To identify trees infected with oak wilt, watch for wilting leaves in the upper canopy. Wilting leaves may develop yellow margins, while the interior portion of the leaf remains green. As the tree continues to wilt, leaves turn brown and fall from the tree.
Trees in the red oak group generally die rapidly, usually within weeks or months after infection. Treatment will not save an infected red oak. Trees in the White oak subgenus, such as Burr oak and Chinkapin oak, can generally be treated for oak wilt and will recover within a year of treatment.
A good indicator of oak wilt infection is a carpet of leaves under an oak in mid-summer. Red oaks infected with oak wilt lose about 90 percent or more of their leaves in 1–2 months; bur and white oaks quickly lose leaves from a few branches during the summer over several years.
White oaks can be treated with propiconazole once they display early symptoms of oak wilt.
Wilting symptoms are observed from June through September, most commonly in July and August. Leaves of the infected trees turn dull green or bronze, and look water-soaked. Unlike normal leaf fall, the leaves of infected trees drop to the ground when they are still partially-green.
To stop the spread of oak wilt through the root system, trenches can be placed around a group of trees, at least 100 feet away from the dripline of infected trees and at least four feet deep, or deeper, to sever all root connections.
If there is concern that a red oak may have been infected with oak wilt, then its wood should be immediately burned, buried, or chipped. Fire will kill oak wilt, and there is no risk of spores being transported by smoke.
All live oaks in a stand are likely connected to each other and other live oaks as far as 200 feet away. If oak wilt infects one of the trees in the stand, the disease spreads through the common root system to adjacent trees and an oak wilt infection center begins.
Firewood from unknown origins should be burned during the winter months. If possible, do not store it to the next season. (Note: Burning firewood with oak wilt spores does not spread the fungus! Fungal spores burn, too.
Oak Wilt Identification
Leaves will turn a pale green, wilt, and then suffer bronzing or tanning along the edges and inward to the main vein. Leaves may droop, roll lengthwise and wilt. Leaves may also die from the tip down. As the disease progresses down the tree's canopy, upper leaves will turn yellow and fall off.
Disease Characteristics
Oak wilt is identifiable by the rapid pattern of wilting starting from the top of the tree and progressively dying down to the bottom, and on specific leaves, wilting from the edges to the base.
Oak wilt is a major problem in Illinois, Iowa, Michigan, Minnesota, Texas, and Wisconsin.
Leaves Turn Brown and Wilt
While it might seem counterintuitive, overwatering can lead to brown and wilted leaves just as underwatering can. Excessive moisture prevents the roots from absorbing nutrients properly, causing the leaves to lose color and vitality.
The oak wilt fungus moves from tree to tree in two ways: underground through the roots or overland by insect vectors. Most new tree infections occur when the fungus moves from an infected tree to a nearby healthy tree through connected root systems in a process called local spread (figure 3, lower pathway).
Propiconazole is the only fungicide scientifically tested and proven effective (when properly applied prior to infection) for use as a preventative treatment to protect live oaks.
Cutting down a tree with oak wilt that is still partially alive can cause the disease to spread, but removing it during the least vulnerable months can prevent this.
Biology & Symptoms:
Trees in this group completely defoliate and die within three to six weeks after initial wilt symptoms occur. Some oaks, including live oaks, develop brown veins in their leaves, although green tissue remains. Early symptoms are wilting, bronzing, and shedding of leaves at the ends of branches.
Once an Oak has Oak Wilt there is no cure, no matter what anyone tells you. You may be able to slow the decline down for a bit by treating it with the fungicide Propiconazole but eventually the tree will succumb to the fungus. Propiconazole is a very strong fungicide and is used more as a preventative than a treatment.
While the best time for tree treatment may be in the spring after the new leaves have set, successful treatments can be made at any time that there are sufficient green leaves on the tree to be treated. We've found that if the Oak Wilt threat is imminent, a treatment should happen soon to avoid major damage.
How much does treatment cost? Fungicide treatments are based on the diameter of the tree and therefore vary quite a bit. The average tree costs about $250 to treat once. However, there are discounts available for larger projects.
Girdling has shown in recent testing to be an effective means in stopping the spread of oak wilt into the tree's roots and, in turn, into adjacent oak trees.
Oak wilt can form spore mats on cut firewood as long as the pieces still have intact bark. Firewood cut from freshly killed oak wilt trees (which just died this summer) can be used but should be burned by next April 1.
Symptoms start from the tip and outer edges of leaves and move toward the midrib and base of leaves, often with a distinct margin (fig. 3). First, leaves turn dull green or bronze, can appear water-soaked, and wilt. Later, the leaves turn yellow and/or brown, curl around the midrib, and are shed at branch tips.
The disease is a particularly serious problem for species in the red oak group such as northern red, northern pin and black oaks. Once wilting symptoms are apparent on a red oak, the infected tree will lose most of its leaves and die within approximately one month.