Disease

Oak Wilt Control: A Thousand-Foot View

Posted on by akabelowsky

A view of two pockets of oak wilt treatment areas using the girdle-herbicide treatment, one from the previous year (bottom, yellow circle) and one from the current year (top, blue circle), captured from an airplane. / Photo Credit: Wisconsin DNR

By Alex Hornung, Forest Health Specialist, Plover
Alexandra.Hornung@wisconsin.gov

An aerial survey was conducted in late August over parts of Lincoln and Taylor counties, primarily looking for new oak wilt infection centers.

A rare glimpse of what oak wilt control can look like was captured from the vantage point of the airplane. The photo below shows a girdle-herbicide oak wilt control treatment from this year, with trees dying; and another from last year, with dead trees standing.

Continue reading “Oak Wilt Control: A Thousand-Foot View” →

Sapstreak Of Sugar Maple An Oldie But A Goodie

Posted on by akabelowsky

DNR Forester Paul Etten stands at the base of a sugar maple tree that has small leaves and a thin crown. The tree was dissected and found to have extensive sapwood streaking. / Photo Credit: Wisconsin DNR

By Alex Hornung, DNR Forest Health Specialist, Plover
Alexandra.Hornung@wisconsin.gov

There are some tree diseases that really get a lot of attention and others that get lost in historical documents, only popping up now and again to ignite excitement within the local plant pathologist community.

Maple sapstreak is one of the latter. This year, a site in Shawano County was found to have several trees experiencing symptoms of the disease. The symptoms led to a positive identification of maple sapstreak.

Continue reading “Sapstreak Of Sugar Maple An Oldie But A Goodie” →

Needle Rust On Spruce, Fir And Hemlock

Posted on by akabelowsky

closeu[p of balsam fir needles with needle rust

Balsam fir needles infected with needle rust turn bright yellow. / Photo Credit: Wisconsin DNR

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

Although this summer has been dry in north-central Wisconsin, there was enough moisture this spring to promote some needle rusts. Needle rusts are fungi that infect needles of various conifers. Most rusts require two hosts to complete their life cycle.

A look at common types of needle rust:

Continue reading “Needle Rust On Spruce, Fir And Hemlock” →

DNR Participates In Beech Leaf Disease Detection Survey

Posted on by akabelowsky
Map of Wisconsin

A Wisconsin map showing results of a beech leaf disease survey that ran from 2023 to 2026. / Graphic Credit: Wisconsin DNR

By Art Kabelowsky, DNR Forest Health Communications, Fitchburg;
Arthur.Kabelowsky@wisconsin.gov; 608-335-0167

To date, there have been no confirmed discoveries of beech leaf disease (BLD) in Wisconsin landscape, and we are on the lookout for this disease.

The disease is primarily found on American beech (Fagus grandifolia) but can also be found on ornamental species like European, Oriental, and Chinese beech (F. sylvatica, F. orientalis, and F. engleriana). Although it has not yet been found in Wisconsin, recent discoveries of the disease in Michigan and other states highlight the continued importance of monitoring BLD’s expansion.

Continue reading “DNR Participates In Beech Leaf Disease Detection Survey” →

Tamaracks Busy Refoliating After Larch Casebearer Defoliation

Posted on by akabelowsky
Wide view of stand of trees

Tamarack trees showing the effect of larch casebearer defoliation. / Photo Credit: Wisconsin DNR

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

Earlier this spring, scattered areas of tamarack in North Central and Northeastern Wisconsin showed tan or light brown foliage. Sometimes that is due to frost, but this year it was the work of an insect called larch casebearer.

As the foliage started to expand this spring, the larch casebearer became active and began to feed. The tiny caterpillars mine out the needles of tamarack and use them as “cases” to protect themselves, thus their name.

Continue reading “Tamaracks Busy Refoliating After Larch Casebearer Defoliation” →

When Can I Prune That Elm Tree?

Posted on by janness

By Elton Rogers, DNR Urban Forestry Coordinator
Elton.Rogers@wisconsin.gov or 414-294-8675

Most urban foresters and arborists are aware of the historical significance of Dutch elm disease (DED). Introduced to the United States in the 1930s, this deadly fungus wreaked havoc on urban and rural forests throughout the United States. In many ways, DED was the impetus for many of the best practices of urban forestry we see today, such as species diversification and modern plant health care practices.

Likewise, most urban forest practitioners understand DED is still very much prevalent today, and if they are managing any lingering American elms, they know they can help decrease their susceptibility to the disease by not actively pruning or wounding them during the growing season. Pruning these elms during the growing season attracts bark beetles, which can carry thousands of DED fungal spores on their bodies. But elm is a complex genus; there are over 30 different species of elm and hundreds (if not thousands) of hybrids and cultivars in the landscape. So, which of these species and cultivars can you prune during the growing season, and which shall you not? Continue reading “When Can I Prune That Elm Tree?” →

Upcoming Training & Events

Posted on by janness
group of adult learning to identify trees

Photo Credit: Wisconsin DNR

* These training opportunities are provided as an information service only and do not constitute endorsement by the Wisconsin Department of Natural Resources (DNR).

See below for information on training topics and events, including biopesticides, invasives, urban trees and fire, tree diseases, ecological restoration, safety and woodland trees and management. Continue reading “Upcoming Training & Events” →

Forest Health Team Debuts Two New Fact Sheets

Posted on by akabelowsky
An image of the front page of a DNR fact sheet.

The front page of the Wisconsin DNR’s new Forest Health Fact Sheet on oak decline. / Graphic Credit: Wisconsin DNR

By Art Kabelowsky, DNR Outreach and Communications, Fitchburg
Arthur.Kabelowsky@wisconsin.gov or 608-335-0167

The Wisconsin Department of Natural Resources Forest Health team has published two new fact sheets, bringing the total count of its fact sheet offerings to 26.

The new fact sheets:

  • Fall Webworm: DNR Publication FR-622u discusses basic information on the biology of the insect (Hyphantria cunea), its life cycle, potential lookalike caterpillars and management options.
  • Oak Decline: DNR Publication FR-622v covers the tricky topic of oak decline, which is not caused by one distinct forest pest or problem but instead is the result of multiple factors — some predisposing, some inciting and some contributing — that work in combination to cause decline.

The fact sheets are designed in full color, with photos, and are meant to be printed out for reference or to carry into the field. They can also be viewed online. They can be printed on standard letter-sized paper. Most fact sheets are two pages, meant for two-sided printing; a few are four-page documents.

A complete directory of links to all DNR Forest Health fact sheets is available online.

Oak Decline May Continue In 2026

Posted on by akabelowsky
A wide view of declining oak trees following spongy moth defoliation and drought.

Oak trees with thin canopies experiencing decline following spongy moth defoliation and drought in the prior few years. / Photo Credit: Wisconsin DNR

By River Mathieu, DNR Forest Health Specialist, Fitchburg
River.Mathieu@wisconsin.gov

Oak decline is a complicated forest health issue impacting oak forests in Wisconsin.

In the past few years, oak decline has been recorded in several regions of Wisconsin, primarily linked to spongy moth defoliation and drought conditions that occurred from 2022-2024.

As the summer months of 2026 continue to unfold, oak decline may continue in areas previously impacted by spongy moth and drought.

Continue reading “Oak Decline May Continue In 2026” →

Maple Leaf Blister And Anthracnose: Two Diseases Of Maple Leaves

Posted on by janness

By Nathaniel Walton, Michigan State University Extension

Updated from an original article written by Diane Brown and Lori Imboden

Irregularly shaped anthracnose lesions on a red maple (Acer rubrum) leaf. Photo by Rebecca Finneran, MSU Extension.

Each year, Michigan State University Extension receives questions from the public regarding spots on the leaves of their favorite maple trees. Both anthracnose and maple leaf blister can be the cause of late spring and early summer damage to leaves of these popular landscape trees. Continue reading “Maple Leaf Blister And Anthracnose: Two Diseases Of Maple Leaves” →