By Janelle Rietz-Kamenar, Master Gardener
Did you know that the greatest concentration of Oak Wilt in Minnesota is found right in our own backyards? Yes, Sherburne, Anoka, Isanti, and northwestern Dakota Counties have the greatest concentration of oak wilt within the state. To learn more about how to prevent this fungus from spreading, you need to know when to prune oak trees. Read this article to help save a precious tree.
Oak Wilt is caused by a fungus called “Ceratocystis fagacearum.” It can kill all species of oak trees, but is especially harmful to the red oak group such as northern red oak and northern pin oak. Once infected, the fungus rapidly transports through the tree’s water conducting system. In red oaks, the leaves begin to turn a bronze to reddish-brown color beginning with the tip and margins and progressing toward the base of the leaf. Usually this starts at the top of the tree or the outer portions of the tree crown and moves down the tree. Moreover, a water-soaked look can also appear on dark green leaves which you might find on the ground around the dying tree. Another sign of oak wilt in red oaks is a dark bluish-gray discoloration when you peel back the bark from an infected branch. Complete oak wilt and leaf loss can occur within 4 weeks of infection in red oaks.
White Oaks can also succumb to oak wilt but usually over a longer period of time – like 2 to 5 years. In white oaks, a single main branch or fork can show signs of wilting leaves during the summer but you may not notice further symptoms until the following year or years. Bronzing and browning of leaves is also seen in white oaks but symptoms can be irregular. If in doubt whether or not your tree has oak wilt, you can consult an arborist.
So, what can you do to prevent the spread of this fungus?
- Prune oaks during November through March when the tree is dormant. Absolutely avoid pruning from April through July.
- Properly dispose of infected logs and avoid moving firewood from known oak wilt infected areas like northwestern parts of Dakota County.
- Stop below-ground spread by cutting root connections of the infected tree so that it doesn’t spread to other oak trees.
- Use of fungicides on high-value, un-infected trees can also help.
For more information consult the University of Minnesota Extension website.