Oak wilt discovered in St. Croix State Park

From the Minnesota DNR

Oak wilt, an aggressive disease that affects many species of oak, has been discovered throughout St. Croix State Park in Pine County, according to the Minnesota Department of Natural Resources.

Several wilting red oaks were found by DNR staff recently. The University of Minnesota Plant Disease Diagnostic Lab confirmed the samples from two wilting trees in the park contained the oak wilt fungus.

“This is the most northern known oak wilt infestation in Pine County,” said Ryan Blaedow, DNR forest health specialist. “The oak wilt fungus was likely introduced to the park immediately following the blowdown event that occurred in July 2011.”

Oak wilt kills thousands of oak trees each year in forests, wood lots and home landscapes in the eastern United States. Trees in the red oak group are more susceptible to oak wilt than trees in the white oak group. In Minnesota, black oak, northern pin oak and northern red oak belong to the red oak group, while bur oak, swamp white oak and white oak belong to the white oak group.

Oak wilt is caused by a fungus that invades the water-conducting vessels of oak trees. Oak trees respond to the invasion by plugging their vessels, which causes leaves to wilt from the top of the crown downward. Infected trees in the red oak group often shed their leaves and start to die within a few weeks of symptom onset, while trees in the white oak group usually die slowly over one to many years.

According to Blaedow, oak wilt spreads from infested trees to healthy trees in two ways:

• The fungus moves through connected root systems. The roots of trees can graft to roots of other trees of the same species, forming a continuous underground network. When an infested tree dies, the fungus spreads through the connected root system and invades healthy trees. This is the most common way oak wilt spreads.

• Insects carry spores of the fungus to healthy trees. Several different species of sap-feeding beetles feed on oak wilt spore mats that form on trees killed by the fungus the previous year. As the beetles feed, oak wilt spores attach to the surface of their bodies. When a beetle that is covered in oak wilt spores feeds on sap oozing from wounds on healthy trees, spores enter the tree at the wounded site and a new oak wilt infestation is initiated.

“The key to keep oak wilt from spreading is to prevent the development of oak wilt spore mats on diseased trees and by preventing the transfer of spores to healthy trees,” said Blaedow.

To prevent the spread of oak wilt, people should follow these management guidelines:

• Trim oak trees during winter, if possible; it is best to remove diseased trees in the winter to avoid wounding neighboring trees.

• Remove diseased trees before the following spring to prevent spore mat development.

• Avoid wounding oak trees between April and July when sap beetles actively feed.

• Treat wounds immediately with pruning paint if trees are wounded during the spring and summer.

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