Oaks can be categorized into two groups; the red oak group and
the white oak group. The red oak group (black, northern red, northern pin and others with pointed leaf margins)
is more susceptible. When oak wilt affects the red oak group, the entire tree may wilt, or lose all of its leaves,
in just a couple weeks. Most often this occurs from late June throughout July. Many times diseased trees occur
in groups. The white oak group (white, bur, swamp white and others with rounded leaf margins), is less susceptible.
When oak wilt affects the white oak group, trees usually die slowly, many times only a branch or two may die each
year. Most often it takes a few years before the entire tree succumbs to the disease. In both groups, the tree
typically dies from the top down, and leaves turn brown from the tips to the base.
The disease can be spread in one of two ways; by root grafts and by insect vectors. The roots of individual oak
trees can graft to each other, and if a diseased tree’s roots are grafted to a healthy tree, most times, especially
in the red oak group, the disease is taken up by the healthy tree. The disease can keep spreading this way from
oak to oak. There are beetles that are attracted to fungal mats that some diseased trees produce. When the beetles
feed on the sweet smelling mats, fungal spores stick to the beetles bodies. If the beetle then moves onto a healthy,
but freshly wounded oak tree, the disease-causing spores are introduced to that tree.
The best control measures against the disease is prevention. Don’t injure, prune, or cut oaks from April 15 to
July 1. It is during this time that oaks are most susceptible. Oaks can still get infected during the rest of the
summer months, but it’s less likely. If an oak is wounded during this period, it’s one of the few times it’s recommended
to apply tree wound paint to the injured area. If pruning oaks around your house, or clearing an oak lot for a
house, it’s advisable to do so during the winter months.
Severing root graphs between healthy and infected oaks is one way of stopping the spread of the disease. A tree
care professional and Diggers Hotline should be contacted before this method is attempted. The wood of a newly
infected tree that is cut down, needs to be debarked, burned, buried, or covered with a tarp and the sides sealed
for a year. This will prevent the beetles from feeding on any fungal mats that may form in the firewood. There
are other oak wilt management strategies available, such as fungicides that help prevent the disease, that a tree
care professional, city forester, or county extension agent may be able to offer or suggest to you.
To confirm that your tree has oak wilt, samples can be sent to the Wisconsin Department of Agriculture Plant Industry
Laboratory. For more information on how to do this, or for any other questions pertaining to oak wilt, please feel
free to E-Mail
the Stevens Point Forestry Department, or call us at (715) 346-1532. |