Forest Insect and Disease Newsletter - August 2019

Older bur oaks dying in forests from multiple stressors

By Brian Schwingle, Central Region Forest Health Specialist

Over the last few years, concerned landowners have asked the forest health team to investigate the cause of scattered bur oak death in several wooded locations. Most sites have been in the southern half of Minnesota, but some investigations were in the north as well. In all cases, a noticeable percentage of bur oaks has died over the last few years, usually scattered across the forest. Death was gradual, starting with the outer branches or a single limb. After a few years, entire trees finally died. We think this scattered death is a result of drought and excessive precipitation stressing old trees, which eventually get finished off by a couple of opportunistic pests.

Understanding why a certain tree species is fading out of a forest is complicated, particularly for bur oaks. Oak wilt and bur oak blight are common on bur oak (particularly in southern and central Minnesota), so the first step in our investigations was to rule out these as the cause of death. We found Armillaria on the lower trunk and root collar of many dying and dead oaks, and we also found frequent indications of two-lined chestnut borer.

 oaks showing sign of decline

Scattered bur oaks dying from stress, Armillaria, and twolined chestnut borer.

Armillaria is a common fungal root pathogen present in the soil. It usually is a secondary problem, only killing roots and lower trunks when a tree is stressed or dying. Likewise, twolined chestnut borer is a native cambium-feeding beetle that is commonly a secondary problem, successfully attacking oaks that are stressed or dying.

We have found no consistent primary tree killer such as oak wilt on these sites. We also do not believe bur oak blight or herbicide drift are fundamental parts of the problem. We suspect these trees have simply been stressed to the point at which they cannot fend off Armillaria and twolined chestnut borer, both opportunists that attack stressed trees. Besides old age possibly predisposing these oaks to stress, we've had several growing seasons during which precipitation was either too low or excessive. Drought occurred in various years in east-central, south-central, and southeast Minnesota from 2006 to 2013. Much higher-than-normal precipitation happened in the years afterward, which in some instances clearly helped kill oaks near wetter areas.

dead oat trees

Armillaria is killing these bur oaks because the site has gotten too wet.

view under the bark showing white fans

Armillaria forms white, fan-like sheets under the bark of dying trees.

two-lined chestnut borer under bark

Twolined chestnut borer larva with characteristic feeding galleries.

Unfortunately, nothing can be done to slow the decline of these bur oaks other than not adding additional stress. In many cases, invasive plants are dominating areas in the forests where bur oaks are dying, and that is a major concern for sustaining a healthy forest ecosystem. We recommend the following:

  • Wait a few years after a drought before thinning a forest.
  • Do not thin a forest if there is excessive precipitation and flooding. Heavy equipment can increase the effects of flooding.
  • Help young trees get established in canopy gaps. Plant trees appropriate for the native plant community, taking into consideration their tolerance for shade and soil moisture.
  • Protect planted trees or naturally-occurring seedlings and saplings from deer browse.
  • Control competing vegetation around the trees you want to eventually take over the canopy. The DNR website shares helpful information on controlling invasive buckthorn

Bur oak blight starting to show

By Brian Schwingle, Central Region Forest Health Specialist

Given our abundant precipitation this year, it is not a surprise that DNR forest health specialists have started fielding calls about bur oak blight. Abundant spring and summer rains promote the fungal leaf pathogen that causes this disease, so we predict that some bur oaks will continue to show more and more symptoms as the growing season progresses. Fortunately, very few bur oaks were showing any symptoms of bur oak blight in late July.

If your bur oak’s lower and inner canopy first turned brown in later July or August, and that brown is spreading outward, there is a good chance it has bur oak blight. But just because a bur oak has bur oak blight doesn’t mean it will decline in health. A healthy bur oak can tolerate heavy bur oak blight for well over a decade. Although the symptoms look concerning, healthy trees with heavy bur oak blight in the autumn can look totally healthy the following May.

If an oak starts dying back or growing epicormic shoots (little branches growing out of trunks and bigger limbs), that is a sign of stress. That stress may or may not have been caused by repeated years of heavy bur oak blight. In many cases though, the main stress is from something else such as soil compaction, flooding, or attack by twolined chestnut borer or Armillaria root disease.

You can find additional information on the DNR bur oak blight website. Identification and management are covered in separate links found on the left-hand side of the webpage.

large oak showing signs of BOB

A bur oak with bur oak blight. Photo by Laurie Wilhelm.

Do your trees have oak wilt?

By Rachael Nicoll, Northwest Region Forest Health Specialist

Most of you have heard of oak wilt, a nonnative and devastating fungal disease that affects all species of oaks in Minnesota. Unfortunately, it can be difficult to know whether your trees have oak wilt because symptoms can resemble damage from the common and native twolined chestnut borer (TLCB) that also attacks all Minnesota oak trees.

Both oak wilt and TLCB can create pockets of dead and dying trees, kill trees quickly, and cause browning of leaves that begins in the outer crown. However, the management options for these two issues are very different, and oak wilt control is expensive, so it is important to know which issue you’re dealing with.

The table summarizes the major differences between oak wilt and TLCB:

Diagnostic clues

Oak wilt

Twolined chestnut borer


Central and southern Minnesota



Pressure pads and fungal mats

Larvae, galleries, exit holes


Rapid wilt, premature leaf fall

Dead leaves hang on tree


Expanding circles of dead oaks

Scattered trees


Heavy leaf fall May-September

Symptoms mid-summer to early fall

Tree mortality

Primary killer; always fatal to red oaks

Secondary killer

Location: Where you are located is critical. Oak wilt was recently confirmed as far north as Morrison County and northern Pine County. Due to the difficulty in finding isolated, infected trees, oak wilt could be in southern Crow Wing County and Carlton County. Twolined chestnut borer is present throughout the state.

Signs of the causal organism: Tree bark can crack for many reasons, but hard-to-see bark cracks are present in roughly half of red oaks with oak wilt. These cracks are caused by fruity-smelling pressure pads and gray fungal mats formed under the bark. TLCB larvae, on the other hand, create meandering galleries under the bark of trees they infest, and the larvae themselves are present in the galleries during the growing season. When they emerge as adults, they create small, D-shaped exit holes in the bark (just like their nonnative relative, the emerald ash borer). Trees can have oak wilt and TLCB; just because your tree has signs of TLCB doesn't mean that it doesn't also have oak wilt.

Patterns in woodlands and forests: Oak wilt occurs in isolated trees and in expanding pockets of dying or dead oaks. TLCB often occurs in scattered trees or pockets and is associated with stress events (e.g., drought, construction, wind damage), often a couple of years after these events.

Symptoms in individual trees: Symptoms of both oak wilt and TLCB start in the outer tree crown. However, oak wilt symptoms include wilting of the entire crown in red oaks and individual branches in white oaks, leaves browning from tip and margin inward, and a water-soaked or bronzed appearance of leaves. Oak wilt can also cause brown streaking in the new wood of wilting twigs, but the primary distinguishing symptom of oak wilt is rapid, premature leaf fall. A carpet of leaves under an oak tree before fall is a good indicator of oak wilt. Symptoms of TLCB also start in the upper crown and on individual branches. This eventually leads to browning of entire leaves without leaf fall and brown, dead leaves that remain on the tree past the growing season. Three years of TLCB infestation creates trees with dead and bare branches at the top of the tree, dead and red leaves in the middle, and green leaves at the bottom. During extreme drought, TLCB can kill oaks in a single growing season.

Road oak showing signs of oak wilt

Red oak tree infected by oak wilt showing canopy wilt and leaf bronzing

Red oak showing signs of having two-lined chestnut borer

Red oak tree with symptoms of twolined chestnut borer infestation: dead top, red middle canopy, and a green lower canopy.


Timing: Oak wilt symptoms can appear at any time during the growing season, and heavy to complete leaf fall happens from May to September. TLCB symptoms appear from mid-summer to early fall.

Tree mortality: Oak wilt is always fatal to red oaks, and death usually happens within two months. Bur oaks succumb one to seven years after infection, and white oaks can die anywhere from one to 20 years after infection—but white oaks also can recover. TLCB can kill trees but it is usually associated with other stress factors and multiple years of repeated attacks.

Although the above symptoms are very concerning, don't panic if you see them on your oak trees. A variety of organisms attack oak trees, and a visit from a professional arborist often is needed to confirm oak wilt, especially in bur and white oaks. There is a much higher probability of TLCB attacking trees on your property than oak wilt, but please contact a local DNR forester if you see symptoms of oak wilt out of its current range.

To prevent oak wilt on your property, always remember to avoid wounding oak trees during the high-risk period for oak wilt infection, generally April through mid-July. Prevent oak wilt from moving across the state: Don't move firewood! To prevent TLCB damage, don't thin your oak woods during drought or excessively wet periods or for several years after those stressful events.

Jack pine budworm populations at rock bottom in north-central Minnesota

By Rachael Nicoll, Northwest Region Forest Health Specialist

Do you have jack pine trees on your property?*

If so, first, the bad news:
This is the time of year jack pine budworm damage shows up in your trees. Jack pine budworm is a nondescript moth as an adult, but its caterpillars aggressively feed on jack pine and occasionally other pines. When their populations are high, defoliation from jack pine budworm can lead to widespread loss in tree vigor and high levels of topkill and tree mortality, especially in older stands with numerous male flowers. Jack pine budworm damage also increases the risk of wildfire in these stands.

Now, the good news:
It's likely jack pine budworm will be only a minor problem this year. During June surveys of Crow Wing and Cass Counties, forest health program staff found only three caterpillars. Jack pine budworm populations crashed in 2018, and our surveys indicate they have not yet begun to rebuild. Jack pine budworm populations are cyclical and peak every 8-10 years in Minnesota. Based on past populations, we expect another outbreak in central Minnesota around 2025.

For more information about jack pine budworm and management options, see the How to Manage Jack Pine brochure from the USDA Forest Service.

*(If you're not sure how to identify a jack pine, find helpful information at the Minnesota Conservation Volunteer Minnesota Profile).

Young jack pine budworm larvae feeding on needles in a shelter made from webbing on a jack pine shoot.

Young jack pine budworm larva feeding on needles in a shelter made from webbing on a jack pine shoot.

Bumps, bladders, and growths: galls on leaves and branches

By Megan O'Neil, Northwest Forest Health Specialist

Galls come in many shapes, sizes, and colors. Perhaps as a result, the insect and disease team receives a lot of questions about them.

Galls on tree leaves and branches are a common sight in the summer but are rarely cause for concern. They develop as a result of abnormal plant tissue growth, typically caused by insects laying eggs and feeding. The growth provides food and shelter for the organism inside. Once the insect is fully developed, it emerges from the gall.

Luckily these growths, while unattractive to some homeowners, usually do not require management. Some species of gall-making insects overwinter in leaf litter, so raking and destroying fallen leaves may provide some control. Galls can change color from green to yellow to reddish-purple. Some of them grow large and fall off while others are barely noticeable. Here are some of the common galls you may see in Minnesota:

maple leaf showing small bladder galls
ash leaves with gall
oak leaf with apple gall

Maple leaf bladder gall: These small bladder galls are common on silver and red maple leaves. Inside each of these bladders is a single mite. Bladder galls start out small and green and as the leaf expands they get bigger and change color. In the fall the adult mites emerge from the gall and move to the bark to overwinter. During the worst infestations the leaves may curl up and fall off early, but no management is recommended for maple leaf bladder galls.

Ash flower fall: This strange-looking growth is caused by a mite and only affects male ash trees. Galls turn dark brown or black in late summer and can stay on the tree for more than one season. Ash flower galls do not negatively affect the health of the tree and require no treatment.   

Oak apple gall: This gall resembles a small apple and is usually found on the stem or mid-vein of an oak leaf. It is caused by a tiny wasp that lays eggs on the developing leaf bud; the larva develops inside the gall. Growths can be as large as two inches in diameter and are found on many species of oak. Photo shows the gall-making wasp next to the gall.

Leaf mining in birch trees

By Eric Otto, Northeast Forest Health Specialist

Birch tree damage has been fairly abundant in northern Minnesota this summer. If you take a close look at damaged birch leaves, you might see something interesting. When held up to the light, the leaves appear transparent. If you look more closely, you will see how the leaf has been "mined" by the larva of a birch leaf miner.

The birch leaf miner is a type of sawfly whose larvae feed on, or "mine," the inside of leaves for the chlorophyll-bearing cells between the upper and lower leaf surfaces. The feeding can be quite extensive, but typically does not cause lasting damage to a tree. In fact, birch can withstand leaf miner damage to as much as 60 percent of its leaves. The only time for concern is if a tree already is stressed by other factors, such as drought. In that case, birch leaf miner damage could result in the death of the tree, but that's rare. Watering your birch during dry periods can help limit stress and prevent lasting damage.

Although the birch leaf mining business is booming, the cause for concern is low.

paper birch leaf with damage from birch leaf miner

Paper birch leaf with damage from birch leaf miner.

Spruce budworm damage continues

By Eric Otto, Northeast Forest Health Specialist

Spruce budworm damage is picking up where it left off last year.

In 2018, spruce budworm impacted just under 200,000 acres, the highest amount by any forest pest in Minnesota that year. This year will be similar.

Most of the spruce budworm feeding damage happened in June in Lake County. The larvae pupated and emerged as moths in mid-July. After emerging, moths lay eggs on needles and eventually die in late summer.

Most of the mortality from spruce budworm happens on balsam fir, which the larvae prefer over spruce. It takes three to four consecutive years of feeding before balsam fir die. Residents of northeast Minnesota have noticed the damage, but not much can be done to manage spruce budworm once it has started feeding on trees. Salvaging fir and spruce trees can be an option before the wood becomes unusable. Don't forget to encourage or plant tree species other than fir to create a future forest that is more resilient to budworm damage.

Spruce budworm is running its course in northeast Minnesota and will likely be present again next year.

More information on the identification and management of spruce budworm can be found on the DNR spruce budworm website.

Defoliation spruce trees from spruce budworm

Defoliation from spruce budworm.

Name that creature!

By Val Cervenka, Forest Health Program Consultant

The forest health team is frequently called upon to identify organisms other than forest pests. Our skills were tested recently when we received a photo of something most of us had never seen—and maybe the same goes for you.

Rachael Nicoll reached into her memory of weird and wonderful organisms and quickly came up with a name for this bizarre critter: chocolate tube slime mold, also called tree hair or pipe-cleaner slime.

Slime molds are not fungi, but the chocolate slime mold (Stemonitis species) has pipe cleaner-like stalks that bear spores similar to fungal spore-bearing structures. Using a mass of protoplasm, the slime mold can move about like an amoeba over surfaces and absorb particles of food. They are usually found on wood, although the one pictured must have found something good to eat on a windowsill! Within days it had dried up, crumbled, and was washed away by rain.

You learn something new every day.

Chocolate tube slime mold. Photo by Amber MacLaughlin Photography, Perham, MN.