Oak leaves from an oak wilt infected tree. The outer portions of the leaf will be brown or have a water-soaked appearance. Part of the leaf remains green even though the leaf has dropped off the tree.

Trees that were infected with the oak wilt fungus this spring, (whether from overland spread by beetles or underground spread by connected root systems) are beginning to drop their leaves. Leaves can drop anytime between July and September. This wilting and dropping of the leaves happens fairly quickly, and trees can go from looking nice and healthy to having lost most of their leaves within just a few weeks. This year I saw my first wilting oaks on June 28, although in areas further south the leaf drop may have begun earlier. Oak wilt is a non-curable, fungal disease specific to oaks. Once the fungus infects a tree it will begin to spread outward from the roots of the infected tree through grafted roots and into the roots of neighboring oaks, eventually killing the neighboring oaks. In this way pockets of dead oak will be created as each year more oaks die. For more information on oak wilt biology, prevention, and control check out the WI DNR’s oak wilt page. 

Firewood from trees that have died from oak wilt will remain infectious for 1 full year (12 months) after the tree has died. There are many areas of northern Wisconsin where oak wilt is not common. Please do not move firewood long distances because you could move oak wilt into a new area.

Many northern counties don’t have oak wilt or have only a few known infections. This map shows townships in the north where oak wilt has been identified. In the red counties oak wilt is considered to be scattered throughout the county, although it will not be found in every stand.

Written by: Linda Williams, forest health specialist, Woodruff, (Linda.Williams@wisconsin.gov), 715-356-5211 x232.