Prune Oaks in Winter to Avoid Oak Wilt Disease

What is Oak Wilt Disease, and How is it Spread?

Oak wilt disease (OWD) is caused by a fungus (Bretziella fagacearum) that plugs up a tree’s "plumbing" (a tissue called xylem) and keeps it from moving water from the roots to the branches and to the top of the tree. This causes the tree to die back rapidly from the crown downwards. OWD can spread through the roots of oaks growing in close proximity. Above ground, OWD is spread by sap beetles and oak bark beetles. This insect threat is more likely to be of concern to a residential gardener. 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.

Where in Pennsylvania is Oak Wilt Disease Found?

Oak wilt disease has been a problem in western Pennsylvania for many years. Recent detections of OWD in eastern Pennsylvania counties have proved to be false positives—confirmed cases of OWD remain west of the Susquehanna River. The northwest Pennsylvania counties of Crawford, Forest, McKean, Mercer, and Warren appear to have been spared (thus far). Because OWD has been found in so much of Pennsylvania, all state residents should watch for it and follow the pruning precautions described below. To see where OWD and other pests and diseases have been detected, see Pennsylvania Forest Health Highlights.

How Can Delayed Pruning Help?

Beetles that spread OWD are highly attracted to fresh cuts and wounds on oak trees, and in some cases, can 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 live in the affected areas and need to prune an oak tree, December through March is the best time to do so, since the transmitting insects are not active during cold periods. If you must do emergency pruning or have storm damage in warm months, it is best to cover those wounds with a tree wound dressing as quickly as possible. (Note: In all other cases of tree pruning, wound dressing is not needed or advisable.)

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

Stay Alert for Oak Wilt!

Oak Wilt in Eastern Forests