Forest Health News

Bur Oak Blight Confirmed In Calumet County

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Photo of a wedge-shaped area of dead tissue at the tip of a bur oak leaf from a tree infected with bur oak blight.

A wedge-shaped area of dead tissue at the tip of a bur oak leaf from a tree infected with bur oak blight. / Photo Credit: Wisconsin DNR

By Bill McNee, DNR Forest Health Specialist, Oshkosh
Bill.McNee@wisconsin.gov

Recent laboratory examination has confirmed the presence of bur oak blight in Calumet County. The disease affects only bur oaks and is caused by the fungus, Tubakia iowensis.

Calumet County is the 35th Wisconsin county to have a confirmed detection of bur oak blight. The disease has not been microscopically confirmed in all counties where bur oak grows and is likely present in additional counties. Bur oak blight was first reported in Midwestern states in the 1990s.

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HRD Conks Found In Different Shapes And Sizes

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Photo from above showing Heterobasidion root disease growing from an infected tree stump.

Heterobasidion root disease grows from an infected tree stump. / Photo Credit: Wisconsin DNR

By Kyoko Scanlon, DNR Forest Pathologist, Fitchburg
Kyoko.Scanlon@wisconsin.gov

Fall is a great time to look for mushrooms, including conks from trees infested with Heterobasidion root disease (HRD).

Considered one of the most destructive diseases of conifers in the northern hemisphere, the fungus causing HRD is very difficult to eradicate once established. Infestation of a conifer stand may significantly impact stand management, making early detection of the disease extremely important.

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Fall Webworm Spins Its Way To Attention

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Photo of fall webworms form a tent on a tree at Kohler-Andrae State Park in September 2025.

Fall webworms form a tent on a tree at Kohler-Andrae State Park in September 2025. / Photo Credit: Wisconsin DNR

By Bill McNee, DNR Forest Health specialist, Oshkosh
Bill.McNee@wisconsin.gov, 920-360-0942

The native insect known as fall webworm (Hyphantria cunea) has been generating calls and emails from curious property owners over the past few weeks.

The caterpillars don’t draw much attention on their own; instead, what catches the eye at this time of year are the large webs (or “tents”) that they create on the tips of tree branches.

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Invasive Japanese Stiltgrass Detected In Three More Counties

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Wide-angle photo of a mat of Japanese stiltgrass in Rock County, Wisconsin, in July 2025

A mat of Japanese stiltgrass in Rock County, Wisconsin, in July 2025. / Photo Credit: John Zaborsky, University of Wisconsin-Madison Renz Weed Science Lab

By Wisconsin DNR

Thanks to the help of eagle-eyed observers across southern Wisconsin, the harmful invasive plant Japanese stiltgrass (Microstegium vimineum) has now been detected in three additional counties. That brings the total number of Wisconsin counties to five.

The most recent detections of the non-native, annual grass came in Dane, Grant and Walworth counties. The Dane County find was in Middleton, northwest of Madison; in Grant County, it was found southeast of Wyalusing State Park; in Walworth County, it was found north of Geneva Lake.

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Autumn Is A Great Time To Treat Invasive Plants

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By Wisconsin DNR

Photo of rosettes on a garlic mustard plant. / Photo Credit: Wisconsin DNR

Rosettes on a garlic mustard plant. / Photo Credit: Wisconsin DNR

As temperatures begin to fall the leaves of many plants and trees have begun to change color. Some trees in the northern part of the state have already started to drop their leaves.

The Wisconsin Department of Natural Resources (DNR) reminds the public that fall is more than a great time to enjoy the changing hues in the woods. It also presents a good opportunity to spot the invasive plants persisting among the native plants and trees.

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Imported Willow Leaf Beetle Spotted

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Closeup photo of an imported willow leaf beetle adult and larva.

An imported willow leaf beetle adult and larvae. / Photo Credit: Wisconsin DNR

By Linda Williams, DNR Forest Health Specialist, Woodruff
Linda.Williams@wisconsin.gov, 920-360-0665

Imported willow leaf beetle (Plagiodera versicolor) is a small, shiny, blue-black beetle as an adult, and the larvae are small, solid black and spikey. Both feed on the foliage of willow and poplar species. Two generations per year are possible.

The invasive pest was first detected in the United States in 1915, likely coming from eastern Europe.

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Introduced Pine Sawfly Found On White Pine

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By Linda Williams, DNR Forest Health Specialist, Woodruff
Linda.Williams@wisconsin.gov, 920-360-0665 

Photo of an introduced pine sawfly larva.

Introduced pine sawfly larva. / Photo Credit: Wisconsin DNR

The introduced pine sawfly was first found in the United States in 1914 and was first detected in Wisconsin in 1944.

Introduced pine sawfly larvae prefer feeding on white pine needles, although literature says they can also feed on jack pine, red pine and scotch pine. Larvae often feed singly, rather than in groups (like redheaded pine sawfly or European pine sawfly do). Larval sawflies can look like caterpillars, but these larvae develop into adult sawflies, which look like a fly or a chunky wasp.

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White Pine Weevil Damage Is Easy To Spot

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A photo showing that white pine weevil has attacked and killed the terminal leader on a young white pine.

White pine weevil attacked and killed the terminal leader on a young white pine. / Photo Credit: Wisconsin DNR

By Linda Williams, DNR Forest Health Specialist, Woodruff
Linda.Williams@wisconsin.gov,  920-360-0665

 Symptoms of white pine weevil attack are easily detected at this time of year, as damaged leaders turn dark brown and branch tips can curl or droop.

White pine weevil (Pissodes strobi) is a native insect that can attack and kill the terminal leader of white pine, jack pine and spruce. Another common name for this insect is “tip weevil.”

These insects prefer to attack stout terminal leaders.

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Oak Leaf Issues That Are Not Oak Wilt

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Closeup photo showing oak wilt leaf symptoms on a red oak leaf. Leaves often show browning on the outer portions of the leaf while the base of the leaf will still have some normal green color.

Oak wilt leaf symptoms on a red oak leaf often show browning on the outer portions of the leaf while the base of the leaf will still have some normal green color. / Photo Credit: Wisconsin DNR

By Linda Williams, DNR Forest Health specialist, Woodruff
Linda.Williams@wisconsin.gov or 920-360-0665

Oak wilt symptoms are active right now, but so are several other oak issues that may be mistaken for oak wilt.

Issues including Tubakia leaf spot, mite damage and leaf scorch are all causing problems in northern Wisconsin and may be mistaken for oak wilt.

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Which Witches’ Broom Is It?

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Witches’ broom on serviceberry. The leaves of this infected shrub are dying because of prolific spore production on their undersides. / Photo Credit: Wisconsin DNR

By Tim Shively, DNR Forest Health Specialist, Eau Claire
Tim.Shively@wisconsin.gov or 608-772-6974

Witches’ broom is a term that describes a disease of many different plants. It’s a distinctive, abnormal growth typically caused by a pathogen — though in some cases it can result from insect feeding.

The namesake symptom appears as abnormally dense clusters of stunted shoots, branches, and foliage that strongly resemble a witch’s broom from storybooks and folklore. In Wisconsin, this is not often a critical forest health concern that merits management action, but Wisconsin Department of Natural Resources Forest Health staff recently observed widespread incidence of two particular witches’ broom diseases.

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