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Prune during the dormant season to protect oak trees
Prevent the spread of oak wilt, a fatal disease to oak trees, by pruning now
March 3, 2020
MANHATTAN, Kan. – Oak wilt, a fatal disease spread by a fungus, has been found in a few sites in the eastern third of Kansas over the last decade. While the spread is often slow and sporadic, Kansans should be mindful of the fungus to prevent the spread of the fatal disease.
If infected, rapid death can come to such red oaks as northern red, shumard, blackjack and pin oaks.
Ryan Armbrust, the forest health specialist with the Kansas Forest Service, said one way to prevent the spread of the fatal disease is to only prune trees in the winter. Winter pruning also makes it easier to observe the framework of limbs that make up the tree’s canopy.
“Pruning before the growing season begins leaves only a short time before the vigorous growth of spring begins, sealing off the wound left from pruning,” Armbrust said. “Reducing the time the tree has an open wound from pruning reduces the chance that the fungus will find its way into the tree.”
If an oak tree is infected with oak wilt, the most obvious symptom of this disease is a bronzing and wilting of leaves in late spring and early summer. The tips and outer halves of the leaves will often appear scorched, with a distinct line between healthy and affected leaf tissue.
“There are many reasons for green leaves to bronze or brown, including the onset of autumn and stress brought on by drought or heat,” Armbrust said. “However, leaves that are yellow or brown in midsummer could be a cause for concern.”
He added that several diseases can affect the leaves of oak trees, making it hard to distinguish the disease, so confirmation by a plant pathology lab is recommended before taking any action.
If you suspect oak wilt in your trees, collect a sample of a wilting branch at least six inches long and half an inch thick. Keep the sample away from heat, and take it to the local extension office, or submit it to the K-State plant pathology diagnostic lab for positive confirmation of the fungus before considering treatment.
Armbrust said if oak wilt appears in a stand of trees, the disease can quickly spread via root grafts. Trenching to a depth of three feet can sever roots and may limit the spread between trees. Treating uninfected trees with a fungicide (such as propiconazole) may help protect these trees, but there are few options for treatment of infected trees: removal and destruction of those trees is recommended.
For the long-term health of oaks and to reduce the threat of oak wilt and other pests and diseases, avoid moving firewood, especially oak wood from areas that may have the disease.
Armbrust suggests taking a walk outside to scout trees and plan for pruning. For those considering hiring an arborist to do the pruning for you, ask if they’ve been certified by the International Society of Arboriculture or the Kansas Arborist Association. A certified arborist is trained in the best practices of the industry.