Forest Insect and Disease Newsletter
From the O.A.K. Corral
Oak wilt confirmed in St. Croix State Park
by Ryan Blaedow
Oak wilt has been discovered throughout St. Croix State Park in Pine County, representing the northern-most find of this vascular wilt disease in Minnesota. Several wilting red oaks were observed in June and July, and while oak wilt had never been observed in the park before, park and forestry staff recognized the characteristic symptoms of this important disease and reported it to the Forest Health Unit for confirmation. The University of Minnesota Plant Insect and Disease Diagnostic Lab quickly confirmed that samples from suspect trees in the park did in fact have the oak wilt fungus (Ceratocystis fagacearum).
Oak wilt is considered to be the most important disease of oaks in the eastern U.S. and is responsible for killing thousands of oaks each year from Texas to the Lake States. While all species of oaks are susceptible, trees in the red oak group are more susceptible to oak wilt and are killed more rapidly 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 is now believed to be non-native, although its origin is still unknown and the disease has never been found outside the U.S. The fungus invades the water-conducting vessels of oak trees, and infected trees respond by plugging up their vessels to prevent the fungus from spreading. As a result, these trees are unable to transport water and they quickly begin to wilt. Diseased trees in the red oak group often shed their leaves and die within a few weeks of symptom onset. Few if any forest pathogens are capable of killing trees so quickly. Trees in the white oak group may die more slowly over a period of several years.
Oak wilt was first discovered in Minnesota in the 1940"s in the southeastern part of the state. Since that time, the disease has slowly spread north and westward, but it has never before been found so far north in Pine County. So how did it reach St. Croix State Park? To answer that, you need to understand how oak wilt spreads.
The most common way oak wilt spreads is when the fungus moves through connected root systems. The roots of trees commonly graft to roots of trees of the same species, forming a continuous underground network. When an infected tree dies, the fungus spreads through the inter-connected root systems and invades healthy trees. While this only allows the oak wilt fungus to spread relatively short distances through a forest stand, large pockets of wilting and dead trees can form over many years and are known as "disease centers."
After a tree is killed, the oak wilt fungus produces specialized structures called spore mats beneath the bark. These mats are dense clumps of fungal mycelia covered in millions of sticky spores. They are also very sweet and fruity smelling like bananas or bubble gum, and they crack the bark open to lure in certain species of beetles with their delicious aroma. Sap-feeding feeding beetles (above) are among the most frequent visitors that feed on the fungal mats, and they become covered in oak wilt spores. These beetles also like to feed on the sap oozing from fresh tree wounds. Spores can be transmitted from the beetle to the wound and the fungus will then spread into the tree's vascular system.
In late July last year, a massive storm destroyed thousands of acres of forests in and around St. Croix State Park. Not only were trees blown down, but many were wounded during the fury and chaos of this storm. These fresh wounds likely attracted sap-feeding beetles, contaminated with oak wilt spores, from the surrounding area. While no oak wilt disease centers have ever been reported in the vicinity of the park, it is believed that isolated diseased trees or small disease centers are present throughout the region. The blow-down event provided the perfect opportunity for this devastating fungus to spread into the park and infect dozens of the oaks that managed to survive the storm.
Because this is currently the northern-most known occurrence of oak wilt in Minnesota, park staff will be aggressively managing the disease not only to limit its impact to the park, but to minimize the risk of additional spread throughout the state. Management in the park will consist of both removing and destroying diseased trees so that spore mats cannot form, and trenching around known disease centers to sever root grafts that might lead to additional spread. While dozens of infected trees have been identified during disease surveys conducted this summer, DNR staff are optimistic that oak wilt can be effectively managed in the park.
The discovery of oak wilt in St. Croix State Park highlights the need to take precautions to prevent the spread of this destructive forest disease throughout Minnesota, including those areas where oak wilt is not currently known to occur. To prevent the spread of oak wilt, follow these management guidelines:
- Only prune oak trees during the winter months
- Avoid wounding oak trees between April and July when sap beetles are actively feeding
- If trees are wounded during the spring and summer months, wounds should be immediately treated with pruning paint.
- Remove diseased oaks before the following spring to prevent spore mat development. It is best to remove trees in winter to avoid wounding neighboring trees.
- Prior to removing dead and diseased trees, sever root connections to healthy trees by trenching around diseased trees with a vibratory plow equipped with a five-foot blade.
- Destroy wood from diseased trees on site by debarking or chipping. If wood cannot be destroyed, dry it properly by covering split wood with a plastic tarp and burying the edges of the tarp for at least six months to kill the oak wilt fungus and any insect vectors.