Forest Insect and Disease Newsletter July 2015

Brain Schwingle, Forest Health Specialist, Central Region

Forest Health Farewell to Minnesota's Forest Health Gurus

Mike and Jana Albers are probably boating off, right now, into a mosquito-obscured sunset on a northern Minnesota lake. They retired in early June after helping foresters and landowners figure out what was killing their trees for a combined 73 years. Their helpfulness, guidance, and experience will be sorely missed for many years. In their absence, do not hesitate to contact Brian Schwingle, central region forest health specialist, for puzzling, significant, and concerning forest death and sickness in northern Minnesota.

Mike Albers
Mike Albers
Jana Albers
Jana Albers


So Far So Good on Red Pine Shoot Blight in 2015

photo of red pine lateral shoots

2015 red pine lateral shoots overtopping the 2014 shoots killed by Diplodia. Diplodia could kill these 2015 shoots later on this growing season, or the disease may lie dormant until a stressful time for this sapling.

In 2014 there was an abundance of early growing season Diplodia shoot blight over most of Minnesota which cast a great shadow of doubt on the survival of any understory red pines. Despite this disease abundance, the incidence of Diplodia shoot blight on 2015 red pine growth is much lower than in 2014. Even though the understory red pines look spritely right now, past experience paints a dismal outcome down the road for the red pines that showed so much shoot blight in 2014.


Anthracnose on Oaks Popping Up

photo of Lower canopy leaves of an oak showing typical anthracnose symptoms

Lower canopy leaves of an oak showing typical anthracnose symptoms, Chippewa Co. MN, mid-June 2015 (photo by G. Russell, MN DNR)

Anthracnose, a spring and early summer leaf disease on broadleaf trees, is common on white and bur oaks across much of Minnesota. Anthracnose causes distorted, brown areas of leaves; the most severe disease occurs in the lower canopy. Some infected trees look ugly, but the good news is that healthy broadleaf trees can tolerate anthracnose. The worst that will occur on healthy white oaks is some minor twig dieback. They will produce a second set of leaves to replace the older infected ones.

For bur oaks that have suffered from severe bur oak blight (BOB) over consecutive years, a heavy anthracnose infection adds insult to injury. Bur oak blight is a leaf disease distinct from anthracnose, and symptoms of BOB won't show up until late July or August. For valuable ornamental bur oaks that are showing symptoms of long-term stress, tree owners could consider knocking back anthracnose and BOB for several years with an injected fungicide. Symptoms of stress are dieback and epicormic branches. Tree owners should consult an experienced ISA-certified arborist if they are interested in chemically protecting their bur oaks.


Lack of Leaves in Ash, Elm, and Maple Crowns

photo silver maple

A silver maple that produced an immense amount of seed in spring 2015 now has an extremely thin crown with expanding leaves in mid-June. This maple will have noticeable dieback in its crown for a few years.

Scattered ash, Siberian elms, and maples across Minnesota have thin crowns. Red maples, silver maples, and Siberian elms produced large amounts of seed, making their crowns look thin. Widely scattered silver maples produced immense amounts of seed, leaving their crowns appear almost bare. Nearly all of these trees should recover normal leaf density over the next several years, but dieback will occur in some parts of their crowns. The reason for the large seed crop could be because of a natural seed cycle or a variety of stressful environmental events, or both.

Ash trees across a large area dropped many leaflets suddenly over Memorial Day weekend. We confirmed symptoms of anthracnose in many of the affected ash crowns, and anthracnose is the likely culprit for this sudden leaf drop. The above-average warmth coupled with precipitation on the weekend of May 16 and 17, followed by below-average cold, followed by warmth and moisture on Memorial Day weekend, likely set the perfect stage for anthracnose. Expect these ash trees to recover.


Light Leaf-feeding on Basswoods

A secretive creature lightly nibbled on basswood leaves across a large portion of central and southern Minnesota. I found no caterpillars or caterpillar frass on these basswoods, but what I did find one night trying to catch "pretty bugs" for my daughter's class were many June bugs (a.k.a. May beetles) feeding on these leaves. These portly beetles pose no threat to the trees. They do threaten one's eyes when searching for them with a headlamp, as they are highly attracted to light and have to fear of flying full speed into a stationary, blinking object.


No Forest Tent Caterpillar Defoliation Reports Thus Far

I am not aware of any report of sizeable defoliation by forest tent caterpillars yet this spring anywhere in Minnesota. Older forest tent caterpillars can now be seen marching across forest floors, roads, and resting on buildings, which means we are beyond the point where we will see large outbreaks this year: good news again!


Lecanium Scale Outbreak in West-Central Minnesota: Expect Sooty Mold!

The quantity of lecanium scales sucking on broadleaf branches is remarkable in Meeker and Kandiyohi counties. Lecanium scales look like warts along branches. This outbreak will be quashed in 2016 from a variety of natural factors. In the meantime, expect sooty mold underneath infested trees. Sooty mold is a black fungus that grows on honeydew, which is excreted by the scales, and is generally not a concern for trees. In extreme situations, a few heavily infested understory saplings may die, and dieback may occur on overstory trees. This article on lecanium scale includes control options for high populations of the scale.

photo of Honeydew dripping out of lecanium scales

Honeydew dripping out of lecanium scales. All the wart-like bumps along this branch are lecanium scales, Kandiyohi Co. MN, early-June 2015.

photo of Honeydew dripping out of lecanium scales

A basswood leaf flecked with honeydew from a lecanium scaled-infested branch above.


White Fluffy Stuff on White Pine Bark

photo of Honeydew dripping out of lecanium scales

It is common to see scattered, large white pines covered with pine bark adelgids in southeast Minnesota, like the tree in the foreground. Note the two white pines on the far left of the photo in the background that are white-washed from adelgids high on their trunks.

Ever see a white pine trunk covered in white fluffy stuff? From a distance, the trunks look white-washed. It is not unusual to see a few white-washed white pines in every plantation in southeastern Minnesota, and recently I got a report of some on saplings near Mille Lacs Lake. The fluffy stuff is the pine bark adelgid, which is not a concern for healthy, large white pines, but it can kill stressed saplings. Concerned shade tree owners should consult this pine bark adelgid article for control options.


The Only Significantly Noticeable Tree-Defoliating Caterpillar in Spring 2015

Drum roll! And the prize for the Only Significantly Noticeable Tree-Defoliating Caterpillar in Minnesota in Spring 2015 goes to...the locust leafroller (probably Sciota subcaesiella). That's right folks, the locust leafroller really chewed away some leaves on black locusts (and honey locusts) from the Twin Cities Metro area up to at least Mora. Don't worry though; I'm certain our black locusts will survive this slight.

photo of Honeydew dripping out of lecanium scales

The locust leafroller is a black-headed, green caterpillar that ties leaves together and eats them.

photo of Honeydew dripping out of lecanium scales

A grove of black locust trees moderately defoliated by the locust leafroller, St. Paul, early June 2015