Don't Prune Oaks Now?

Oak trees are majestic, but some are in danger of 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 oak trees 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. A truck injured the tree's branches, 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, oak trees 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 the end of 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 definite diagnosis is necessary in many cases.

The University of Illinois Plant Clinic can do that diagnosis. Go to their website at for information on how to submit a sample. There you will also find an Oak Problems fact sheet.

Oak problems are the topic of a University of Illinois Extension Four Seasons Garden webinar by Diane Plewa, Diagnostician at the University of Illinois Plant Clinic. It is available as a free taped webinar on the University of Illinois Extension Horticulture YouTube channel.