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Septoria (leaf-spot and) canker of poplars

Written by N. Feau Revision 2024

General information and importance

Sphaerulina musiva (teleomorph Mycosphaerella populorum) is an endemic fungal pathogen of native eastern cottonwood (Populus deltoides) in northeastern and north central regions of North America. It only causes leaf spots that have a relatively minor impact on tree growth and survival. The stem and branch canker symptom caused by this pathogen was described for the first time in the 1920s on poplars of Asian origin and their hybrids that were introduced in North America for their hardiness to winter cold and summer drought. Since then, several canker outbreaks have been reported in plantations of mature, susceptible hybrids in the Great Lakes area and from Quebec and Ontario to the Canadian Prairies. The pathogen has spread from its endemic range to western areas of North America and into South America, likely through the transport of infected planting material (cuttings).

Distribution and hosts

Sphaerulina musiva matches the distribution of eastern cottonwood in north central and northeastern regions of North America. It is epidemic on susceptible exotic species such as Japanese poplar (P. maximowiczii) and black poplar (P. nigra), and susceptible hybrids planted in the same area. North American balsam poplars, including balsam poplar (P. balsamifera) and black cottonwood (P. trichocarpa), are reported as minor hosts. However, a high incidence of canker infections caused by S. musiva has been observed on balsam poplar (and its hybrids), previously thought to be a non-host, in a plantation within its native range in northern Alberta. Sphaerulina musiva was recently introduced in different regions of the Pacific Northwest. Since 2006, Septoria canker has been observed on a regular basis in hybrid poplar nurseries and plantations, black cottonwood provenance tests, native black cottonwood trees located in the Upper Fraser Valley of British Columbia, and some plantations of hybrid clones in the Okanagan valley in British Columbia. Septoria canker has also been reported in Mexico and South America (Argentina, Brazil, and Chile). Due to its invasiveness, S. musiva is considered a high-priority quarantine pathogen for Europe. Leaf spots caused by S. musiva have also been observed on shining willow (Salix lucida Muhl. spp. lucida) in Quebec.

Tree parts affected

Buds, leaf petioles, leaves, and stems

Symptoms and signs

The most commonly visible life stage of the pathogen is its asexual phase, which occurs during the growing season. Necrotic spots on poplar leaves may develop within 3 to 4 weeks of leaf expansion and usually appear first on the foliage of lower branches. Number and size of leaf spots depend on the host species and the hybrid clone that is infected. Leaf spots are generally circular to angular and 1 to15 millimetres in diameter. Their colour ranges from yellow to reddish brown, and they sometimes have a silver to whitish centre. They usually become larger and more widely distributed as the growing season proceeds. Globose, ostiolate pycnidia that are brown to black in colour are often visible within spots on either leaf surface. Conidia exudates (pink masses or tendrils) can erupt from pycnidia during moist conditions. Conidia are hyaline, cylindric, and straight or slightly curved. They are variously septate (1 to 6), and measure 17 to 56 micrometres × 3 to 4 micrometres. Heavily affected leaves may turn yellow and drop prematurely.

Canker lesions occur mainly on the stems and young branches of susceptible host species and hybrids. They may also form on the slightly lignified leaf petioles. Green wood tissues of the current year’s growth appear to be particularly vulnerable to infection. The development of stem and branch lesions always begins with tissue discolouration. The lesions enlarge to form elongate, often depressed, dark brown to black necrotic cankers. Stems may look constricted due to the production of callus. Pycnidia are sometimes observed on young, current-year cankers. Morphology of cankers on older stems is variable and may be influenced by the secondary colonization of other canker-forming and decay fungi. Canker-weakened stems often break, and adventitious shoots are produced below damaged areas. In canker-tolerant clones, the development of lesions is slowed, and a defence response is activated. This results in the formation of lignified swellings with enlarged margins.

Disease cycle

The life cycle of S. musiva is completed in 1 year, with a sexual and asexual stage. The winter-spring part of the cycle is characterized by the production and dispersal of meiotic ascospores, differentiated in pseudothecia on leaves of the litter. Ascospores (hyaline, 1-septate, 17 to 24 micrometres × 3 to 6 micrometres) constitute the primary inoculum and are dispersed across short distances by rain splash, explaining the high incidence of the disease on the foliage of lower branches in spring. Ascospores penetrate leaves of young growth through the stomata and stems through wounds, lenticels, stipules, and leaf petioles. The initial lesions appear 3 to 4 weeks after bud burst. The summer part of the cycle is dominated by the spread of asexual (mitosporic) conidia, produced in pycnidia that are visible on symptomatic tissues (leaves and sometimes current-year cankers). Conidia are also dispersed by rain splash and cause new infections on leaves and stems. Like ascospores, the conidia penetrate buds, leaf petioles, and leaves through the stomata. In the autumn, spermogonia are produced from leaf spots on both senescent leaves still on the tree and on fallen leaves. Spermogonia will release spermatia at the end of the winter season that, after mating, will produce a pseudothecium containing asci and ascospores to complete the cycle.

Damage

Septoria canker is recognized as a major threat to exotic and hybrid poplar plantations in North America. The disease has affected poplar plantations on more than 44,000 hectares in Canada and 45,000 hectares in the United States. On the endemic host, eastern cottonwood, leaf spot damage is minor, and cankers are absent. On susceptible exotic and hybrid poplars, the pathogen can cause severe incidence of leaf spot that results in premature defoliation and reduction of tree growth, especially if repeated over multiple years. Canker attacks on susceptible trees affect tree growth and wood quality. They also cause a predisposition to wind breakage, and infection by secondary fungi (e.g., Phomopsis and Cytospora) and decay agents. Defoliation and branch and stem breakage can lead to a complete loss of highly susceptible clones and plantation failure.

Prevention and management

Pest management strategies for a particular pest vary depending on several factors. These include:

Decisions about pest management strategies require information about each of these factors for informed decision-making. These various factors should then be weighed carefully in terms of costs and benefits before action is taken against a particular pest.

Efforts to manage this disease have included cultural, chemical, and biological control practices. Removing leaf litter in nurseries has the potential to reduce primary inoculum (ascospores). The selection, breeding, and planting of disease resistant or tolerant genotypes constitute the most effective methods used to manage the damage caused by this pathogen. Application of a registered fungicide can be efficient to reduce propagation of inoculum. These practices have proven to be effective in nurseries and stoolbeds but remain ineffective or too expensive to be used during a typical 12- to 20-year rotation in a plantation. Fungicides registered for use against Septoria canker under specific situations may change from year to year for various reasons. Therefore, please search Health Canada’s Pesticide Product Information Database for currently registered fungicides and product information for use against Septoria canker. The application of any registered product should be based on population size and applied only when necessary and against the indicated life stage. It is also recommended to consult a local tree care professional. Chemical pesticides may be toxic to humans, animals, birds, fish, and other beneficial insects. Apply registered products only as necessary and according to all directions and precautions noted on the manufacturer’s label. In some jurisdictions and situations, only a licensed professional can apply pesticides. Consulting relevant local authorities to determine local regulations that are in place is recommended.

Selected references

Callan, B.E.; Leal, I.; Foord, B.; Dennis, J.J.; van Oosten, C. 2007. Septoria musiva isolated from cankered stems in hybrid poplar stool beds, Fraser Valley, British Columbia. North American Fungi 2: 1–9.  

EFSA Panel on Plant Health (EFSA PLH Panel). 2018. Pest categorisation of Sphaerulina musiva. EFSA Journal 16(4): e5247. https://doi.org/10.2903/j.efsa.2018.5247

European and Mediterranean Plant Protection Organization (EPPO). 1997. Data sheets on quarantine pests: Mycosphaerella populorum. In I.M. Smith, D.G. McNamara, P.R. Scott, and M. Holderness (editors). Quarantine pests for EUROPE, 2nd edition. CAB International. Wallingford, United Kingdom. 1425 p.

Feau, N.; Bernier, L. 2004. First report of Shining Willow as a host plant for Septoria musiva. Plant Disease 88(7): 770. https://doi.org/10.1094/PDIS.2004.88.7.770B

Herath, P.; Beauseigle, S.; Dhillon, B.; Ojeda, D.I.; Bilodeau, G.; Isabel, N.; Gros-Louis, M.-C.; Kope, H.; Zeglen, S.; Hamelin, R.C.; Feau, N. 2016. Anthropogenic signature in the incidence and distribution of an emerging pathogen of poplars. Biological Invasions 18: 1147–1161. https://doi.org/10.1007/s10530-015-1051-8

LeBoldus, J.M.; Blenis, P.V.; Thomas, B.R.; Feau, N.; Bernier, L. 2009. Susceptibility of Populus balsamifera to Septoria musiva: a field study and greenhouse experiment. Plant Disease 93(11): 1146–1150. https://doi.org/10.1094/PDIS-93-11-1146

Luley, C.J.; McNabb, H.S., Jr. 1989. Ascospore production, release, germination, and infection of Populus by Mycosphaerella populorum. Phytopathology 79(10): 1013–1018.

Ostry, M.E. 1987. Biology of Septoria musiva and Marssonina brunnea in hybrid Populus plantations and control of Septoria canker in nurseries. European Journal of Forest Pathology 17(3): 158–165. https://doi.org/10.1111/j.1439-0329.1987.tb00741.x

Ostry, M.E.; Wilson, L.F.; McNabb, H.F. Jr. 1989. Impact and control of Septoria musiva on hybrid poplars. United States Department of Agriculture, Forest Service, North Central Forest Experiment Station. St. Paul, Minnesota. General Technical Report NC-133. https://doi.org/10.2737/NC-GTR-133

Qin, R.; LeBoldus, J.M. 2014. The infection biology of Sphaerulina musiva: clues to understanding a forest pathogen. PLoS ONE 9(7): e103477. https://doi.org/10.1371/journal.pone.0103477

Sakalidis, M.L.; Feau, N.; Dhillon, B.; Hamelin, R.C. 2016. Genetic patterns reveal historical and contemporary dispersal of a tree pathogen. Biological Invasions 18: 1781–1799. https://doi.org/10.1007/s10530-016-1120-7

Sivanesan, A. 1990. CMI description sheets: Set 99. Mycopatholgia 109: 41–62. https://doi.org/10.1007/BF00437005

Søndreli, K.L.; Keriö, S.; Frost, K.; Muchero, W.; Chen, J.-G.; Haiby, K.; Gantz, C.; Tuskan, G.; LeBoldus, J.M. 2020. Outbreak of Septoria canker caused by Sphaerulina musiva on Populus trichocarpa in eastern Oregon. Plant Disease 104(12): 3266. https://doi.org/10.1094/PDIS-03-20-0494-PDN

Strobl, S.; Fraser, K. 1989. Incidence of Septoria canker of hybrid poplars in eastern Ontario. Canadian Plant Disease Survey 69(2): 109–112.

Tabima, J.F.; Søndreli, K.L.; Keriö, S.; Feau, N.; Sakalidis, M.L.; Hamelin, R.C.; LeBoldus, J.M. 2020. Population genomic analyses reveal connectivity via human-mediated transport across Populus plantations in North America and an undescribed subpopulation of Sphaerulina musiva. Molecular Plant-Microbe Interactions 33(2): 189–199. https://doi.org/10.1094/MPMI-05-19-0131-R

Cite this fact sheet

Feau, N. 2024. Septoria (leaf-spot and) canker of poplars. In J.P. Brandt, B.I. Daigle, J.-L. St-Germain, A.C. Skinner, B.C. Callan, and V.G. Nealis, editors. Trees, insects, mites, and diseases of Canada’s forests. Natural Resources Canada, Canadian Forest Service, Headquarters. Ottawa, Ontario.