Pathogen
Fabrella needle blight is caused by the fungal pathogen Fabrella tsugae (previously known as Didymascella and Keithia) (Boyce 1961; Hepting 1971; IndexFungorum 2025).
Hosts
Fabrella tsugae occurs on eastern hemlock (Tsuga canadensis), western hemlock (T. heterophylla), and Carolina hemlock (T. caroliniana) across northern North America (Hepting 1971; Farr et al. 2026).
Distribution, Symptoms, & Signs
Overall, relatively little is known about Fabrella needle blight. The fungus appears to be widespread throughout the range of hemlocks in North America but typically causes only minor damage (Boyce 1961; Hepting 1971). At times, localized outbreaks can develop and some early reports of Fabrella needle blight describe serious damage (Spaulding 1914; Boyce 1961; Kistler et al. 1977; Pennsylvania DCNR 2026). Yet, across its range, Fabrella needle blight has never been a sustained health issue for hemlock, and the pathogen lacks mention in Sinclair and Lyon (2005).
Like many needle blight pathogens of conifers, Fabrella attacks the needles through the stomata. Once established, the fungus causes rapid browning and death of scattered needles throughout the canopy (Hepting 1971). Cool and wet weather in the spring is believed to favor disease development (Pennsylvania DCNR 2026). Infected needles appear yellow brown to reddish-brown, creating symptoms that are usually distinct from other issues, such as winter burn (Kistler et al. 1977; Pennsylvania DCNR 2026). Brown-colored pads of fungal tissue (apothecia) erupt through the underside of the needle to disperse spores (ascospores). One feature of this disease is that blighted needles are often retained on the shoots throughout the summer and even into the following year (Hepting 1971). For that reason, “needle blight” is a better descriptor than “needle cast” for this disease.
Management
Because Fabrella is a secondary pathogen that is most damaging to weakened hemlocks, treating underlying stress should be the initial management strategy. Infestation by the hemlock woolly adelgid (Adelges tsugae) and elongate hemlock scale (Fiorinia externa) are major stresses for landscape hemlocks. In addition, because of their shallow root system and desire for moist soils with abundant organic matter, eastern hemlocks are susceptible to drought stress during hot and dry conditions during the summer. Hemlocks in managed landscapes should have a thick layer of bark mulch or wood chips over the root zone. If an outbreak of Fabrella needle blight develops, discarded needles can be covered with mulch or compost to reduce inoculum and promote decomposition. There are no chemicals labeled specifically for Fabrella, and fungicide treatment is not recommended for this disease.
Literature Cited
Boyce, JS. 1961. Forest Pathology, 3rd edn. McGraw-Hill Book Company, Inc., New York, NY.
Farr DF, Rossman AY, and Castlebury LA. 2026. United States National Fungal Databases. USDA-ARS Mycology and Nematology Genetic Diversity and Biological Laboratory. Retrieved March 12, 2026 from https://fungi.ars.usda.gov
Hepting GH. 1971. Diseases of Forest and Shade Trees of the United States. Washington, DC: USDA Agricultural Handbook No. 386.
IndexFungorum (2025). The Royal Botanic Gardens, Kew, UK. [Retrieved 16 March 2026]. http://www.indexfungorum.org
Kistler BR, Merrill W, Longenecker J, and Wagner V. 1977. Fabrella needle cast of hemlock: a new record for Pennsylvania. Plant Disease Reporter 61(10): 863. https://hdl.handle.net/2027/chi.23661245
Pennsylvania DCNR. 2026. Other Insects and Diseases: Fabrella Needle Blight. https://www.pa.gov/agencies/dcnr/conservation/forests-and-tree/insects-and-diseases/other-insects-and-diseases
Sinclair WA and Lyon HH. 2005. Diseases of Trees and Shrubs, 2nd edn. Cornell University Press, Ithaca, NY.
Spaulding, P. 1914. Diseases of the eastern hemlock. Proceedings of the Society of American Foresters. 9(2): 245–256. https://hdl.handle.net/2027/umn.31951d00303429f