer months, sealing the wound is important to protect the area from the insects that spread oak wilt disease—primarily sap beetles and oak bark beetles. These insects are highly attracted to fresh cuts and wounds on oak trees and in some cases, may arrive just 15–20 minutes after a tree has been damaged. If a healthy oak is pruned in warm months while these insects are active, they may come from an infected tree and spread the disease's fungal spores to the cuts in the healthy tree.

If you must trim an oak tree between April through November, perhaps because of storm damage, use pruning paint or even latex paint to seal the fresh cut as quickly as possible. In addition, it is always a good idea to sanitize pruning materials in between cuts. See the article Disinfecting Tools, Equipment, Pots, Flats, and Benches about disinfecting tools and equipment by Penn State professor emeritus Gary W. Moorman.

To summarize best practices regarding oak wilt disease and pruning:

• Plan oak tree pruning for winter—between December and March.
• If oak trees must be pruned in warmer months, immediately cover the fresh cuts with pruning paint (or latex paint).
• Do not use pruning paint on fresh cuts or wounds on trees in any other situation.

For more information about OWD symptoms, what to do if your oak tree is infected, and safe pruning practices, check out these articles from Penn State Extension Forestry and Horticulture Educators and Michigan State University Extension:

• Stay Alert for Oak Wilt!
• Oak Wilt in Eastern Forests
• Stop Pruning Oak Trees Now to Avoid Oak Wilt