Pathogen
Asterosporium canker is caused by Asterosporium asterospermum (Hughes 1958; Tanaka et al. 2010). The fungus lives primarily as an endophyte, but at times is a weak pathogen of twigs and branches (Sieber 2007; Danti et al. 2022; Tropf et al. 2025). Asterosporium has been reported from North America, Europe, and Asia (Farr et al. 2026). Phylogenetic analysis has shown that Asterosporium groups with other known cankering fungi of trees (Fan et al. 2018) but is not mentioned by Hepting (1971) or Sinclair and Lyon (2005).
Hosts
Asterosporium asterospermum occurs on American beech (Fagus grandifolia) in North America (N.A.), European beech (F. sylvatica) in N.A., Europe and Asia, and Siebold’s beech (F. crenata) in Japan (Tanaka et al. 2010; Farr et al. 2026). The fungus has been found on fossilized beech samples dating back millions of years, illustrating a long host relationship with genus Fagus (Worobiec et al. 2023).
Symptoms & Signs
Relatively little is known about Asterosporium and its role as a cankering pathogen of beech. However, the fungus can be found on large and old American and European beech across managed landscapes in southern New England, attacking small diameter twigs and larger branches (i.e. >6" in diameter). Typically, these are trees with a variety of other health issues. Yet, because it’s an endophyte, Asterosporium is likely present in the canopies of otherwise healthy trees. When trees become stressed, some endophytes become parasitic and take advantage of the weakened host to cause further dieback (Sieber 2007). Overall, Asterosporium appears to be associated with trees suffering from drought stress, beech bark disease (for F. grandifolia), coral-spot canker (Nectria), and infestation by boring insects like ambrosia beetles (e.g. Xylosandrus spp.), among other stresses.
In a survey of fungal endophytes on twigs of European beech in Italy, Asterosporium was uncommon, but had a higher frequency on trees with symptoms of cankering and dieback (Danti et al. 2002). Another study from Germany found that while Asterosporium was present in xylem and cambium from twigs, branches, and the main trunk of European beech, it was found on both healthy and declining trees with a low overall frequency (<1% of all samples) (Tropf et al. 2025).
Asterosporium can produce large masses of dark-colored, stellate conidia on infected bark tissue (Tanaka et al. 2010). These may be readily visible on branches with symptoms of cankering (i.e. cracking, splitting, and sloughing bark tissue).
Management
As a general practice for managing any potential cankering disease, prune and discard all dead branches from the canopy, if possible. This helps to reduce fungal inoculum (plant tissues that harbor the pathogen and allow it to spread to healthy tissues) and limit future infections. Continue to regularly scout the tree and remove deadwood as it develops. A lower trunk drench with phosphites could be beneficial as well. This systemic fungicide can be translocated into the canopy where latent may be present. Overall, because Asterosporium is an endophyte that behaves as a pathogen when trees are stressed, maintaining high tree vigor and addressing more serious health issues should be the primary management focus.
Literature Cited
Danti R, Sieber TN, and Sanguineti G. 2002. Endophytic mycobiota in bark of European beech (Fagus sylvatica) in the Apennines. Mycological Research 106(11): 1343–1348. https://doi.org/10.1017/S0953756202006779
Fan XL, Bezerra JD, Tian CM, and Crous PW. 2018. Families and genera of diaporthalean fungi associated with canker and dieback of tree hosts. Persoonia 40(1): 119–134. https://doi.org/10.3767/persoonia.2018.40.05
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 19, 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.
Hughes SJ. 1958. Revisiones Hyphomycetum aliquot cum appendice de nominibus rejiciendis. Canadian Journal of Botany 36(6): 726–836.
Sieber TN. 2007. Endophytic fungi in forest trees: are they mutualists? Fungal Biology Reviews 21(2-3): 75–89. https://doi.org/10.1016/j.fbr.2007.05.004
Sinclair WA and Lyon HH. 2005. Diseases of Trees and Shrubs, 2nd edn. Cornell University Press, Ithaca, NY.
Tanaka K, Mel’nik VA, Kamiyama M, Hirayama K, and Shirouzu T. 2010. Molecular phylogeny of two coelomycetous fungal genera with stellate conidia, Prosthemium and Asterosporium, on Fagales trees. Botany 88(12): 1057–1071. https://doi.org/10.1139/B10-078
Tropf J. Bien S, Bußkamp J., Langer GJ, and Langer EJ. 2025. Fungi associated with vitality loss of European beech in central Germany. Mycological Progress 24(1): 53. https://doi.org/10.1007/s11557-025-02041-y
Worobiec G, Worobiec E, Gedl P, Kowalski R, Peryt D, and Tietz O. 2023. Fossil history of fungus host-specificity: Association of conidia of fossil Asterosporium asterospermum with macro-and microremains of Fagus. Fungal Biology 127(9): 1312–1320. https://doi.org/10.1016/j.funbio.2023.07.005