Pathogen
Cytospora canker is caused by numerous species of Cytospora (syn. Leucostoma and Valsa) that collectively have a global distribution (Rossman et al. 2015; Lin et al. 2024).
Hosts
Cytospora has a very broad host range among conifers and hardwoods (Sinclair and Lyon 2005; Lin et al. 2024). Across North America, Cytospora causes the most damage to maple (Acer), blue spruce (Picea pungens), aspen / poplar (Populus), and willow (Salix) (Sinclair and Lyon 2005). While some Cytospora species are believed to be relatively host specific (Kepley and Jacobi 2000; Fan et al. 2020), the tremendous diversity of species makes it difficult to understand the complete host range for many (Ilyukhin and Markovskaja 2025). Also, some Cytospora species can readily co-occur on individual trees (Dudley et al. 2020).
Based on samples submitted to the UMass Plant Diagnostic Laboratory, some of the most common hosts in southern New England include true fir (Abies), maple (Acer; especially A. palmatum), spruce (Picea; especially P. pungens), pine (Pinus; especially P. strobus), cherry & peach (Prunus), willow (Salix), eastern hemlock (Tsuga canadensis), and American elm (Ulmus americana). Cytospora is also very common on recently dead trees and shrubs as a saprophyte.
Symptoms & Signs
The fungus most often invades smaller diameter twigs and branches, sometimes colonizing larger diameter branches, where it creates small, eruptive cankers (Sinclair and Lyon 2005). The cankers expand and coalesce, killing the sugar-rich cambium tissue. The underlying sapwood is also colonized and becomes stained as infections spread and advance. The disease leads to yellowing or browning of foliage due to water starvation, premature leaf or needle shedding, stunted growth of shoots and foliage, and scattered twig and branch dieback (Sinclair and Lyon 2005). Bark at the site of the cankers may be sunken and stained with sap or resin or may show few if any symptoms of the disease. A prominent symptom of Cytospora canker on spruce is the presence of hardened resin that has oozed from the site of the cankers and rough wound wood tissue on the margins. On cherry and peach, amber-colored gum may be produced near the site of the cankers, which is a generalized defense response (Sinclair and Lyon 2005).
Signs of the pathogen include very small, dark colored fruiting bodies that rupture through the bark at the site of the cankers. They can be difficult to see without magnification but at times can be seen extruding spores masses of varying colors (clear, white, yellow, and orange) (Sinclair and Lyon 2005). Shade and free moisture favor the pathogen and during extended periods of wet weather in late spring and early summer, Cytospora produces vast amounts of spores that are spread by wind and splashing rainwater. The fungus can utilize nearly any possible wound in the bark to establish. Infections on conifers typically begin on older branches towards the base of the tree and gradually expand upward in the canopy. On deciduous hardwoods, they can develop throughout the canopy depending on where stress is most acute. On fruit trees, infections tend to develop near pruning wounds (Proffer 2014).
Management
Like most fungal cankering pathogens, Cytospora is an opportunist, colonizing stressed, weakened and injured hosts (Sinclair and Lyon 2005). Common predisposing stresses include drought, excessive pruning, transplant shock, insect damage, other diseases, and various compounding stresses common in managed landscapes (e.g. poor quality and compacted soils, girdling roots, frequent mechanical damage from lawn equipment, etc.). As with any opportunistic pathogen, minimizing stresses that predispose woody plants to infection is critical. Maintaining high vigor is important since natural defenses are often sufficient to minimize damage by the pathogen. To that end, irrigate susceptible trees and shrubs on regular intervals during extended dry periods to minimize drought stress, fertilize if soil nutrients are lacking, ensure the root zone is mulched to help retain soil moisture, limit mechanical wounds (e.g. string trimmers), avoid excessive pruning, especially on recently transplanted trees and shrubs, and avoid needless damage to the canopy and roots. For certain trees and shrubs, especially those with thin and easily wounded bark (e.g. Japanese maple and true fir), regular sanitation pruning should take place to remove dead twigs from the canopy. If possible, avoid pruning during wet periods in the spring as this is the time when the fungus is most actively sporulating, and sanitize pruning tools after working with plants known or suspected of being infected by Cytospora. Pruning of cankered stems and branches, at least 6–12 inches away from the blighted tissue (if possible), and removal of the diseased material from the site is the best management practice. Repeated scouting and pruning are often required, and complete eradication is difficult to achieve (Moorman and Lease 1999).
For large and mature trees and shrubs, fungicides may also be helpful in suppressing the pathogen once the disease has been identified. It can be difficult to control cankering fungi like Cytospora with fungicides because the fungus lives beneath the bark and may be present on branches that show no symptoms of disease. However, if chemical management is desired, azoxystrobin, copper hydroxide, copper salts of fatty and rosin acids, mancozeb, phosphites, and thiophanate-methyl may have utility against the pathogen. They should be applied as new growth is developing in the spring to help protect these sensitive plant parts from becoming infected. Keep in mind that spores can travel long distances and Cytospora is common in the environment, making eradication of the pathogen difficult to impossible. If pruning can effectively remove all cankered stems then fungicide application may be unnecessary. Avoid fungicide application on plants that are actively flowering as the chemicals are detrimental to pollinators.
Literature Cited
Dudley MM, Tisserat NA, Jacobi WR, Negrón J, and Stewart JE. 2020. Pathogenicity and distribution of two species of Cytospora on Populus tremuloides in portions of the Rocky Mountains and Midwest in the United States. Forest Ecology and Management 468: 118168. https://doi.org/10.1016/j.foreco.2020.118168
Fan XL, Bezerra JDP, Tian CM, and Crous PW. 2020. Cytospora (Diaporthales) in China. Persoonia 45(1):1–45. https://doi.org/10.3767/persoonia.2020.45.01
Ilyukhin E and Markovskaja S. 2025. DNA Barcoding as a tool for surveying Cytospora species associated with branch dieback and canker diseases of woody plants in Canada. DNA 5(2): 20. https://doi.org/10.3390/dna5020020
Kepley JB and Jacobi WR. 2000. Pathogenicity of Cytospora fungi on six hardwood species. Arboriculture & Urban Forestry 26(6): 326–333. https://doi.org/10.48044/jauf.2000.040
Lin L, Fan XL, Groenewald JZ, Jami F, Wingfield MJ, Voglmayr H, Jaklitsch W, Castlebury LA, Tian CM, and Crous PW. 2024. Cytospora: an important genus of canker pathogens. Studies in Mycology 109: 323–401. https://doi.org/10.3114/sim.2024.109.05
Moorman GW and Lease RJ. 1999. Effects of pruning in the management of dogwood and pine branch dieback in the landscape. Journal of Arboriculture 25(5): 274–277. https://doi.org/10.48044/jauf.1999.037
Proffer TJ. 2014. Leucostoma canker. In: Compendium of Apple and Pear Diseases and Pests, 2nd edn. APS Press, St. Paul, MN. 56–57.
Rossman AY, Adams GC, Cannon PF, Castlebury LA, Crous PW, Gryzenhout M, Jaklitsch WM, Mejia LC, Stoykov D, Udayanga D, and Voglmayr H. 2015. Recommendations of generic names in Diaporthales competing for protection or use. IMA fungus 6(1): 145–154. https://doi.org/10.5598/imafungus.2015.06.01.09
Sinclair WA and Lyon HH. 2005. Diseases of Trees and Shrubs, 2nd edn. Cornell University Press, Ithaca, NY.