Septoria leaf spot of poplars

Written by B.E. Callan Revision 2024

• French disease name: Tache foliaire des peupliers baumiers
• Pathogen name: Sphaerulina populicola (Peck) Quaedvl., Verkley & Crous
• Kingdom: Fungi
• Phylum: Ascomycota
• Class: Dothideomycetes
• Order: Mycosphaerellales
• Family: Mycosphaerellaceae
• Partial list of synonyms:
  • Mycosphaerella populicola G.E. Thompson
  • Septoria populicola Peck

General information and importance

Septoria leaf spot of poplars can be found in stands of susceptible poplar (Populus) hosts each year. It occasionally causes serious outbreaks in balsam poplar (P. balsamifera) and black cottonwood (P. trichocarpa) in Canada.

Susceptible stands are most heavily affected in years with cool, wet springs followed by an early onset of warm summer weather. Under these conditions, almost every tree can suffer severe defoliation. Unlike the closely related species, S. populorum (formerly known as Septoria musiva), S. populicola does not cause cankers on its poplar hosts.

Distribution and hosts

Sphaerulina populicola is endemic to North America. Its natural distribution follows that of its two main hosts, black cottonwood (P. trichocarpa) in western North America, and balsam poplar (P. balsamifera) throughout its range from the Atlantic provinces to the eastern slopes of the Rockies in Alberta. In Canada, it has been reported in Alberta, British Columbia, Manitoba, New Brunswick, Nova Scotia, Ontario, Quebec, Saskatchewan, and the Northwest Territories. It is especially prevalent in black cottonwood stands in the Pacific Northwest, and in balsam poplar stands in central North America.

Black cottonwood is the most common host in western Canada, while balsam poplar is the primary host in the rest of Canada. The disease also causes damage on susceptible hybrid poplars, such as crosses with black cottonwood and eastern cottonwood (P. deltoides), and black cottonwood with Japanese poplar (P. maximowiczii). Populus tremuloides (trembling aspen) has been reported as a host on rare occasions, but recent research indicates that the Sphaerulina occurring in this host-association might in fact be an undescribed species.

Tree parts affected

Sphaerulina populicola causes only leaf spots in natural field conditions. Heavy infestations occasionally cause spots on leaf petioles as well. Cankers have been induced on P. balsamifera ssp. balsamifera in greenhouse inoculation experiments.

Symptoms and signs

The overwintering sexual state of the fungus is produced in dead leaves on the ground. These leaves bear many clusters of tiny subepidermal pseudothecia that are 96 to 160 micrometres in diameter, blackish brown, and globose, with a slightly protruding central opening (ostiole). The asci develop within the inner pseudothecial tissue rather than on the typical hymenium produced by a perithecium. The pseudothecial tissue surrounding the asci disintegrates as the asci mature. Asci are bitunicate (double-walled), cylindrical to club-shaped, 64 to 110 micrometres long × 13 to 16 micrometres wide, rounded at the apex, and contain eight ascospores. Ascospores are subcylindrical to fusiform (tapered at either end), straight or slightly curved, and two-celled, with a single septum close to the centre of the spore. They are hyaline, smooth-walled, and measure 22 to 32 micormetres × 6 to 6.5 micrometres.

Infections on young leaves are first evident in early summer when yellow lesions begin to develop. These lesions eventually turn dark brown to black, with lighter coloured, pale brown to tan centres where the pycnidia develop. Highly susceptible trees show overall leaf yellowing associated with the spots. Leaf spots are initially small (1 to 15 millimetres in diameter). They often coalesce with adjacent spots as they expand, resulting in the necrosis of the entire leaf. Individual spots have rounded margins, and the spots often form in rows parallel to the long axis of the leaf. Globose, black pycnidia are 48 to 128 micrometres in diameter, each with a circular central opening (ostiole). They develop within the leaf spots, usually on the upper leaf surface, but occasionally on the leaf undersurface. Pycnidia ooze pinkish tendrils of conidia during periods of high humidity. Conidia are hyaline, long, sinuous, and 60 to 110 micrometres long × 3.5 to 4.5 micrometres wide. They have 3 to 6 septa (cross walls) spaced evenly along the length of the spores.

Unlike the closely related Sphaerulina populorum, S. populicola does not cause cankers on its host. Sphaerulina populorum can be distinguished from S. populicola by this symptom, but also from the morphology of its conidia. Its conidia are much smaller, measuring 28 to 54 micrometres long x 4 micrometres wide, on average.

Disease cycle

Dead, overwintered leaves on the ground below trees are the primary source of inoculum in the spring. During daytime periods of warm, wet weather, ascospores are forcibly ejected from the pseudothecia on the dead leaves. They infect the first flush of young leaves on branches closest to the ground (the source of inoculum). Well-spaced, mature trees lacking branches close to the ground are less affected by leaf spot due to the greater distance from the ascospore inoculum.

Leaf symptoms first appear in early summer. During the growing season, infections intensify, moving upward and eventually involving leaves throughout the crown and spreading from rain splashed conidia that are produced in the pycnidia on the leaf spots. The conidia penetrate the host leaves via stomates. At the end of the growing season, pycnidial production ceases and pseudothecia start to develop on the leaves. The pseudothecia overwinter on fallen leaves and mature to produce asci and ascospores in the spring.

Damage

Leaf spot incidence is highest in poplar stands that experience cool, wet spring weather that is followed by an early onset of warm weather in the summer. When poplar stands are exposed to such conditions, infections are often heavy, with leaf yellowing and premature defoliation affecting most trees. An outbreak in the Pacific Northwest had a range of leaf spotting from isolated to severe spots (coalesced spots on chlorotic or necrotic leaves with some spots on petioles). Up to 50% of the leaves in the crowns of some trees were severely infected by late August.

Heavy defoliation can cause death of young saplings. In older trees, it causes a reduction of growth and wood fibre.

Prevention and management

Removal and destruction of dead leaves in late autumn will reduce the springtime source of inoculum. This may only be practical in a nursery setting with young trees. Eastern cottonwood is resistant to S. populicola. Selection of resistant hybrid poplars for plantations is a practical method of preventing Septoria leaf spot. Many hybrid poplar clones propagated from crosses of black cottonwood and eastern cottonwood show resistance to S. populicola.  

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

• the population level of the pest (i.e., how numerous the pest is on the affected host[s]);
• the expected damage or other negative consequences of the pest’s activity (either to the host, property, or the environment);
• an understanding of the pest’s life cycle, its various life stages, and the various natural or abiotic agents that affect population levels;
• how many individual host specimens are affected (an individual tree, small groups of trees, plantations, forests);
• the value of the host(s) versus the costs of pest management approaches;
• consideration of the various silvicultural, mechanical, chemical, biological, and natural control approaches available and their various advantages and disadvantages; and
• and consideration of the various silvicultural, mechanical, chemical, biological, and natural control approaches available and their various advantages and disadvantages.

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.

Photos

Leaf spots on hybrid poplar caused by the fungus Sphaerulina populicola.

Brenda Callan

Selected references

Callan, B.E. 1998. Diseases of Populus in British Columbia: a diagnostic manual. Natural Resources Canada, Canadian Forest Service, Pacific Forestry Centre. Victoria, British Columbia. 157 p.

Feau, N.; Hamelin, R.; Bernier, L. 2006. Attributes and congruence of three molecular data sets: Inferring phylogenies among Septoria-related species from woody perennial plants. Molecular Phylogenetics and Evolution 40: 808–29. https://doi.org/10.1016/j.ympev.2006.03.029 

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

Newcombe, G.; Bradshaw, H.D. Jr. 1996. Quantitative trait loci conferring resistance in hybrid poplar to Septoria populicola, the cause of leaf spot. Canadian Journal of Forest Research 26(11): 1943–1950. https://doi.org/10.1139/x26-219

Newcombe, G.; Chastagner, G.A.; Callan, B.E.; Ostry, M.E. 1995. An epidemic of Septoria leaf spot on Populus trichocarpa in the Pacific Northwest in 1993. Plant Disease 79(2): 212.

Quaedvlieg, W.; Verkley, G.J.M.; Shin, H.D.; Barreto, R.W.; Alfenas, A.C.; Swart, W.J.; Groenewald, J.Z.; Crous, P.W. 2013. Sizing up Septoria. Studies in Mycology 75(1): 307–390.  https://doi.org/10.3114/sim0017

Thompson, G.E. 1941. Leaf-spot disease of poplars caused by Septoria musiva and S. populicola. Phytopathology 31: 241–254.

Zalasky, H. 1978. Stem and leaf spot infections caused by Septoria musiva and Septoria populicola on poplar (Populus balsamifera) seedlings. Phytoprotection 59(1): 43–50.

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