Forest & Grassland Health

Sirococcus Shoot Blight

Sirococcus tsugae Rossman, Castlebury, D.F. Farr & Stanosz

Host(s) in Alaska:

mountain hemlock (Tsuga mertensiana)
western hemlock (T. heterophylla)
Sitka spruce (Picea sitchensis)

Habitat(s): shoots, needles

Photos

Click on the image for a larger version.

Western hemlock shoot recently killed by Sirococcus shoot blight, causing needles to turn red-brown.

A western hemlock shoot recently killed by Sirococcus shoot blight, causing needles to turn red-brown.

Chronic Sirococcus shoot blight on several mountain hemlock trees.

Chronic Sirococcus shoot blight on several mountain hemlock trees in a mountain valley on Douglas Island near Juneau.

Tan mountain hemlock needles with Sirococcus tsugae fruiting structures.

Dead mountain hemlock needles with black fruiting structures of Sirococcus tsugae.

Multiple western hemlock shoots recently killed by Sirococcus shoot blight.

Multiple western hemlock shoots recently killed by Sirococcus shoot blight.

Chronic Sirococcus shoot blight of mountain hemlock branches from below.

Chronic Sirococcus shoot blight of mountain hemlock branches from below.

Current Status & Distribution in Alaska (2020 Update)

Sirococcus shoot blight activity was more pronounced near Juneau in 2020 than in a decade or more. It is expected that the disease was also widespread in unsurveyed areas throughout the Panhandle. The causal fungus, which kills shoots of western and mountain hemlocks and occasionally spruce in Southeast Alaska (see Distribution Map), benefits from the cool, wet conditions that were common this year. Sirococcus damage was negligible in 2019 following a second year of drought. Hemlocks with evidence of repeated years of shoot dieback and compromised tree form are most often found along creeks and in mountain bowls. Chronic shoot disease observed in landscape plantings suggests that non-native hemlock varieties may be more susceptible to this disease than hosts in natural settings.

Historic Activity

Although this disease has recently caused widespread damage to western hemlock and mountain hemlock shoots in Southeast Alaska, this disease is not typically considered economically or ecologically important. More often, chronic shoot blight occurs to ornamental mountain hemlock plantings, or to mountain hemlock in riparian areas. Suppression mortality of chronically diseased saplings may occur. In the 1970s, there was concern about the impact of this disease to western hemlock regeneration (especially around Thomas Bay near Petersburg, Alaska), but it was determined that affected crop trees would recover with little long-term damage, especially after pre-commercial thinning. See the linked publications below for more information about this research.

Symptoms, Biology & Impacts

This disease of young lateral or terminal shoots occurs in Southeast Alaska on both western and mountain hemlock (rarely spruce). Mountain hemlock is considered more susceptible, but shoot symptoms have recently been widespread on both hemlock species.

Infection occurs through young needles and moves into developing shoots, causing canker formation, distorted shoot growth, and shoot mortality. Spores are dispersed by rain splash from small, circular fruiting bodies.

There appears to be a correlation between cold air drainage and higher disease incidence; disease has been pronounced on forest edges, riparian areas, and bowls or depressions in mountain valleys. Riparian zones with apparently conducive infection conditions, such as Montana Creek and Eagle River near Juneau, often show evidence of repeated years of shoot dieback resulting in compromised tree form. Ornamental mountain hemlocks are sometimes chronically diseased; this may be due to the genetic source of landscape trees conferring greater susceptibility.

Management suggestions include avoiding offsite plantings. In Southeast Alaska, forestry relies on natural regeneration, so seed source is not an issue.

Survey Method 

Disease severity and distribution information primarily comes from informal ground observation.

Distribution in Alaska

Observations of Sirococcus shoot blight in Alaska as of 2020.

Georeferenced observations of Sirococcus tsugae in Alaska as of 2020 with the modeled distribution of western hemlock and mountain hemlock. Host tree distributions were developed by the Forest Health Technology Enterprise Team in 2011 (240-m resolution, based on dominant tree cover by diameter).

Links to Resources & Publications

Shaw, C. G., III; Laurent, T. H.; Israelson, S. 1981. Development of Sirococcus shoot blight following thinning in western hemlock regeneration. Research Note PNW-RN-387. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Forest and Range Experiment Station. 6 p. Available here.

Wicker, E. F.; Laurent, T. H.; Israelson, S. 1978. Sirococcus shoot blight damage to western hemlock regeneration at Thomas Bay, Alaska. USDA Forest Service Research Paper INT-198. USDA Intermountain Forest and Range Experiment Station. 11 p. Available here.

 

Content prepared by Robin Mulvey, Forest Health Protection, robin.mulvey@usda.gov.

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