Sirococcus Shoot Blight

Overall, shoot blight damage to hemlock was not severe or notable. Another related disease, Sirococcus blight of conifers (Sirococcus sp.), was detected on Sitka spruce cones on the ground near Sitka. Another superficially similar-looking saprophyte of spruce cones, Phragmotrichum sp., was identified from a different collection site in Sitka.  

2024 Ground Detection Survey Observations: 8 on western hemlock in Southeast Alaska from Haines, Juneau, and Mitkof and Revillagigedo Islands. 

2024 iNaturalist Observations: 6 on western hemlock, 1 from Juneau and 5 from Sitka. 

Sirococcus shoot blight affects western and mountain hemlock (occasionally spruce) across Southeast Alaska (see Detection Map). 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. 

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.

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

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 Links to Resources below for more information about this research.

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

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.