Linda Williams, forest health specialist, Woodruff, Linda.Williams@wisconsin.gov, 920-360-0665 

Trees in the red oak group (those with points on their leaves) that were infected with the oak wilt fungus this spring will rapidly drop their leaves from July to September and be dead by fall. This wilting and dropping of the green leaves happens quickly. Once it starts, the tree will drop most of its leaves within just a few weeks.

Tree dying from oak wilt with rapidly dropping leaves. The tree was injured in May, attracting the beetles that help spread the fungus, and the tree was dead by the end of the year. Photo was taken in August.

Red oak leaf from infected tree. Leaves are often green at the base, with the outer portions of the leaf appearing water-soaked or brownish.

Oak wilt is a non-curable, fungal disease specific to oaks. Once the fungus infects a tree it spreads outward from the roots of the infected tree into the roots of neighboring oaks, eventually infecting and killing them. In this way pockets of dead oak will expand outward as more oaks die each year. For more information on oak wilt biology, prevention and control check out the WI DNR’s oak wilt page.

Firewood from oak wilt-killed trees remains infectious for one full year (12 months) after the tree has died. Moving infected firewood during that time can introduce the fungus to new areas. You should let oak wilt-infected firewood age for a minimum of 12 months before moving or buy firewood near where you will use it or that is certified to be free of insects and diseases. For more information on firewood, including a list of certified vendors, visit the DNR firewood page.

Oak wilt is considered uncommon in many northern counties in Wisconsin.

There are many areas of northern Wisconsin where oak wilt is still uncommon. If you think you have found oak wilt and you are in one of the counties NOT colored red in the map, please contact the DNR forest health specialist for your area so it can be verified. You can also send a sample to the UW Extension Plant Disease Diagnostic Clinic for verification. It is important to track locations of this disease so that the map can be updated and appropriate management can be taken to prevent further spread and mortality.