Forest Insect and Disease Newsletter - Wet Summer 2017

Statewide Forest Health Issues

One-year-old containerized red pine death in a plantation

By Brian Schwingle, DNR forest health specialist

On June 14 I inspected the forest health unit's red pine planting trial at General Andrews Nursery in Willow River, where we're analyzing the long-term impact of latent Diplodia infection in red pine (Forest Insect & Disease Newsletter, May 2017). We planted one-year-old containerized red pine stock as a control (containerized stock refers to seedlings grown in a greenhouse and planted with a small plug of soil). Roughly 20 percent of the containerized stock has died since we planted in late April, compared to nine percent of the DNR 3-0 bare-root stock (grown for three years at a nursery without transplanting). The dying containerized stock appeared to have a root disease, and I submitted samples for analysis in mid-June. Watch our future newsletters for any lab results.

Jack pine budworm update

Jack Pine budworm next to a penny

By Mike Parisio, DNR forest health specialist

We conducted roadside surveys for jack pine budworm caterpillars the third week in June in southwest Cass County and northwest Morrison County. We found caterpillars large enough to be causing significant damage, but few visible signs of current-year defoliation is an indication that populations have decreased in that area. A larger infestation from last year still appears to be somewhat active just east of Bemidji, though not as significant as last year. In most areas, we've noticed that jack pines heavily defoliated in 2016 failed to produce large crops of pollen cones in 2017, which should help limit jack pine budworm survival this year. We'll continue to monitor for growing populations shifting to the northern portions of the Northwest Region; however, no visible defoliation was noticed during roadside surveys in the Beltrami Island State Forest as of the first week of July.

map of minnesota showing central region

Central Region

By Brian Schwingle, DNR forest health specialist

Widespread dieback and mortality of hybrid poplars

hybrid poplars dieback

Dieback and mortality on scattered hybrid poplars is occurring from Pine and Morrison County to the Iowa border. Symptoms include cankers (sunken, dead areas of bark) and crown dieback. The University of Minnesota Plant Disease Clinic identified Cryptodiaporthe canker on some branches with dieback collected from Winona County in late June. Other canker-causing fungi such as Valsa and Leucostoma may also be contributing to these symptoms. Age, past stress (e.g., drought, roadside conditions), and moist conditions are probably promoting this epidemic. In yard situations, I recommend planting a different tree species that has a longer life. If dieback is less than 50 percent of the crown, the tree may still be able to bounce back with appropriate watering. Dead and dying branches could be pruned away in the winter.

Mystery dieback of green ash

green ash with mystery dieback

In 2016 and 2017, I've been noticing scattered dieback on green ash from Eagan to Sibley State Park to St. James to New Ulm. Dieback severity is variable and is not associated with emerald ash borer. Due to its widespread nature, the cause of the dieback has to be either past unfavorable weather events or some non-native mystery species that we haven't identified yet. I doubt it is the latter.
I submitted a wilting ash sprout from Sibley State Park to the University of Minnesota Plant Disease Clinic, and staff isolated a fungus called Seimatosporium. Isn't that a mouthful? This fungus is not a reported pathogen of ash, and it is probably a secondary or saprophytic fungus living on dead tissue. I have no recommendation at this time. It's a mystery.

Widespread, minor dieback on junipers

Junipers showing dieback

Minor dieback scattered across individual shrub or tree crowns is occurring throughout the region on multiple species of junipers. I've looked at fruiting bodies and spores, and they are representative of either Kabatina or Sclerophoma blight (read the Kabatina article below for more information). Dieback has usually not been a serious issue on eastern red cedars. The cause of this fungal blight is probably several consecutive wet springs as well as last year's drenched summer.

Leaf disease on white and bur oaks

Bur oak showing leaf diseasePhoto by: Gina Hugo, Sherburne County Soil and Water Conservation District

In late June I received many reports of deformed and browned leaves on white and bur oaks. Reports came in from Sherburne, Kanabec, Morrison, and Pine counties. This issue is not bur oak blight; bur oak blight symptoms generally don't appear until July, at the earliest, and the causal fungus is different.

This problem is caused by a fungal leaf pathogen, and the disease it causes is called anthracnose. On some isolated oaks, anthracnose has affected leaves across the tree's entire crown, but in most instances, diseased leaves are most concentrated in the lower crown. As long as anthracnose affects roughly 40 percent or less of the leaf area, the tree should not be overly stressed. Several consecutive years of anthracnose impacting more than 50 percent of the leaf area would stress the tree and make it susceptible to other problems.

map of minnesota showing northeast regionNortheast Region

By Jess Hartshorn, DNR forest health specialist

Kabatina tip blight on northern white cedar

Venturia leaf and shoot blight on bigtooth aspen regeneration

Residents across the state saw widespread dieback of cedars (Thuja) from tip blight, likely caused by Kabatina. In some parts of northeastern Minnesota the disease is rapidly causing whole-tree browning of cedars along roadsides. Phomopsis and Sclerophoma also cause tip blight on cedars; however, Sclerophoma typically doesn't cause the level of damage observed, and Phomopsis doesn't infect mature growth. It's also possible that multiple tip blights are affecting cedars across the state. These shoot blight fungi usually attack trees stressed by wind, winter injury, or insect damage. When planting cedars, select tip blight-resistant varieties, and space trees far enough apart to allow for sufficient air flow. Homeowners can remove affected shoots and avoid wounding trees to prevent additional introduction of fungal spores.

Region-wide dieback and mortality of bur oaks

Botryosphaeria on oak leavesBotryosphaeria on bur oak

Northeastern Minnesota is in another year of bur oak dieback and mortality. The factors playing into this situation are varied and inconsistent and are not following a specific pattern or distribution. Many yard tree and woodlot oaks in the Aitkin area are simply over-mature and reaching the point at which they can no longer support their own growth. Some younger oaks are still experiencing second-hand symptoms of severe drought from 2011. The younger oaks often have symptoms of two-lined chestnut borer, oak bark beetles, Armillaria, and previous Botryosphaeria twig blight. Some may recover with continued precipitation and others will continue to decline over the next several years.

Needle cast of pines

Needlecast on pine trees

Due to the wet conditions of spring and summer 2016, pines across northeastern Minnesota are now showing symptoms of needle cast, most likely Dothistroma or Lophodermium. In mature trees, symptoms occur mostly in the lower crown, where accumulated moisture promotes shooting spores that infect new growth in late summer and fall. Depending on the needle cast present, needles turn yellow and then brown either in the fall or the following spring. Needle casts are differentiated from other similar symptoms such as salt damage by the presence of a distinct brown or red band separating the dead and live portions of the needle (Dothistroma) and black spots (containing spores) that most often appear late in the summer. Lophodermium and Dothistroma can be problems in nurseries and Christmas tree plantations and only cause minor damage on older trees. On a small scale or in situations where tree aesthetics are important, removing infected material and controlling weeds to allow for better air flow and prevent future infections may be possible. In commercial situations with high disease incidence, a registered fungicide might be necessary for control.

map of minnesota showing northwest region regionNorthwest Region

By Mike Parisio, DNR forest health specialist

Venturia leaf and shoot blight on bigtooth aspen regeneration

Venturia leaf and shoot blight on bigtooth aspen

Foresters at the Brainerd area office recently reported pockets of bigtooth aspen regeneration with blackened, dead leaves. These symptoms are being caused by Venturia leaf and shoot blight, a fungal disease that spreads from leaves through leaf petioles and into new shoots. Wilted shoots form a distinct shepherd's crook, with discolored leaves remaining attached for some time. The disease results from wet weather while shoots are elongating and does not cause significant damage unless the leader is killed for multiple years. The saplings we observed were already pushing out a second set of leaves on affected shoots and are expected to pull through.