Dothistroma Needle Blight

Dothistroma septosporum (Dorog.) M. Morelet

Host(s) in Alaska:

Shore pine (Pinus contorta subsp. contorta)
Lodgepole pine (Pinus contorta subsp. latifolia)

Habitat(s): pine needles

General Distribution in Alaska: Throughout the natural range of pines in Southeast Alaska, and in some pine plantations in Southcentral Alaska

Current Status & Distribution in Alaska (2024 Update)

Dothistroma needle blight foliage discoloration symptoms and premature needle shed have been elevated since 2021. Moderate to severe disease has impacted clumps of pine in muskegs of central Southeast Alaska resulting in foliage discoloration and dieback in the lower and middle tree crown and limited tree mortality. Near Talkeetna, three years of crown symptoms were reported on plantation lodgepole pines, including: foliage discoloration, premature needle shed, and branch dieback in the lower, middle, and upper tree crowns with significant tree mortality. Among other endophytes and decomposition fungi, D. septosporum fruiting bodies and conidia were identified by Dr. Glen Stanosz (University of Wisconsin – Madison) on the foliage. It is likely that this disease was brought in with the seedlings and environmental conditions have precipitated this event. Pine plantations like this have been installed to evaluate new timber sources; the disease will continue to be monitored by Forest Health Protection with the aid of University of Alaska Cooperative Extension, Natural Resources Conservation Service, and Forest Service Cooperative Forestry. Planting lodgepole pine at more northerly locations outside of its native range necessitates evaluation of diseases like Dothistroma needle blight to understand the potential longevity and resilience of the plantations.  

2024 Ground Detection Survey Observations: 18 statewide, 3 in Southcentral Alaska near Talkeetna on planted lodgepole pine; 2 on natural lodgepole pine near Haines, and 13 on shore pine in Southeast Alaska near Juneau and on Mitkof, Kupreanof, and Wrangell Islands.  

2024 ADS Observations: Dothistroma needle blight was mapped on 280 acres around the northern Lynn Canal in Southeast Alaska along the Haines Highway, Chilkat River, Skagway River, and Dewey Lakes. 

Symptoms, Biology & Impacts

Dothistroma needle blight affects a wide range of pine hosts worldwide. 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.

Dothistroma needle blight causes varying levels of premature needle shed throughout the range of pine (Pinus contorta ssp. contorta and P. contorta ssp. latifolia) 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). 

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. The combination of mortality and limited pine regeneration in this area is likely to reduce the abundance of shore pine in the flatland pine-spruce-cottonwood forests near Gustavus. Monitoring plots were also installed near Haines in 2015/2016 to track disease severity, tree survival and changes in forest composition, but that outbreak subsided with minimal mortality. Long-term impacts depend on the duration of these outbreaks and other factors related to forest succession. 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.

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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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)