Pathogen
Sirococcus shoot blight is caused by Sirococcus conigenus (formerly S. strobilinus), S. tsugae, and S. piceicola (Sinclair and Lyon 2005; Rossman et al. 2008).
Hosts
Sirococcus has a broad host range among conifers and hosts overlap among the three dominant species in North America. Specifically, S. conigenus is known to attack true fir (Abies), true cedar (Cedrus), larch (Larix), spruce (Picea), two- and three-needle pine (Pinus), and Douglas-fir (Pseudotsuga), while S. tsugae is found on hemlock (Tsuga) and true cedar, and S. piceicola is found on spruce (Sinclair and Lyon 2005; Rossman et al. 2008). Further study may reveal additional hosts for S. tsugae and S. piceicola.
Across New England, S. tsugae appears to be the dominant species present (Munck et al. 2018). Based on samples submitted to the UMass Plant Diagnostic Laboratory, eastern hemlock (T. canadensis) and true cedar are the most common hosts. On non-hemlock hosts, Siroccocus is most often found in coastal areas with cool, moist climates and abundant fog.
Symptoms & Signs
Sirococcus causes disease of young, succulent shoots in the spring, shortly after budbreak. Infected shoots can die rapidly, and the stunted, wilted needles first appear pale green to yellow-brown while the shoots can be irregularly curved. Over time, the needles become darker brown and desiccated. After symptoms appear, the fungus produces small black pads of tissue on the diseased needles and shoots. When moisture and humidity are abundant, the pads rupture to release large masses of spores, which may be splashed or blown onto nearby shoots or trees, where new infections are initiated. Older shoots (>1 year old) remain unaffected and for trees with indeterminate growth rates (e.g. hemlock), new growth produced later in the growing season may be unaffected. The dead shoot tips can linger in the canopy, allowing the pathogen to readily overwinter in the tree.
Sirococcus shoot blight of eastern hemlock is a relatively new disease in New England and outbreaks in managed landscapes are uncommon. A significant outbreak of the disease occurred throughout the northeast in 2010, and subsequent regional surveys concluded the pathogen was widespread in New England forests (Miller-Weeks and Ostrofsky 2010; Munck et al. 2018). Despite its prevalence, the disease caused relatively minor injury at most locations. However, in some settings disease severity was high and resulted in significant defoliation of understory trees (Munck et al. 2018). While Sirococcus shoot tip blight readily occurs in forests, it has been an uncommon disease on hemlocks in managed landscapes. This is most likely due to better light and air flow outside of dense stands of hemlock. The outbreak in 2010 was fueled by a very wet summer the previous year. The summer of 2009 was one of the wettest on record in New England, with the 2-month June and July precipitation total ranking among the top four wettest on record for CT, ME, MA, NH, and RI (NOAA 2026). A similar trend occurred in 2024, with high disease incidence and severity across southern New England due to very wet conditions in July of 2023.
Management
Pruning dead shoot tips and removing the diseased material from the site is the first step in controlling the disease. For large and mature trees with significant outbreaks, this can be a daunting task, but inoculum reduction is important. Fungicide application early in the growing season as new shoots are flushing can help to protect these sensitive tissues. Chemicals labeled for use against Sirococcus include: thiophanate-methyl, mancozeb and copper hydroxide. Chemical control can be challenging with shoot blight pathogens since they live within woody tissues and can be difficult to contact. Therefore, once the tissues are fully hardened off, a locally systemic fungicide may be required if applications occur mid-season. Keeping trees healthy and robust is important, so provide supplemental irrigation (if possible) during extended dry periods to avoid drought stress. This is the primary predisposing stress that helps facilitate cankering pathogens. Because major outbreaks are caused by very wet summers the previous year (e.g. wet summer in 2023 > outbreak in 2024), in some cases little can be done to prevent disease development. Overall, this is not a lethal disease and infected trees can recover.
Literature Cited
Miller-Weeks M and Ostrofsky WD. 2010. Sirococcus tsugae Tip Blight on Eastern Hemlocks. Pest Alert NA–PR–01–10, Newtown Square, PA: USDA Forest Service, Northeastern Research Station. 2 p.
Munck IA, Morin RS, Ostrofsky WD, Searles W, Smith DR, and Stanosz GR. 2018. Impact of Sirococcus shoot blight (Sirococcus tsugae) and other damaging agents on eastern hemlock (Tsuga canadensis) regeneration in Northeastern USA. Forest Ecology and Management 429: 449–456. https://doi.org/10.1016/j.foreco.2018.07.043
NOAA National Centers for Environmental information, Climate at a Glance: Statewide Rankings, published March 2026, retrieved on March 17, 2026 from https://www.ncei.noaa.gov/access/monitoring/climate-at-a-glance/statewide/rankings
Rossman AY, Castlebury LA, Farr DF, and Stanosz GR. 2008. Sirococcus conigenus, Sirococcus piceicola sp. nov. and Sirococcus tsugae sp. nov. on conifers: anamorphic fungi in the Gnomoniaceae, Diaporthales. Forest Pathology 38(1): 47–60. https://doi.org/10.1111/j.1439-0329.2007.00529.x
Sinclair WA and Lyon HH. 2005. Diseases of Trees and Shrubs, 2nd edn. Cornell University Press, Ithaca, NY.