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Vilas County and Oak Wilt / Yellow Flag Iris

Deadly Oak Wilt First Appears on Developed Lakeshores
in Northern Wisconsin
Article by Brian Schwingle, WI DNR Forest Health Specialist, 715-536-0889

The DNR detected Oak Wilt for the first time last year in Vilas, Lincoln and Sawyer counties. Oneida County was found to have this fatal fungal tree disease for the first time in 2010. In each case the infected tree was growing on a lakeshore or river edge property, and the affected property was surrounded by hundreds of seasonal homes.

It is noteworthy that none of these counties in northern Wisconsin has an Oak Wilt find in an undeveloped forested area (see map below). The reason for this is not because foresters and forest disease experts are not actively surveying for this disease, but rather, it is because seasonal homeowners, developers, and ornamental tree care workers are guilty of spreading this damaging disease. Eventually this disease will march into undeveloped forests, as it has in southern and central Wisconsin.

So what should lakeshore property owners do about Oak Wilt? Besides learning about what infected oaks look like (see Figures 2 and 3), they need to know that humans can easily prevent this disease from entering an area. After all, the beetle that carries this disease only flies up to a quarter mile from the originally infested firewood pile or tree, and it rarely flies that far. By simply not transporting fresh oak firewood (less than 2 years old) from an infested area to a clean area, people can prevent Oak Wilt. Not wounding oaks in any way between April and July is also an important strategy to avoid oak wilt. Lastly if oaks are damaged in any way from April through October, those wounds should be cut clean and the cut surface painted with a latex-based paint to prevent disease transfer.

For as long as communities, property owners, developers, and tree care professionals ignore the easy strategies to prevent oak wilt, the disease will continue to spread into clean areas of Wisconsin, decreasing property values and killing beautiful oaks. Please prevent oak wilt and report suspect infected oak trees to a professional forester or Extension agent. Educate your neighbors, community leaders, property
developers, and tree care professionals about the easy ways to prevent oak wilt.

An oak tree with oak wilt at the end of August. This tree was perfectly healthy in May. Now it is dead. Other diseases, insects, and severe drought can kill oaks, but none will kill a healthy oak in only a few months.

Fallen leaves will be totally green or bronze, or they will have brown tips.

Additional information about oak wilt can be found online at

Here you will find information on the distribution, biology, impact, symptoms and signs, prevention, and management of Oak Wilt.

Vilas County and Yellow Flag Iris

Non-native yellow flag iris (Iris pseudacorus) is becoming more common each year along northwoods lakes, streams and in wetlands.  Native blue flag iris (Iris virginica), also common in the northwoods, is much smaller and more delicate and does not present an environmental threat.  Yellow iris grows much taller and displays large, beautiful, bright yellow flowers.  It is very aggressive and capable of quickly overtaking native vegetation and altering shoreline/wetland habitat depended on by wildlife.
Does this sound something like purple loosestrife?  Look for yellow iris in the same areas.  But unlike the pretty purple flowered plants, there is no known biological agent we can use to control yellow iris.
Pulling young plants while their roots are relatively easy to remove is a better choice than waiting until plants are large with root masses the size of Volkswagen Beetles before trying to remove them, which would likely cause harmful and illegal shoreline disturbance (permits are needed before doing backhoe work, which is about what would be needed to extract the roots of a well established patch!). 
Yellow flag iris is classified as a noxious weed in many states.  It is proposed to be added to Wisconsin’s NR40 classification as a “Restricted” plant in the rule’s next update.