Oak Wilt Disease is Encroaching on South County

A tree trimmer hard at work in an oak tree in the spring.

By Hailey Black

Editor’s note: Oak Wilt disease can be controlled somewhat by timing pruning of oaks. Michigan DNR suggests no pruning from early April through June 15. The “safe to prune” time is November through March. The following story explains the situation with the disease.

When the first white settlers came upon what is now called Schoolcraft in the early 1800s, they noted a vast prairie with surrounding dense oak forests, inhabited by the native Potowatomi. The south Kalamazoo area we know and love today may look quite different, but evidence of those ancient oak forests still surrounds our communities.

Today, the most notorious threat to our beloved oak trees lies in oak wilt disease.

Oak wilt disease is caused by a fungus which is almost always fatal to oak trees. Once infected, the tree dies within months or even weeks. It first appeared in the U.S. around the 1970s, and the source is still unclear. Possibly, the fungus was introduced from Mexico, Central or South America. It has since spread throughout the Midwest and Texas. Despite the spread, “the number of confirmed cases is declining,” according to Linda Whitlock of the Consumer Horticulture and Master Gardener Program at the Kalamazoo County Michigan State University Extension Office. She attributes much of this to education and outreach efforts.

Part of the outreach is teaching how the disease spreads. This can happen one of two ways: When an oak tree is wounded, either via pruning or storm damage, the wound releases a pheromone. Whitlock notes that the pheromone “smells like rotting fruit” that attracts “picnic beetles”, which can carry the fungus from one tree to another. The disease can also be spread from an infected tree’s complex underground root grafts, 50 to 100 feet apart.

Red oaks (with the pointed leaves) are most susceptible to oak wilt disease and can die within weeks of symptoms being noted. White oaks (with rounded leaves) tend to be more resistant to the fungus and can take months to die. Phillip Kurzeja, forest health technician with the Oak Wilt Mapping and Community Outreach Forest Resource Division of the Michigan DNR notes that occasionally, if caught early enough, white oak trees infected with wilt can be treated, but often the disease is fatal. Treatment itself occurs to prevent the spread of disease to other nearby oaks.

This is what makes oak wilt one of the more unique tree diseases. Treatment options don’t necessarily save a property owner’s tree, but proper management can prevent the fungus from spreading throughout the owner’s and neighboring properties. Whitlock states that proper management may include “trenching” an infected tree to prevent spread through root grafts. Another form of management may include carefully and tightly covering infected oak wood with a tarp to prevent spread via a dead tree. Kurzeja said that not moving firewood is important, because “If it is moved by a person – in infected firewood – it can start a new epicenter anywhere there is host and suitable conditions.”

Kurzeja added that the Michigan DNR has confirmed cases of oak wilt near the south county area (in Ramona Park and the Gourdneck State Game area), though he did not confirm cases within Schoolcraft or Vicksburg. He noted that sometimes potential cases go unreported by property owners, and the Michigan DNR cannot directly manage private property.

Both Whitlock and Kurzeja agreed that oak wilt can be mistaken for other oak diseases, so early and correct identification is important. Kurzeja said, “A healthy tree’s foliage can begin to brown from the tips of the leaves toward the stem, usually starting at the top of the tree and in a matter of a few weeks to a month, can go from fully leafed out to almost completely bare. These symptoms can begin as early as May extending through the summer into September and October is some parts of the state.” Two other oak pests, the two lined chestnut borer (TLCB) and Armillaria root rot, are sometimes mistaken for oak wilt disease.

Although Kurzeja wasn’t aware of any impact studies done by the state to analyze oak wilt’s effect, MSU’s tree experts acknowledge the impact of oak wilt on property values, and estimates that each tree lost may represent thousands of dollars. He also estimates that “trenching” two trees could cost about $300, and tree removal could range from $2,000-$7,000.

With those numbers, it’s easy to see why experts say that prevention is key. For folks in South County, keeping a watchful eye on their own oaks can preserve their properties and their neighbors’, as well as a bit of the heritage of these well-loved communities.

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