Don't Prune Oaks Now?

This article was originally published on April 23, 2012 and expired on July 1, 2012. It is provided here for archival purposes and may contain dated information.

Oak trees are majestic, but some are in danger from a disease. Rhonda Ferree, University of Illinois Extension educator, horticulture, says that one of the best ways to protect oak trees is to prune at the proper time.

You have probably heard that it is not wise to prune oaks during the active growing season. The actual act of pruning does not harm the tree. The problem involves what you will attract to the tree—insects that may carry the oak wilt fungus.

The Forest Service recommends that we halt any pruning of oak trees during April, May, and June. Others extend that ban through July. Fresh cuts in those months produce sap that attracts sap-feeding nitidulid beetles that may have visited sporulating mats of the oak wilt fungus on diseased trees. If that is the case, they bring the oak wilt fungus to your tree. If you are certain there is no oak wilt disease in your area, you do not have to follow this guideline.

Ferree says that she lost a red oak to oak wilt several years ago. "It died very quickly once it was infected. The tree's branches were injured from a truck and we didn't even notice it until the top started to brown and die." Since then, Rhonda says that she has seen several oaks die in her neighborhood.

Become familiar with oak wilt symptoms so that you can recognize it in your area.

Leaf symptoms vary depending on the oak species involved. Generally oaks in the red or black oak group (pointed leaf lobes) develop discolored and wilted leaves at the top of the tree or at the tips of the lateral branches in late spring and early summer. The leaves curl slightly and turn a dull pale green, bronze, or tan, starting at the margins. Usually by late summer, an infected tree has dropped all its leaves. In some years, we have seen red oaks progress from scorched foliage to total defoliation in as little as three weeks.

The white and bur oak group (rounded leaf lobes) generally shows symptoms on scattered branches of the crown. The disease is often confused with general dieback and decline. Leaves on infected white oaks become light brown or straw-colored from the leaf tip toward the base. The leaves curl and remain attached to the branches. This tree group may die in one season but is much more likely to survive for many years with dieback and stressed appearance.

The most dependable symptom, present in all affected tree species, is vascular staining. Go to http://urbanext.illinois.edu/focus/oakwilt.cfm for more information.

Oak wilt is particularly threatening because there is no complete control or cure once the fungus infects. The fungus infects through fresh wounds by a beetle vector, and it can spread by root grafts between trees. The infected tree cannot be saved; but you may be able to save surrounding trees, so a positive diagnosis is important in many cases. The University of Illinois Plant Clinic can do that diagnosis. Go to their website at http://web.extension.illinois.edu/plantclinic/ for information on how to submit a sample.

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Adapted from article by Nancy Pataky, Retired Director, University of Illinois Plant Clinic

Source: Rhonda J. Ferree, Extension Educator, Horticulture, ferreer@illinois.edu

Pull date: July 1, 2012