The Forest Service recommends not pruning oak trees during the period of April through July. (Photo courtesy University of Illinois Extension)

Oak trees are majestic, but some are in danger from a disease. One of the best ways to protect oak trees is to prune them 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 beetles that may have visited nearby diseased trees. If that is the case, they bring the oak wilt fungus to your tree.

I lost a black 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, I’ve seen several oaks die in my area.

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 side branches in late spring. 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.

Oak wilt is particularly threatening because there is no complete control or cure once the fungus infects. The fungus infects through fresh wounds through the beetle, 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. There you will also find an Oak Problems fact sheet.

Oak problems are the topic of an upcoming University of Illinois Extension Four Seasons Garden webinar series. Join Diane Plewa, diagnostician at the University of Illinois Plant Clinic, as she discusses some of the most common problems seen on oaks across Illinois.

Topics will include serious diseases such as oak wilt and bur oak blight, and less serious (but can look concerning) problems like tubakia leaf spot and galls. A few common insects that cause problems on oaks will also be discussed.

For more information visit the website at http://web.extension.illinois.edu/fmpt or call (309) 543-3308.

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

— Mid-summer is not time for pruning oak trees —