Sirococcus Shoot Blight

Sirococcus tsugae Castl., D.F. Farr & Stanosz

Host(s) in Alaska:

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

Habitat(s): shoots, needles, cones

Current Status & Distribution in Alaska (2022 Update)

Sirococcus shoot blight affects western and mountain hemlock (occasionally spruce) across Southeast Alaska (see Detection Map). The outbreak of western blackheaded budworm, which also primarily damages western hemlock shoot tips, made it difficult to detect Sirococcus shoot blight. It was only recorded at two locations on Revillagigedo and Annette Island in 2023. There was an uptick in other shoot blight diseases associated with wet spring conditions in coastal Alaska, so the true incidence of this disease may have been higher than the low number of recorded observations suggests. Symptom severity and compromised tree form worsen with repeated years of shoot dieback. Cool, wet conditions that favor chronic infection are most often found along creeks and in mountain bowls. Severe shoot disease observed in landscape plantings suggests greater susceptibility among non-native planted hemlock varieties. 

Another species of Sirococcus was documented causing low-severity cone disease of Lutz spruce near Kenai Lake. Specimens were viewed microscopically and sent to Oregon State University Plant Clinic for species confirmation

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.

Detection Map

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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