Forest health


August 2016 newsletter - Forest Insect and Disease Newsletter

Oak wilt confirmed in far northern Pine County and found in Waseca County for the first time

By Brian Schwingle, Central Region forest health specialist

Known range of oak wilt in Minnesota.
Known range of oak wilt in Minnesota. All oaks in red zones are at great risk of contracting oak wilt if wounded from April through mid-July.

The known range of oak wilt took a large jump northward in Pine County in early August with its discovery on private property near Kerrick. DNR is working closely with the landowner to help eradicate or manage the infected zone. Pine County Soil & Water Conservation District and the DNR are currently working on sending out an alert to nearby residences. In northern Minnesota between April 15 and July 15, people within 20 miles of known oak wilt should avoid wounding oaks or thinning oak forests. Not injuring oaks in that timeframe is a highly effective way to avoid infection.

In addition, oak wilt was confirmed for the first time in Waseca County at the end of June. The property in question has a large mortality center of oak wilt in a bur oak woodlot. DNR has been working with the landowner on control.


Bur oak blight back again

By Brian Schwingle, Central Region forest health specialist

As in previous years, bur oak blight has reared its ugly face on scattered bur oaks for the last few weeks in Minnesota. Symptoms will continue to increase until autumn leaf-drop, but those oaks will produce a healthy set of new leaves next spring, so don't cut down a bur oak just because of bur oak blight. For additional information, read "Bur Oak Blight: the New Norm" in our August 2015 newsletter.


Not always BOB - Botryosphaeria twig canker on bur oak

By Mike Parisio, Northwest Region forest health specialist

Although some of us might be quick to dismiss the site of scattered dead leaves on a bur oak as just one more case of seemingly ever-present bur oak blight (BOB), a closer inspection might reveal another disease at work that can sometimes look like BOB from a distance - Botryosphaeria twig canker. This was the case during our recent outing to inspect some ailing oaks near the northern end of Itasca State Park.

Canker wound on bur oak twig with visible fruiting bodies caused by Botryosphaeria
Canker wound on bur oak twig with visible fruiting bodies caused by Botryosphaeria.

Botryosphaeria twig canker is identified by dark lesions or wounds on the bark of twigs that extend 4 to 6 inches inward from the tips of affected branches. Wood tissue beneath the canker will be discolored and leaves at twig tips or within or at the end of the canker zone will be wilted and brown but remain attached to the tree. Discoloration of wilted leaves is uniform and will lack the distinct wedge-shaped leaf lesions associated with BOB. The small black fruiting bodies of the Botryosphaeria fungus responsible for spore production can also be found on twigs within the canker zone and usually become visible during late summer and fall.

Dead, wilted leaves on multiple twigs of a single branch affected by Botryosphaeria fungus
Dead, wilted leaves on multiple twigs of a single branch affected by Botryosphaeria fungus

Though Botryosphaeria twig canker can be quite extensive throughout affected stands and discolored leaves may be abundant in the canopy of affected trees, twig canker infections remain localized to branch tips and generally cause little damage to overall tree health. Like several other tree diseases affecting Minnesota's forests, stressful environmental conditions can increase both the spread of Botryosphaeria and widespread appearance of visible symptoms. We suspect that a major stress event in 2015 and abundant rainfall this season are responsible for the dramatic appearance of the bur oak stand pictured here. Fortunately, localized outbreaks such as this will usually subside after a year or two.


Scattered sickly green ash in central and southern Minnesota

By Brian Schwingle, Central Region forest health specialist

Green ash with upper and lower canopy dieback.
Green ash with upper and lower canopy dieback.

Green ash trees that look sick are widely scattered throughout much of central and southern Minnesota. They have dieback in various parts of their canopies, usually starting in the lower canopy. Before we get started here, you might be thinking, "Isn't EAB found everywhere by now?" The short answer is, "No, not at all." (Check out the Minnesota EAB distribution map This link leads to an external site. if you want to know where EAB has been found.)

The likely cause of this dieback is consecutive years of heavy seed crops, severe anthracnose, and late spring frosts.

Many tree species occasionally produce seed crops so large that individual branches are killed. It often appears that green ash produce the most seeds in their lower canopies, but heavy seed production also happens on upper canopy branches.

Anthracnose is a fungal leaf infection that increases in abundance with overly wet conditions during leaf emergence, and can completely defoliate ash during leaf elongation periods. During this same time, frost can cause some trees to simply drop their leaflets. Lower canopies can be severely impacted by anthracnose and frost damage, both of which affected communities across large portions of Minnesota in 2015 and 2016.

Some of these ash trees lost over 50 percent of the canopy they ought to have. Taking a positive outlook, those trees provide fantastic opportunities for communities to reduce their street ash tree population to prepare for the inevitable, but unpredictable, arrival of emerald ash borer.


Spruce needle rust

By Jess Hartshorn, Northeast Region forest health specialist

The wetter-than-average summer has brought with it a widespread bout of spruce needle rust. This disease is caused by at least ten native species of Chrysomyxa fungus. Colorado blue spruce, white spruce, and black spruce are all susceptible, while Norway spruce is only occasionally infected. The rust requires an alternate host which, in Minnesota, is often Labrador tea, leatherleaf, and bearberry.

Yellow and orange needles with projections growing from infected needles.
Yellow and orange needles with projections growing from infected needles. Photo by Joe O'Brien, US Forest Service.
Witches' broom symptom in white spruce from infection with spruce needle rust.
Witches' broom symptom in white spruce from infection with spruce needle rust. Photo by Joe O'Brien, US Forest Service.

While spores can form every year, they are more abundant in cool, wet springs. Spores are released from infected leaves on alternate hosts and go on to infect young spruce needles. Current year needles turn yellow in the summer and white to orange blisters appear on infected needles in July or August. Infected needles fall off in September, leaving spruce trees thinner in appearance. Most of these rust species release powdery spores in late summer that overwinter in an alternate host. Some species, however, can cause symptoms called witches' brooms and overwinter in spruce needles. Neither of these instances results in a re-infection of the damaged spruce trees the following year.

In most cases, spruce needle rust is cosmetic and does not require management. To reduce disease, consider redirecting lawn sprinklers away from spruce trees. When planting, provide adequate spacing between trees for proper air flow. Unsightly witches' brooms can be pruned out. Removing alternate hosts within 1,000 feet of spruce trees may reduce disease occurrence but is often impractical. Severe infections do not typically occur many years in a row so chemical control is not necessary.


Weird curled leaves on yard black ash in southern Minnesota

By Brian Schwingle, Central Region forest health specialist

Black ash yard trees in southern Minnesota? Yup. They're actually lovely trees.

In late July and early August I received three reports of unhealthy black ash yard trees in southern Minnesota. Though this isn't a concern whatsoever for the forest as a whole, it's such an interesting incident I have to describe it for you.

A black ash yard tree with distorted leaves.
A black ash yard tree with distorted leaves.

The cause of the deformed, partially-browned, curled leaves is feeding by a non-native insect called the cottony ash psyllid. The nymphal stage makes its home under a cottony "house" it produces along leaf edges that curl over or along leaf mid-veins and feeds there using its piercing-sucking mouthparts. I can't predict the impact the psyllids will have on black ash trees, but I recommend that homeowners take steps to keep trees healthy and well-watered, or heavily-infested trees may die within a year or two of infestation. More information may be found in this flier on cottony ash psyllids This link leads to an external site.PDF.


A "Tinge"-idae of discoloration

By Mike Parisio, Northwest Region forest health specialist

Stippled appearance of cherry leaves demonstrating different degrees of discoloration caused by cherry lace bug feeding damage.
Stippled appearance of cherry leaves demonstrating different degrees of discoloration caused by cherry lace bug feeding damage.

Named for the delicate appearance of their outer wings, lace bugs in the insect family Tingidae are common throughout Minnesota and can sometimes be responsible for the leaf discoloration of numerous species of trees and shrubs. The surfaces of leaves discolored by lace bug feeding take on a distinct spotted or stippled appearance, caused as immature and adult lace bugs create countless feeding injuries on the leaf underside. These injuries are made by specialized piercing mouthparts common to all true bugs, which are inserted into plant tissues and used to suck out liquid nutrients. When lace bug populations are high enough, significant discoloration of trees can become quite apparent on the landscape.

Underside of bur oak leaf showing oak lace bug nymphs, adults, and characteristic dark excrement spots.
Underside of bur oak leaf showing oak lace bug nymphs, adults, and characteristic dark excrement spots.

If you happen upon this type of leaf discoloration at this time of year, take a moment to examine the undersides of affected leaves for lace bug evidence. At just a few millimeters in length, adults are small but easily identified by their distinct lacelike wings held flat over the body. Less distinct are the wingless lace bug nymphs, which can easily be confused with immature life stages of other common insects. Fortunately, lace bug nymphs and adults are frequently found together, making field identification of nymphs somewhat less puzzling. Other tell-tale signs of lace bugs to look for include outer skins shed during molting and dark excrement spots, both of which remain attached to the underside leaf surface.

Different species of lace bugs have specific hosts or small groups of host plants and can be found on a wide variety of trees and shrubs. Examples of host plants include alder, basswood, birch, cherry, elm, hackberry, hawthorn, oak, willow, and others. So far in 2016, increased lace bug activity has been observed in northern Clearwater County on bur oaks and in Pine County on cherries. Despite substantial discoloration, lace bug feeding damage is largely a cosmetic concern and in most cases trees suffer little or no ill effects. If you decide to attempt lace bug control on ornamental trees or shrubs, consider non-chemical methods such as dislodging flightless nymphs from the undersides of leaves with a garden hose. Methods like this preserve natural enemies that can help reduce and regulate lace bug populations in the future.


Yellownecked caterpillars

By Jess Hartshorn, Northeast Region forest health specialist

Cluster of mature yellownecked caterpillars feeding on blueberry and presenting a defensive posture.
Cluster of mature yellownecked caterpillars feeding on blueberry and presenting a defensive posture. Photo by Jerry A. Payne, USDA Agricultural Research Service.

This month I have received a few emails from concerned homeowners in south St. Louis County reporting yard tree defoliation, mostly on mountain ash. The culprits have been yellownecked caterpillars. These common, native insects are found across the eastern United States and Canada. In the late summer and early fall they can be key pests of shade trees such as basswood, mountain ash, elm, and various fruit trees. Ornamentals can be seriously defoliated while forest trees are typically not victims. Adult moths emerge in June with mating and egg-laying taking place soon after. Eggs hatch within a few days and young larvae cluster at the ends of twigs and branches, feeding in large groups. Like many other gregarious larvae, they rear up into a defensive position when disturbed. These young larvae are light-colored with stripes of various colors along their bodies, and have black heads. As they mature they become darker, eventually turning black with white to yellow stripes. Mature larvae drop to the ground and overwinter 2-4 inches into the soil.

Leaf skeletonization by yellownecked caterpillars.
Leaf skeletonization by yellownecked caterpillars. Photo by Lacy L. Hyche, Auburn University.

Damage occurs when young larvae skeletonize foliage at the tips of branches. Older larvae continue to feed and devour whole leaves except for large main veins. Damage is first noticed at the edges of crowns and later includes a wider area of the tree. Healthy trees can survive light to moderate defoliation but heavy defoliation can have negative effects on ornamentals. Significant feeding can lead to slower tree growth, smaller future buds, and branch dieback. Several years of heavy defoliation can eventually result in tree death.

Egg mass on underside of walnut leaf.
Egg mass on underside of walnut leaf. Photo by Lacy L. Hyche, Auburn University.

Monitor for egg masses in July and remove and destroy infested foliage. If larvae are present in low numbers they can be hand-picked or knocked off infested branches. Knocking them to the ground makes them vulnerable to bird and insect predation. Use of pesticides is not recommended, since these late-season defoliators usually do no long-term damage.


Fall webworms appearing

Fall webworm nests on birch in northeastern Minnesota.
Fall webworm nests on birch in northeastern Minnesota.
Fall webworm caterpillar, hanging out in its pad in northeastern Minnesota.
Fall webworm caterpillar, hanging out in its pad in northeastern Minnesota.

By Brian Schwingle, Central Region forest health specialist

For the last couple of weeks in central and southern Minnesota I have noticed fall webworms making their messy nests at the ends of branches on various hardwoods. Jessica Hartshorn reports the same nest-making activity from fall webworm in parts of northeastern Minnesota. Their current nest size and general level of leaf destruction is just starting to become obvious. In general, the best thing to do on small trees in yard situations is simply rip out the nests and squash caterpillars underfoot. In forests, these insects won't significantly harm trees. For additional information, read this fall webworm This link leads to an external site.fact sheet.



Yellow-headed spruce sawfly

By Jess Hartshorn, Northeast Region forest health specialist

Damage by YHSS on young ornamental white spruce trees.
Damage by YHSS on young ornamental white spruce trees. Photo by Steve Katovich, US Forest Service.

Driving through residential areas of the northeastern part of Minnesota, you may see some sad-looking ornamental spruce that look like they had a run-in with an angry barber. The defoliation was caused by yellow-headed spruce sawflies (YHSS)—native pests that feed on all native spruce as well as Norway spruce, and damage from this insect is easily visible now in late summer. Although the window for control this year has closed, defoliation of young, ornamental spruce trees is a sign that control may be needed next year. Open grown, young (5-9 years old, 3-18 feet tall) ornamental spruce trees are most vulnerable to YHSS attack. Once trees mature and the canopy closes, they are no longer a threat.

Larvae are currently spinning silken cocoons in the top 1-2 inches of the soil under or near spruce trees, waiting to emerge as adults next year in late May and early June. After emerging, adults mate and females lay eggs in the new shoots. Eggs hatch four to twelve days later and young larvae begin feeding on new growth. Larvae mature and go on to feed on previous years' growth, and at this point most of the damaging defoliation has already occurred. Healthy trees can tolerate light to moderate defoliation and not be significantly affected. Heavy defoliation can slow tree growth, kill terminal buds resulting in branch forking, and cause buds to form later and be smaller in following years. Heavy defoliation for three to four years can cause tree death.

YHSS larva showing yellow body with green stripes, orange head, and prolegs.
YHSS larva showing yellow body with green stripes, orange head, and prolegs. Photo by Steve Katovich, US Forest Service.
A group of mountain ash sawflies form defensive S-shapes when startled.
A group of mountain ash sawflies form defensive S-shapes when startled. Photo by Steve Katovich, US Forest Service.

Monitoring is very important in order to accurately identify YHSS and decide on a method of control. In early to mid-June, look for yellow larvae with orange-red heads and six green stripes along their backs. Larvae may look like caterpillars (moth and butterfly larvae), but sawflies are actually stingless wasps. They have six prolegs (the stubby legs along the body of the larva) which are not present on moth larvae such as spruce budworm. Sawflies also have a characteristic defense strategy of curling into an S-shape when startled.

The perfect time to implement control measures is when sawfly larvae are young, as the damage has not yet been done and young larvae are most susceptible to control measures. In years of light infestation, hand-pick or use a heavy stream of water to remove clusters of larvae feeding on new shoots. Knocking them onto the soil makes them vulnerable to predation. If the infestation is severe, pesticides may be used on these groups of young, feeding larvae. There are more than a dozen different chemicals labeled for sawfly control. It is important to stop spraying by late June since these chemicals also kill beneficial parasites and predators that normally reduce sawfly populations, and mid-summer is the time that natural enemies are most abundant. For more information on sawfly control, read the University of Minnesota Extension publication, Sawflies of Trees and Shrubs.This link leads to an external site.