Forest Insect and Disease Newsletter

What's up with that?

Are my spruce trees dying?

photo: Three infected white spruce with spruce needle rust

Three infected white spruce

photo: Close up of infected pine needle with spruce tree rust

Infected current year needles

photo: Infected Blue Spruce with Spruce needle rust

Infected Colorado blue spruce

No, your spruce trees are not dying and, yes, it is too late in the season to spray anything on the trees this year.

Many homeowners in northern Minnesota have noticed their spruce trees turning tan, pink, orange or yellow recently. The needles are infected with a spruce needle rust fungus. It is more of an aesthetic problem and seldom a tree health problem. Infections by the spruce needle rust fungi, Chrysomyxa ledi and Chrysomyxa ledicola, look bad but aren't serious for most trees. However they can be tough on newly planted trees and Christmas trees. The rust is most common on blue spruce but can also infect white and black spruce as well.

The infected current year needles will turn yellow and fall off the tree later this fall. The tree will not grow new needles in place of the needles that have fallen off. However, the tree will have healthy buds on the ends of the branches and these will produce new needles next year. The fungus only infects the current year's needles. It does not infect the shoots or branches of the tree or older needles on the tree. So, healthy trees survive the infection with little or no long term damage. And, next year, homeowners probably will not see any needle rust on their trees.

In some years, like this one, spruce needle rust is very common; but in most years it's hard to find. The fungus requires other plants such as Laborador tea or leather leaf to complete its life cycle. Nature is so cool. In early summer, the rust fungus produces spores on leaves of Laborador tea or leather leaf. If the wind blows these spores onto current year spruce needles and if the weather is wet and cool, the spruce needles become infected and turn yellow, orange or tan in July and August. The rust fungus produces spores on infected spruce needles and these are carried by wind and rain splash to Laborador tea or leather leaf infecting them. The spores produced on spruce trees do not infect other spruce trees. That is the up-side of this disease and it keeps many more trees from becoming infected each year.

Once spruce needle rust becomes obvious on the trees it is too late to use a pesticide. When the needles start to turn color they are already infected and fungicides cannot cure the infected needles. Pesticides are seldom recommended because the infections usually won't damage the tree health. Additionally, because there is no way to predict which years the rust will be a problem, pesticides would have to be applied every year.

The best thing you can do for your trees is to keep them well watered if the weather turns dry. Avoid using sprinklers though because they get the needles wet and can lead to needle and twig disease problems such as needle rust. It is much better to just lay the hose on the ground under the tree to soak the ground. Keep weeds and grass mowed around small trees so winds can dry the needles better and prevent infection. Mulching around yard trees is also recommended because it keeps the soil moist and also helps keep the weeds and grass away from them. Mulching also keeps your lawn mower and weed whip away from your trees there by reducing injury to the stem. These are likely to do a lot more damage to your trees than spruce needle rust ever will.

Additional information can be found at the following web sites:

Larch casebearer damage is down

Although they don't seem to cause much long-term damage, these tiny caterpillars defoliate tamarack by skeletonizing needles and the damage is quite noticeable. Caterpillars are present all season long, so the color of infested foliage starts out green in the spring and progresses through various shades of tan to orange by the end of summer. This season, the acreage of larch casebearer activity seems to have decreased in comparison with the last few years.

photo: Tree with Larch casebearer

Tamarack with Larch casebearer

photo: tig with Larch casebearer

Tamarack tig with Larch casebearer

photo: Group of tree with Larch casebearer

Group of tamarack with Larch casebearer

For more info on larch casebearer

Leaf miners on birch and aspen

photo: Aspen blotchminer- upper surface of leaf

Blotch miner on upper surface of Aspen leaf

Heavy populations of birch leaf miner have turned leaves of paper birch tan along the north shore of Lake Superior. Outbreaks of birch leaf miner sometimes occur in the forest like this year along the North Shore, but they tend to be most damaging, in the form of stress, to open growing trees in yards in urban areas.

There are a couple of different birch leaf miner sawflies in North America. All of them are exotics. Adult birch leaf miners are black, about 3 mm long and look like a small fly or wasp. The female lays eggs in slits she cuts into the leaf epidermis using a saw like structure on the end of her abdomen. The eggs hatch and the larvae mine the leaf, feeding between the leaf surfaces creating a hollowed out area in the leaf. The larvae are flat and, when full gown, are only about 5 mm long. With high populations the larvae may mine out much of the leaf. As summer progresses, the mined areas turn tan or brown. Then mature the larvae drop out of the leaf onto the ground. Depending on the miner species and the length of the growing season, there can be from one to four generations per year. Larvae from the final generation of miners overwinter in the soil.

Heavy populations of aspen blotch miner have also developed in many locations in the northern part of the state. Aspen blotch miner damage looks like light yellow blotches on aspen leaves right now but they will soon turn brown. Aspen blotch miner eggs are laid on the underside of aspen leaves from the spring to mid-summer. After hatching, the larvae bore into the leaf and mine the leaf producing pale yellow round to oval blisters on the underside of the leaf. When full grown, the larvae change to pupae inside the mined out blotch. Shortly before adult emergence, the pupae wiggles until it pushes its way partially through the lower epidermis of the leaf. The pupal case may remain partially sticking out of the leaf for some time after the adult moth emerges. The tiny moths fly in August and overwinter in sheltered locations such as under loose bark scales of jack pine trees.

Wooly alder aphids: Why is my windshield so sweet?

photo: Wooly Alder aphids on alder

Wooly alder aphids on alder

photo: Wooly alder aphids on silver maple

Wooly alder aphids on silver maple

Wooly alder aphids are covered by wool-like waxy filaments that are extruded from its body. They produce dense, white, wooly masses on the leaves and twigs of silver maple and also on their secondary host, alder. Aphids suck sap from the trees. Sap is rich in sugar but poor in nitrogen.The sugar provides energy to the aphids, but they also need nitrogen for protein production. To get enough nitrogen, the aphids have to take in a lot more sap than they need and they excrete this extra sugar as 'honeydew'. The honeydew is attractive to ants that feed on the sugar and the ants protect the aphids from predators. This works out good for the ants and the aphids. However, the extra honeydew also drips onto things people have sitting under silver maple trees, like car windshields, decks, and patio furniture. The sugary honeydew makes the cars, decks and furniture sticky. Black sooty mold fungus sometimes grows on the accumulation of honeydew. I should mention that while the honeydew excreted by the aphids is very sweet it is mixed with aphid waste materials, so licking the honeydew off your car windshield is not recommended.

Aphids sucking sap from leaves may cause leaves to twist and curl, and cause some yellowing of foliage. Wooly alder aphid feeding usually causes no permanent damage. Some leaves may shrivel and drop prematurely but generally not enough to reduce the vigor of healthy trees. Control is usually not warranted or recommended.

To avoid the problem of honeydew making things sticky and to avoid the development of sooty mold, don't park under your car under silver maple trees. Move lawn furniture out from under the trees as well. For things like decks that can't be moved, if lots of honeydew is dripping onto them, either cover then with a sheet of plastic when not using them or rinse them off with the garden hose once a day.

Maple anthracnose

photo: Maple anthracnoseNormally overlooked on sugar maple, this fungal disease was quite common and noticeable this summer. Where maple anthracnose infections were heavy, crowns looked sparse and anthracnose blotches were visible throughout crown on the remaining leaves. Anthracnose infections occur during the spring as buds are breaking and leaves are expanding. A "good" year for anthracnose is when spring is protracted, cool and moist. Drier springs generate much less anthracnose. By the time symptoms are observed it's too late to do anything but rake and burn (or bag) the leaves in the fall to prevent next year's infections.

Bur oak blight

photo: Bur oak leaf showing BOB symptomsThe first bonafide case of bur oak blight (BOB) in Minnesota has been confirmed by Dr. Tom Harrington of Iowa State University. Previously, symptoms of BOB were reported to occur in portions of southern Minnesota, however, the disease was then called Tubakia leafspot and was cited to be caused by the fungus, Tubakia dryina. Since then, Dr. Harrington has completed DNA and pathogencity testing that confirms this disease is caused by a new, and yet unnamed, species of Tubakia, and he has named the disease bur oak blight.

It is not clear if this new species of Tubakia is a recent arrival to this region or if a shift in climate (more early-season rain events) have made this disease more noticeable over the last two decades. To date, BOB is known to occur from eastern Nebraska to central Minnesota and southwestern Wisconsin, and it appears to occur across all of Iowa.

Plant pathologists and arborists have been on the lookout for the new BOB Tubakia species in Minnesota, particularly in central and more northern counties. Jill Pokorny, plant pathologist with the US Forest Service located symptomatic bur oak trees in Mille Lacs and Sherburne counties, collected leaf samples, and identified the fungus, Tubakia, to be present. To determine if it was the new species of Tubakia that causes BOB, she submitted samples to Dr. Harrington for further laboratory testing. The samples tested positive for BOB.

In recent weeks, symptoms of BOB have also been reported on bur oaks located in Hennepin and Ramsey counties. These samples have also been submitted for species-level DNA testing, and we are awaiting test results. Jill Pokorny predicts, "As we continue to investigate symptomatic bur oak trees and more samples are tested, it is expected that BOB will be found in additional Minnesota counties."

For more information, see the recently published Pest Alert on Bur Oak Blight external link

Contact your local Region Forest Health Specialist for more information on diagnosing BOB.


A number of windstorms have damaged trees in Minnesota this year. The largest event so far this year occurred on July 1st in Pine County in and around St. Croix State Park. The same storm caused extensive damage in Wisconsin. St Croix State Park is currently closed because of the damage.  Clean up operations are currently underway both inside and outside the park.

photo: Widnstorm damage at St. Croix State Park photo: storm damage at St. Croix State Park

Windstorm damage at St. Croix State Park

Additional photos and information about the park.

The DNR has a very nice website for landowners with damaged woodlands looking for advice and information to help them recover from the storm.