Dothistroma Needle Blight

Dothistroma septosporum (Dorog.) M. Morelet

Host(s) in Alaska:

Shore pine (Pinus contorta subsp. contorta)

Habitat(s): pine needles

Current Status & Distribution in Alaska (2023 Update)

Dothistroma needle blight foliage discoloration symptoms and premature needle shed have been elevated since 2021. This year, Dothistroma needle blight was aerially detected at two locations along the Outer Coast of Glacier Bay National Park and one on the northern tip of Chichagof Island, less than in recent years. Fourteen observations were made during ground detection surveys scattered between Gustavus to the north and Annette Island to the south. Moderate damage was detected on Mitkof and central Prince of Wales Islands. Three research grade observations were submitted through iNaturalist in Sitka.

Consecutive rainy days and temperatures greater than 62°F are linked to outbreaks. Notable tree mortality occurred in Gustavus during a localized, prolonged outbreak from 2009-2016. Substantial crown loss from the outbreak continues to cause individual tree mortality. In forests heavily affected by the outbreak in Gustavus, 57% of shore pine were killed and dominant trees were left with foliage only in the upper 1-3 ft of tree crowns. In recent years, Dr. Renate Heinzelmann (formerly of University of British Columbia) and PhD student Barbara Wong from Université Laval have evaluated the influence of temperature and moisture regime on the growth rate of Dothistroma isolates collected across western North America, including Southeast Alaska, on nutritional media.

Dothistroma needle blight causes varying levels of premature needle shed throughout the range of shore pine in Alaska (Detection Map), but does not typically cause tree mortality. If ideal disease conditions become more common, this could result in more frequent, prolonged and severe outbreaks. Outbreaks in managed lodgepole pine forest in British Columbia have been linked to climate change (Welsh et al. 2014, Woods et al. 2005, 2016).

Historic Activity

Low to moderate levels of disease causing limited damage are common throughout the range of shore pine in Alaska. The severe outbreaks near Gustavus, Haines, and Skagway, AK are thought to be unprecedented for Alaska. The Dothistroma needle blight outbreak near Gustavus and Glacier Bay National Park (GBNP) that began around 2010, affecting at least 11,000 cumulative acres, has caused significant damage and mortality to shore pine (Pinus contorta subsp. contorta). The bulk of shore pine mortality occurred between 2013 and 2015. Nine permanent plots established in outbreak areas near Gustavus in 2016 found 57% of shore pine trees and 34% of the pine basal area to be dead. Smaller pines were more likely to die, but surviving dominant and co-dominant trees often only retained live foliage in the upper 1-5 feet of the tree crown. In 2017, few Dothistroma fruiting structures were observed on shore pine and mortality rates had slowed, indicating that the outbreak may have run its course. Evaluation of weather data from Gustavus identified a prolonged wet period with temperatures greater than 62°F in late-July 2009 that likely precipitated the outbreak.

In 2016, about 2,200 acres of severe Dothistroma needle blight crown damage was aerially detected in northern Lynn Canal in 2015 and 2016, primarily along the Chilkat River between Haines and Klukwan, and from Skagway north along the Taiya River. The outbreaks near Haines and Skagway decreased in severity without causing significant pine mortality (8 permanent plots established in Haines in 2015/16 were revisited in 2017). Shore pine regeneration was observed in some affected stands, likely associated with stress cone production triggered by the outbreak. A few other places in Southeast Alaska are known hotspots for Dothistroma needle blight, particularly localized muskegs near Juneau (Pt. Bridget State Park and Douglas Island) and Sitka (Gavin Hill Trail).

In the last decade, managed lodgepole pine (Pinus contorta subsp. latifolia) plantations in British Columbia have also experienced mortality from this disease, with elevated disease activity linked to increased summer precipitation and temperature. 

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Symptoms, Biology & Impacts

Dothistroma needle blight affects a wide range of pine hosts worldwide. It occurs throughout the range of shore pine in Southeast Alaska, where wet, mild summers are conducive to many conifer foliage diseases. The causal fungus produces black, pimple-like fruiting bodies and orange-red banding symptoms on needles in spring and early summer. Diseased trees may have sparse crowns and reduced growth from premature needle shed.

This native disease does not usually kill trees; however, multiple years of severe disease can cause substantial tree mortality, as observed near Gustavus. The species of Dothistroma present in Southeast Alaska has recently been confirmed as Dothistroma septosporum (Dorog.) M. Morelet in partnership with Drs. Irene Barnes and Mike Wingfield at the University of Pretoria in South Africa. Dothistroma septosporum population genetics, including samples from Southeast Alaska, are being evaluated by colleagues at the University of British Columbia.

The combination of mortality and limited regeneration is likely to reduce the abundance of shore pine in the flatland pine-spruce-cottonwood forests near Gustavus. Monitoring plots installed near Haines and Gustavus in 2015/2016 will allow us to track disease severity, tree survival and changes in forest composition. The long-term impacts will depend on the duration of these outbreaks. Where substantial mortality occurs, we expect potentially different compositional changes in stands that contain other tree species that may be poised to replace shore pine compared to stands with limited tree diversity.

Survey Method

Severe outbreaks can be mapped by aerial survey. Ground-based plots have been established in severe outbreak areas identified since 2012. In monitoring transects established near Gustavus in 2013 and revisited in 2014, over half of the shore pine trees in severely affected areas were dead and many monitored trees succumbed to disease from 2013-2014 (about 60% of the 204 severely diseased trees flagged for monitoring died within one year). Nine permanent plots were established near Gustavus in 2016 to allow us to more closely track disease severity and tree mortality. In these pine-dominated plots, 57% of shore pine trees and 34% of the pine basal area was dead. When plots were revisited in 2017, additional mortality was detected but the rate of mortality had slowed. Many of the surviving pines had greatly reduced crown heights due to branch dieback from the disease. For more detail, see the Biological Investigation Report. Eight permanent plots were also established near Haines in 2015 and 2016.

Bio-evaluation: Dothistroma Needle Blight near Gustavus, AK, 2016 (PDF)