Pruning and Wilt Diseases

Pruning and Wilt DiseasesOak Wilt is a serious disease of oak trees. red oaks are much more susceptible than white and bur oaks.

It is suggested that oak wilt is transmitted from diseased to healthy trees by two methods: 1) through root grafts, and 2) by insects carrying spores to fresh pruning cuts.

By far, the most common method of disease transfer is through root grafts. In Minnesota, root graft transmission is believed to account for 90 to 95% of disease spread. The importance of insect transmission is less clear. Insects such as oak bark beetles and sap beetles are suspected in carrying oak wilt spores from diseased trees to fresh wounds on healthy trees. The wounds give the fungus access to the tree.

Plant pathologists usually recommend that pruning should be avoided on oaks and elms during late spring and early summer. Fresh cuts are considered attractive to insects that may carry fungal spores. Several factors, however, appear to influence the likelihood of infection.

First, properly made pruning cuts (often referred to as collar, target, Shigo, and Neeley cuts) are less likely to become infected than improperly made (flush) cuts. Trees have natural defense mechanisms that protect them from fungal invasion.

Secondly, the insect vectors of oak wilt and Dutch elm diseases do not travel very far from their brood trees. Figures of about 500 feet are reported for sap beetles, the most important vector of oak wilt in our area. Flying distances for the European elm bark beetle, the primary vector of Dutch elm disease, is also listed as about 500 feet. Therefore, if no active oak wilt or Dutch elm disease infestations are nearby, spread by insects is reduced.

As with all aspects of life, risks are unavoidable. We offer the following guidelines with respect to pruning of oaks and elms.

To minimize the risk of infecting pruning cuts by insects, avoid pruning live wood from oaks and elms from April 1 to July 1. This is especially true of red oaks in known oak wilt areas. The safest time to prune is between November 1 and April 1.

For white, bur and swamp white oaks and elms, proper pruning cuts will substantially reduce the likelihood of infection. If trees are pruned during April, May, or June, a wound dressing can be applied to even further reduce the likelihood of infection. Because of the time and cost of the materials, an extra charge will be incurred if the wounds must be painted.

Davey would never intentionally do anything to jeopardize the health of our clients’ trees. Every client has the opportunity to make the final judgement on when to prune his or her trees. We offer this information to help you make this important decision.

To get more advice on how to handle wilt diseases, contact your local certified arborist today!

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