Forest Insect and Disease Newsletter - October 2019

Too much of a good thing? High precipitation levels impact trees throughout Minnesota

By Rachael Nicoll, Northwest Region Forest Health Specialist

In central and southern Minnesota this spring and summer, you probably noticed a lot of water in your rain gauge. Indeed, this year has been one of the wettest recorded in Minnesota since 1871. On the ground and from the air, we've seen evidence of multiple years of flood damage on trees rimming wetlands. We've also fielded calls about declining and dying trees in water-logged, low-lying areas. Some places in southern and south central Minnesota have had record-breaking levels of precipitation this year. Central Minnesota is also very damp, and only a few areas in Minnesota are very dry, according to the US Drought Monitor.

These wet conditions impact tree species differently. Tolerance of waterlogged soils is not well-understood, but trees that are intolerant of flooding include pines, basswood, white spruce, sugar maple, and red and white oak. Alternatively, elm, red maple, ash, cottonwood, and tamarack can better tolerate having "wet feet," although not forever. Longer duration and frequency of wet conditions are harder on trees, and our abundant precipitation over the last several years has stressed trees in some areas. Also, flood-waters along rivers can dump large amounts of silt and sand over tree roots, causing them to suffocate and die (read "Flooding or high water damage" in the 2018 DNR Forest Health Annual Report to learn how long-term flooding changed the forest in a southern Minnesota state park).

Symptoms of flooding damage include:

  • leaf yellowing and loss
  • small leaves
  • epicormic sprouts (sprouts along the trunk and branches)
  • crown dieback
  • ample seed crop or no seed crop in the years after flooding
  • early fall color
  • death (often after prolonged or severe flooding)

Symptoms of flooding frequently do not show up for a couple of years in relatively flood-tolerant trees, but susceptible trees often die the next spring. Flooding stress can also make trees more susceptible to attack from insects and diseases.

Ultimately, healthy trees are more likely to recover from flooding than unhealthy trees, and maintaining tree vigor is the best way to prevent flooding damage. University of Minnesota Extension has a website sharing recommendations for managing flood damage to trees. However, severely affected trees may succumb to the flooding or opportunistic pests and pathogens. The current 90-day forecast predicts normal levels of precipitation in eastern Minnesota and slightly above average in west-central and southwest Minnesota, but trees are most susceptible to flood damage when they are growing. We will have to wait and see what next year brings. To get the latest updates, the DNR provides monthly information on statewide precipitation trends in the HydroClim newsletter.

 oaks showing sign of decline

Trees died from flooding adjacent to a wetland in central Minnesota

Blighted shoots on white oaks

By Brian Schwingle, Central Region Forest Health Specialist

Starting in August, we noticed a few widely-scattered white oaks (Quercus alba) with heavily blighted canopies around the Twin Cities and southeast to the Iowa border. Shoot blight on oaks can be caused by several pathogens and insects.

We observed that the dead white oak shoots had blackened leaf petioles (the tiny stems that connect leaf blades to twigs) and dark, withered shoot tips. Now (late September) many of these affected white oaks are shedding infected shoots, where the base of the twig is flat, as if it were cut with a knife by a tiny fairy. The University of Minnesota Plant Disease Clinic found a species of Tubakia on samples of affected shoots. Tubakia species are leaf and twig fungal pathogens, and you can read more about them on Iowa State University's Tubakia Leaf Spot in Oaks. Given the abundance of precipitation this year, it is not surprising that this fungal pathogen is causing severe shoot blight on some white oaks.

We suspect that most of these infected oaks will be fine in the long-run and this is only a temporary problem. In general, control of shoot blights on larger oaks is almost never needed for their long-term health. We hope this problem will not worsen over the years due to a wetter climate, as the related pathogen that causes bur oak blight has, and we will monitor these oaks down the road.

In related news, some scattered red and bur oaks also have shoot blight. It appears that a different fungal pathogen, Botryosphaeria, is responsible for the shoot blight on red oak. On bur oaks, bur oak blight certainly is blighting some shoots, but other pathogens could be involved in some cases. As with the white oak story, we suspect this is a temporary malady on scattered oaks.

White oak base of the twig is flat.

Shoot blight on a white oak associated with a Tubakia species.

Shoot blight on a white oak associated with a Tubakia species.

Shed oak twigs with blackened petioles were associated with a Tubakia species.

Shed oak twigs with blackened petioles were associated with a Tubakia species.

Acorns falling early

By Brian Schwingle, Central Region Forest Health Specialist

Starting around mid-July, you might have stepped on many car-flatted acorns on your neighborhood's streets. Maybe you accidently "acorn-skated," similar to slipping on a banana peel. We received a few reports from the Twin Cities metro area and Mankato of abundant acorns dropping early from oaks. We know this was widespread, since our counterparts with Wisconsin DNR reported on it recently.

Similar to Wisconsin, we found acorn "pip" galls on fallen red oak acorns. These galls are found right under the acorn cap, and are made by tiny cynipid wasps. Pip galls cause acorns to fall off the tree, apparently before they are mature. These wasps appear to go through boom and bust cycles, and perhaps their population increased this year due to ample acorn production last year.

Bur and white oaks also were dropping their acorns early. By the time we started investigating these fallen acorns, it was too late to determine the cause, although we were able to rule out weevils as the problem.

A gall from an acorn pip gall wasp.

A gall from an acorn pip gall wasp.

A divot left in the place where the pip gall was.

A divot left in the place where the pip gall was.


Aspen blotch miner

By Eric Otto, Northeast Forest Health Specialist

In northern Minnesota, you may have noticed aspen leaf damage made by a small caterpillar called the aspen blotch miner. Though the aspen trees' appearance is concerning, they can endure the blotch miner's onslaught.

The blotch miner causes crowns of aspen trees to thin. Damage on aspen leaves initially looks like off-color, yellow blotches, but by September the blotches become brown. Infested leaves may eventually curl.

From spring to mid-summer, the aspen blotch miner lays eggs on the undersides of aspen leaves. The eggs hatch and the larvae bore into the leaves and mine the inner contents. Eventually, caterpillars change to pupae inside the leaf, and in August moths emerge. The moths overwinter in protected locations such as under bark scales.

The blotches can look numerous and concerning, but aspen trees can tolerate the damage. After the miner feeds, most aspen trees will send out new leaves. The impact on the overall health of aspen trees is minimal.

Aspen blotch miner

Aspen blotch miner

Start seeing mushrooms

By Eric Otto, Northeast Forest Health Specialist

Click on images to enlarge

With cooling weather and additional rainfall, fall is great time to look for mushrooms. Mushrooms are simply the spore-bearing fruiting bodies of a fungus. It might not be evident to some, but mushrooms are an important component of forest health. They have important roles as decomposers, mycorrhizal associates, pathogens, and wildlife food sources. They also serve as edibles and medicinals for humans. Caution: do not eat mushrooms unless you're certain of their identity.

As decomposers, mushrooms recycle carbon, minerals, and nutrients for other organisms and the soil. Most plants have symbiotic relationships with mycorrhizal fungi that attach to plant roots and increase plant water and nutrient uptake in exchange for carbon. Even though disease-causing fungi can kill trees, they also create dead wood to be recycled and produce gaps in the forest canopy that increase plant diversity. Many mammals and microorganisms also rely on fungi as an important food source.

As you are walking through the woods this fall there are a few mushrooms you might see. When most people close their eyes and think of a mushroom, they often picture the "fly agaric," or Amanita muscaria. This mushroom can be found in aspen-birch forests. It will have a yellow to red cap with white scales. It has gills under the cap and a central stipe (stem). This mushroom has a mycorrhizal association with hardwoods and conifers. This mushroom is poisonous and should not be consumed.

The oyster mushroom (Pleurotus ostreatus) is another mushroom that can be found in aspen-birch forests or hardwood forests. The cap is white to tan with a lateral or absent stipe and gills that run down the stem. These mushrooms are wood-decay fungi and can be found growing on dead and dying trees and logs.

If you see what looks like a white softball or soccer ball in the forest, it could be a giant puffball (Calvatia gigantea). These have white and leathery skin that turns more yellow-tan when mature. They can be found in hardwood forests and serve as mycorrhizal associates. Giant puffballs can produce seven trillion spores that can be disseminated by the wind.

A rare and excellent find is bear's head tooth or the coral tooth fungus (Hericium coralloides). This mushroom has a single stem where it branches into clusters of white spines that point down. With age, the mushroom darkens to yellow or brown. They act as decomposers and decay hardwood logs and thus are found mostly in hardwood forests.

White pines look yellow in northwest Minnesota

By Megan ONeil, Northwest Region Forest Health Specialist

Throughout the middle and later part of the summer I received calls from private landowners and DNR foresters about yellowing white pines. The symptoms occurred too early to be seasonal needle drop, the annual shedding of older needles. The summer reports described straw-colored newer needles, which is unusual. After doing some investigation and ruling out common needle issues, I sent samples to the University of Minnesota Plant Disease Clinic for a closer look.

The clinic found Septorioides strobi, a fungus associated with the diseased needles. The clinic has only diagnosed one other case of Septorioides strobi in Minnesota, but it is not known if it is truly new to the state or just newly reported and diagnosed.

I decided to do some more digging to see what else I could find out about Septoriodes strobi. I quickly learned many states in the Northeast have reported finding Septoriodes strobi on white pines and have suggested that high precipitation in the spring of the previous year created favorable conditions for the pathogen. When found in combination with three other fungi they create a disease complex called white pine needle decline. A changing climate has been linked to the increase in the disease. Luckily only Septoriodes strobi was found on our sample.

As summer quickly turns into fall in northwest Minnesota, I expect to see more yellowing in white pine, and I am sure that I will receive a few phone calls about it. Most of what I will now be seeing can probably be attributed to normal autumn needle drop, but the discovery was a good reminder to keep my eyes peeled for things that are slightly different than what we are used to seeing.


Yellowing tips on white pine


Yellowing white pine


Much ado about nothing

By Val Cervenka, Forest Health Program Consultant

The emails start in August. Concerned citizens contact the DNR Information Center, horrified at the "thing in the tree" or at the trees "dying" along the highway. What are they up-in-arms about? Fall webworm.

In late summer, fall webworm caterpillars construct large, filmy webs at the ends of hardwood tree branches and feed on the leaves within (I would call the nests ugly, but there is already something called an ugly nest caterpillar). They are not picky about what they eat, feeding on some 90 different tree species.

While the nests are noticeable enough to raise alarm—and to be fair, they seem to be more abundant this year than last—the caterpillars do no lasting damage to otherwise healthy trees. They feed gregariously, enlarging their silken home as they eat the leaves within. Caterpillars leave the nest five or six weeks later to form pupae at the base of trees.

Bottom line: the fall webworm caterpillars require no treatment. If you can reach the nest and can't tolerate the site of it, rake it out of the tree or use a stick to pull it down. The webbing usually hangs on until wet snow drags it down. Fall webworms generally don't nest in the same tree year after year—but we will see them again, late next summer.

filmy nest with fall webworm inside on tree limbs

Fall webworm. Photo: Rick Golla