Forest Insect and Disease Newsletter - Spring 2018

Statewide Forest Health Issues

By Val Cervenka, DNR forest health program coordinator

DNR Launches New Firewood Program

firewood pile by fire ring

New firewood regulations went into effect this year to keep invasive pests out of our state parks and state forests.  The new rules simplify things for visitors while keeping our trees safe from forest pests that could kill them.

The law does away with the DNR's list of approved firewood vendors and instead gives the public several firewood options:

  • You can purchase firewood from the DNR.
  • You can purchase firewood from a vendor who verifies it is not ash and was harvested within the same county as the state park or state forest you're visiting (the wood must have a label showing the county of harvest).
  • You may use firewood harvested in Minnesota that the Minnesota Department of Agriculture has certified to be insect-free.
  • You may use firewood that is kiln-dried, clean (unpainted and unstained) dimensional lumber free of metal or foreign substances.
  • You may use manufactured logs

If you purchase firewood to bring onto DNR lands, be sure to get a receipt from the vendor that contains the vendor's name, contact information, the quantity of firewood, the county where the firewood was harvested, and date you purchased it. You will need this receipt to present to DNR staff if asked.

Are these rules really necessary?

Firewood is the number-one way invasive species travel from forest to forest, so it's the best place to target our prevention efforts. We don't want invasive insects hitching a ride on firewood, moving into our state parks and state forests and killing our trees.

If you grew up bringing firewood from home when you went camping, you're probably wondering what all the fuss is about now. The issue is emerald ash borer.

Back in the mid-2000s, emerald ash borer started killing millions of ash trees in states just east of Minnesota. But since the bug wasn't here yet, the Minnesota Department of Agriculture didn't issue a quarantine.

The DNR quickly realized the insects could infiltrate state lands anyway by traveling in on firewood, so in 2007, the legislature passed a law that allowed only "approved" firewood in State Parks. That meant any firewood used in state parks had to be kiln-dried dimensional lumber; certified (heat-treated and insect-free) by the Minnesota Department of Agriculture; sold by a DNR-approved vendor that harvested the wood in Minnesota within 100 miles of the state park where it would be burned (in 2009 that distance was reduced to 50 miles), or sold by the state park itself.

The DNR's Parks and Trails Division oversaw education and enforcement of the law, while the Forestry Division maintained a database of approved firewood vendors.

Why revise the rules?

Over the past decade, things have changed: the Minnesota Department of Agriculture quarantined counties for emerald ash borer and certified more firewood as heat-treated and free of insects—and the Forestry Division's approved-vendor database software became obsolete.

Meantime, people had a hard time figuring out the difference between "DNR-approved" firewood and firewood "certified" by the Minnesota Department of Agriculture.

So, the DNR decided to look into whether the firewood law could be simplified. That led to this year's new firewood rules.

From now on, the Division of Parks and Trails will oversee the firewood program, since most firewood use on DNR lands happens in state parks, state recreation areas, and state forest campgrounds and day-use areas.

The next time you go camping, you can find approved firewood by logging onto Firewood Scout. Just click on the map near your destination and the system will give you the names of several nearby firewood vendors.

We hope the new rules make things easier to find firewood and keep invasive pests out of our state parks and state forests.


map of minnesota showing central region

Central Region

By Brian Schwingle, DNR forest health specialist


Thousand Cankers Disease Update

Red pines with Diplodia shoot blight from a July hail storm Native range of black walnut (Juglans nigra). Source: Atlas of United States Trees.

In the early 2000s in the western United States, thousands of black walnut trees (Juglans nigra) were rapidly dying of an unknown cause. In 2008, scientists identified the cause as thousand cankers disease of walnut (TCD). TCD is a disease involving complex relationships among several organisms including a fungus and its associated walnut twig beetle, both of which appear to be native to the western U.S. The beetle and fungus normally inhabit western walnuts without causing significant damage. However, black walnut trees are not native to the western United States and are more susceptible to TCD than other Juglans species native to the West.

In 2010, a Tennessee Department of Agriculture forester discovered TCD in Tennessee, in the heart of black walnut's native range (see the range map), likely a result of transporting infested black walnut logs, firewood, or nursery stock from the West. People feared that TCD would spread throughout the entire range of black walnut, killing millions of trees and devastating a large commercial industry. Since then, TCD has only been confirmed in limited areas within Maryland, North Carolina, Ohio, Pennsylvania, and Virginia.

TCD is a disease caused by a complex relationship of organisms that scientists are still unraveling, but it has not proved to be the quick-moving, aggressive tree-killer some feared. Virginia Tech professor emeritus of plant pathology Gary Griffin found that a black walnut tree was able to recover from TCD after sufficient rainfall, and Jennifer Juzwik, plant pathologist at the U.S. Forest Service Northern Research Station, found the trees could start to heal over cankers caused by the fungus associated with TCD.

However, since the impact of TCD remains questionable in the eastern United States, and since TCD can also infect Minnesota's endangered butternuts, the Minnesota Department of Agriculture will continue to enforce its external quarantine of regulated walnut materials as a cautious approach to preventing any potential introductions of invasive walnut pests and diseases into the state.

Because many other forest health issues cause symptoms similar to TCD in Minnesota (for example Fusarium canker, frost damage, and drought damage), it's nearly impossible for homeowners to diagnose their own black walnut trees. The DNR forest health unit, with help from the Minnesota Department of Agriculture and the USDA Forest Inventory Analysis, will continue to monitor for TCD in Minnesota.

Needlecast on stressed black spruce not uncommon

Black spruce with sickly middle and lower canopies associated with the needlecast fungus StigminaBlack spruce with sickly middle and lower canopies associated with the needlecast fungus Stigmina.

Colorado blue spruce (Picea pungens) is common in urban landscapes as an ornamental tree and is famous for suffering from needlecast diseases in Minnesota. Several different types of fungi can cause needlecast diseases on spruce. They typically infect needles in the lower and middle canopy, initially turning the needles off-green and eventually causing them to fall off. White spruce (Picea glauca) and Black Hills spruce (Picea glauca 'Densata') may not get needlecast diseases as frequently as Colorado blue spruce, but we commonly see those species suffering from needlecast diseases too, both in rural and urban settings.

On a recent forest health mission into a Pine County tamarack and black spruce bog, Forest Health Specialist Brian Schwingle finally collected samples of something he's commonly seen over the years: black spruce (Picea mariana) with needlecast disease. Brian submitted samples to the University of Minnesota Plant Disease Clinic, who confirmed that a fungus in the genus Stigmina was associated with the sick needles and branches on the samples.

For years, people considered Rhizosphaera kalkhoffii to be the main type of fungus responsible for causing needlecast on spruce in Minnesota. More recently, we have learned that Stigmina lautii is commonly associated with needlecast. In fact, according to the Plant Disease Clinic's records, Stigmina was much more common on samples in 2017 than Rhizosphaera and Lirula (another fungus that causes needlecast).

So what does this mean? We know that both white and blue spruce can be more susceptible to Rhizosphaera needlecast when stressed. From frequent observations of needlecast on black spruce, it seems clear that stressed black spruce are more susceptible to needlecast diseases too. In the case of black spruce, however, it could be Stigmina rather than Rhizosphaera causing the majority of the damage.

In natural forests, needlecast diseases simply signify stress (from drought or insect feeding, for example) or overcrowding. You can decrease needlecast diseases in managed forests, by encouraging a more diverse forest and thinning before trees suffer from overcrowding or spruce budworm defoliation.

In yard settings, in windbreaks, or in privacy plantings, one easy way to avoid needlecast disease is to diversify the trees you plant. Consider planting at least two types of evergreen (if you desire year-round green foliage) and planting different types of evergreen next to each other. If branches in a spruce windbreak are starting to contact each other, consider removing every other tree. For isolated yard spruce, irrigate them with a drip irrigation hose during a drought to reduce stress, and make sure you are not hurting them with herbicides or lawn fertilizers.

Several on-line publications or websites discuss needlecast diseases in more detail. The University of Minnesota Extension discusses spruce diseases in detail in Diseases of Spruce Trees in Minnesota, and Rhizosphaera and Stigmina are the focus in Two Needle Diseases of Spruce. Find a complete list of spruce pest and cultural issues in Spruce Problems.


map of minnesota showing northeast regionNortheast Region

By Jess Hartshorn, DNR forest health specialist


Cold temperatures not a cure-all for pests

Two gypsy moth caterpillars feeding on white oak leavesTwo gypsy moth caterpillars feeding on white oak leaves in State College, Pennsylvania

If you have spent your winter looking for a silver lining behind the incredibly cold temperatures, we have both good and bad news for you. The good news is that some insect pests, like invasive gypsy moths, are not very cold hardy in the absence of snow. Gypsy moth females lay eggs on just about any surface, and when that surface is exposed to temperatures below -20°F, eggs die relatively quickly. When they lay eggs lower on a tree, more of them survive due to the insulating effects of snow. This winter we have seen temperatures reaching -20°F and lower in many places in northern Minnesota. In these areas, we can expect fewer gypsy moth eggs to survive.

The bad news is that these temperatures are not cold enough to kill off all gypsy moths or any other pest species entirely. Arthropods (insects, spiders, ticks, and related animals) have many ways of dealing with the cold, including the creation of their own antifreeze. Many insects and ticks are able to avoid the cold altogether and burrow into the soil to insulate themselves. While we can expect some emerald ash borers to die at temperatures as warm as -14°F, it takes temperatures of at least -30°F to kill a significant number of this invasive insect. This is because wood-boring pests like emerald ash borer spend the winter under the bark of trees. Bark provides insulation against the cold, and if temperatures rebound quickly, they are even less likely to experience mortality.

Overall, we may see lower numbers of a few insect species in 2018, like gypsy moths. This minor setback does not mean much in the long-term, however. This winter will not be enough to kill all emerald ash borer, gypsy moth, or any other pest of concern. For more information, listen to Jess Hartshorn's interview on KAXE Phenology.


map of minnesota showing northwest region regionNorthwest Region

By Mike Parisio, DNR forest health specialist


Tips to Know your Winter Brooms

Identifying eastern dwarf mistletoe (EDM) quickly and confidently in the field is not always an easy task, especially when there are multiple organisms capable of causing brooms on spruce trees. So without a close-up look, how can you tell whether that broom all the way up there is due to EDM or spruce broom rust? Fortunately, the easiest time to tell the two apart is during the winter months, when foresters spend the most time out in the woods.

In winter, and if the tree and the broom-bearing branch are clearly still alive, the key is to determine whether the broom itself is bearing needles. Live brooms caused by EDM will bear needles year-round and will continue to do so through the winter. EDM is a parasitic plant, so if you can figure out a way to get a broom in hand, a close-up look will also reveal small growths known as aerial shoots on the branches of the host tree near the broom.

three image showing various states for winter brooms A. Bemidji area forester Andy Kernan stands beneath a "trophy" EDM broom B. Aerial shoots of EDM growing on a spruce branch C. A dead spruce on the forest floor killed by multiple EDM brooms.

On the other hand, brooms caused by spruce broom rust drop their needles during the winter months, which will regrow the following season, provided the broom has not killed the branch or tree it's growing on. During the rest of the year, needles growing on a broom caused by spruce broom rust are often stunted and very pale green or yellow, eventually developing an orange hue toward late summer when the rust fungus starts to produce spores. Be careful not to confuse these brooms with dead EDM brooms during the winter months—EDM is much more common, so do your best to find live EDM brooms with needles to make a confident diagnosis.

Two images showing fir and spruce broom D. Similar in appearance to spruce broom rust, a broom caused by fir broom rust on a balsam fir that has shed its needles for the season E. Spruce broom rust during the growing season demonstrating stunted and discolored needles. Photo by USDA Forest Service