Mycosphaerella populorum
(septoria leaf spot)

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Identity

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Preferred Scientific Name

• Mycosphaerella populorum G.E. Thomps. 1941

Preferred Common Name

• septoria leaf spot

Other Scientific Names

• Septoria musiva Peck 1884

International Common Names

• English: canker: poplar; septoria canker; septoria canker of poplar

Local Common Names

• Germany: Krebs: Pappel

EPPO code

• MYCOPP (Mycosphaerella populorum)

Summary of Invasiveness

Top of page Detailed studies of invasion are lacking, but the disease has developed rapidly in areas in which cultivation of highly susceptible hosts has been attempted in central and eastern North America. Survival ability, production of both windborne and rainsplashed inoculum, and repeating cycles of disease during the growing season suggest high invasive potential.

Taxonomic Tree

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• Domain: Eukaryota
•     Kingdom: Fungi
•         Phylum: Ascomycota
•             Subphylum: Pezizomycotina
•                 Class: Dothideomycetes
•                     Subclass: Dothideomycetidae
•                         Order: Capnodiales
•                             Family: Mycosphaerellaceae
•                                 Genus: Mycosphaerella
•                                     Species: Mycosphaerella populorum

Notes on Taxonomy and Nomenclature

Top of page The taxonomy followed in this datasheet is the traditional scheme outlined in Kirk et al. (2001). More recently, Ericksson et al. (2003) have placed the Mycosphaerellaceae among Dothidiomycetes Chaetothyriomycetes incertae sedis. These two classes correspond to the Loculoascomycetes of earlier schemes.

Description

Top of page Features of M. populorum have been described or illustrated by Peck (1884), Bier (1939), Thompson (1941), Waterman (1954), Sivanesan (1990b) and others. Ascoma (pseudothecia) are dark, solitary or aggregated, partially erumpent, globose (up to ca 110 µm diam) and ostiolate. Bitunicate asci are approximately 50-70 µm x 12-17 µm with eight, one-septate, hyaline ascospores, 16-28 x 4-6 µm. Conidioma (pycnidia) are dark brown, immersed, globose (up to ca 130 µm diam) to depressed globose and ostiolate. Conidia are cylindrical, tapered toward the ends, straight or slightly curved, (1-) 2-4 (-6) septate, hyaline, 15-55 x 3-4 µm, and may ooze in pinkish masses under moist conditions. Globose spermagonia with hyaline, aseptate spermatia (4.5-6 x 1.5 µm) are also produced.

Distribution

Top of page M. populorum, including its anamorph Septoria musiva, has been reported from across North America and from Argentina. In addition to the states and provinces specifically mentioned in the literature, general statements such as 'eastern North America' (Farr et al., 1989), 'Vermont to Georgia' (Anon., 1960) and 'throughout the Dominion' (Bier, 1939) suggest occurrence in other areas of the USA and Canada. However, many reports are poorly documented or consist only of a mention in a checklist. In particular, occurrence in North America west of the central plains states and prairie provinces should be viewed with scepticism. Susceptible species (Populus trichocarpa) and hybrids grown in Oregon, Washington and British Columbia do not appear to be attacked and M. populorum was not detected in a survey of the region in 1993 (Newcomb et al., 1995).

See also CABI/EPPO (1998, No. 220).

Distribution Table

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The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

Risk of Introduction

Top of page M. populorum is a European and Mediterranean Plant Protection Organization (EPPO) A1 quarantine list organism (EPPO, 2002a).

Hosts/Species Affected

Top of page M. populorum can affect all species of Populus native to North America. However, neither leaf spot nor canker diseases normally cause severe damage to species naturally occurring in the eastern USA and Canada (the aspen species P. grandidentata and P. tremuloides, the eastern cottonwood P. deltoides, and the balsam poplar P. balsamifera). The foliage and stems of clones of black cottonwood (P. trichocarpa [P. balsamifera subsp. trichocarpa]), which is native to western North America, may be severely damaged when artificially inoculated or grown where the pathogen is prevalent.

The incidence and severity of disease have been high in plantings of susceptible hybrids in central and eastern North America. However, susceptibility to either leaf spot or canker disease varies greatly among clones resulting from interspecific hybridization. Among the highly susceptible clones are many with parentage of P. balsamifera subsp. trichocarpa, P. maximowiczii (Japanese poplar) and P. nigra (European black poplar).

The host range and variation among clones in both the incidence and severity of septoria leaf spot and canker diseases is detailed in numerous papers containing anecdotal observations, descriptions of experimental studies and field surveys (e.g., Bier, 1939; Thompson, 1941; Waterman, 1954; Ostry and McNabb, 1985; Hansen et al., 1994). However, conclusions from field surveys have not always been supported by isolation or identification of the pathogen, and some authors have been careful not to attribute canker damage observed in the field solely to M. populorum (Lo et al., 1995). The potential for prediction of long-term canker disease damage from the responses of juvenile poplar clones to inoculation with M. populorum was demonstrated by Weiland et al. (2004).

Growth Stages

Top of page Seedling stage, Vegetative growing stage

Symptoms

Top of page Septoria leaf spot and canker symptoms have been described and illustrated by Bier (1939), Thompson (1941), Waterman (1954) and others, and vary somewhat among susceptible species and hybrids. Necrotic spots on leaves may develop within 3-4 weeks of leaf expansion, and may appear first on the foliage of branches close to the ground. Spots enlarge and become more general in crowns as the growing season proceeds. Spots may be circular to angular, appear grey to silver or become yellow to brown, and coalesce to include large portions of the leaf including the petiole. Conidioma are often produced within spots on either leaf surface. Affected leaves may turn yellow and drop prematurely.

On young, current-year shoots, lesions appear to develop from infection through leaf petioles, at stipules, stipule scars and leaf scars, through lenticels, at wounds, and through uninjured bark. Lesions enlarge to form elongate, often depressed, cankers that are dark brown or black but may include lighter brown or tan areas. Irregular concentric rings may appear in the bark of rapidly expanding cankers on highly susceptible hosts. Stems may be constricted at the canker, or appear swollen due to the production of callus. As cankers enlarge they often become more irregular. The vascular cambium may be killed and underlying wood discoloured to the pith. The stems of highly susceptible clones may be girdled in their first season of growth. Conidioma may develop in cankers within 4 weeks of infection, and pseudothecia have been found in cankers formed in the previous year.

The appearance of cankers on older stems is highly variable, and may be influenced by the invasion of other canker-producing fungi such as species of Cytospora or Phomopsis. The main stems may become infected by the expansion of cankers on lateral branches. Stem cankers may be largely restricted or become perennial to increase in size over time. Some stems persist with multiple disfiguring clones for years, whereas others are rapidly girdled and killed. Decay can develop within cankered stems and weakened stems are often broken, with adventitious shoots produced below damaged areas.

List of Symptoms/Signs

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Biology and Ecology

Top of page M. populorum produces spermagonia on colonized leaves in the autumn. It overwinters on these leaves and ascoma (pseudothecia) are produced in the spring. Both conidioma (pycnidia) and ascoma (pseudothecia) have also been observed on cankers produced during the previous growing season. Thus, primary inoculum may consist of either airborne ascospores or conidia dispersed in water. The germination of ascospores or conidia is followed by infection of the leaves or stems. Although histological details have not been presented, infection of stems reportedly occurs on at least some clones through leaf petioles, at stipules, stipule scars and leaf scars, through lenticels, at wounds, or through uninjured bark. Incipient lesions have been observed within 2 weeks of inoculation of stems and from 7 to 21 days of inoculation of leaves. Conidioma may be formed on leaves from 2 to 4 weeks after inoculation and conidia are secondary inoculum for cycles that can repeat several times during the growing season. For additional information, see Bier (1939), Thompson (1941) and Waterman (1954).

Although inherent resistance or susceptibility of hosts is an important determinant of the incidence and severity of disease, site conditions may also influence canker development. Canker disease ratings often reflected more severe damage to clones growing on harsh sites than on good sites in a clonal field performance trial in the north-central USA (Hansen et al., 1994). In greenhouse experiments, cankers were larger on inoculated, water-stressed poplars than on non-stressed trees (Maxwell et al., 1997).

Means of Movement and Dispersal

Top of page Natural Dispersal

Under natural conditions, M. populorum ascospores are windborne and Septoria musiva conidia are disseminated in water (rainsplash or stem flow). Either form of inoculum could be disseminated from colonized leaves moved by wind or water. The potential for spread by animals has not been studied, but spores could be casually acquired and spread by insects, birds, etc.

Movement in Trade/Transport

In trade, M. populorum could be moved on or in cuttings, infected seedling leaves and stems, cankered bark of trees, and colonized bark or wood of logs or green lumber.

Plant Trade

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Wood Packaging

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Impact

Top of page In central and eastern North America, leaf spot and canker diseases caused by M. populorum result in only minor damage to native poplar species. These diseases can damage P. trichocarpa planted in these regions, and susceptible hybrids can be extensively damaged by cankers. Cankers can significantly reduce economic value, provide means of entry to decay fungi, predispose stems to breakage, and girdle and kill stems. For example, Ostry et al. (1989) reported that 86% of trees in plantation in Michigan had Septoria cankers 5 years after planting, and 69% of trees had broken tops 7 years after planting.

Environmental Impact

Top of page No significant impact to natural environments has been noted in areas in which the pathogen co-occurs with its native hosts in central and eastern North America. The potential for significant environmental impact is great if M. populorum were introduced and established in the ranges of susceptible hosts in western North America, Europe and Asia.

Diagnosis

Top of page It is possible to isolate M. populorum from ascospores projected onto culture media, and from conidia streaked onto culture media. Conidioma and conidia will form in colonies produced on a V-8 vegetable juice medium (180 ml V-8 juice, Campbell Soup Co., Camden, NJ, USA; 2 g calcium carbonate; 20 g agar; 820 ml deionized water) (Krupinsky, 1989). It is possible to isolate the pathogen from canker margins, especially from young cankers, but its presence may be masked by more rapidly-growing fungi because the radial growth of M. populorum is slow. Identification from bark and wood has been enhanced using this medium amended with captan, chlorothalonil, iprodione and streptomycin sulphate (Stanosz and Stanosz, 2002). Conidioma with conidia were produced in colonies on both media within 3 weeks incubation at 21°C in continuous fluorescent light. Ascoma and ascospores also have been produced in culture (Luley et al., 1987).

Detection and Inspection

Top of page Although sometimes recognisable as characteristic, leaf and stem symptoms vary and are not unique. Therefore, M. populorum must be confirmed in plant material by recognition of ascoma with ascospores or conidioma with conidia. Both types of fruiting structures have been observed on leaves and stems (Bier, 1939). Other fungi are often present and fruiting, especially on older cankers, and can interfere with detection of M. populorum.

Similarities to Other Species/Conditions

Top of page Callan (1998) summarized the differences between M. populorum and other Mycosphaerella species associated with poplars in North America. The leaf spot and canker pathogen M. populicola (anamorph = Septoria populicola) has slightly larger ascospores (22-32 x 6-7 µm) and much shorter conidia (28-54 x 4 µm). The leaf saprophyte M. tassiana has similar ascospores (16-29 x 4-8 µm), but its asexual state is Cladosporium herbarum. The leaf pathogen M. populi (anamorph = Septoria populi) has larger ascospores (38-45 x 4-5 µm) and mostly one-septate conidia (30-40 x 3-4 µm) (Sivanesan, 1990a).

Prevention and Control

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Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Phytosanitary Measures

The programme on EPPO pest-specific phytosanitary measures (former specific quarantine requirements) is under complete revision and standard PM 2/17(4) is no longer available and will be replaced in due course by a revised version (EPPO, 2002b). However, the European and Mediterranean Plant Protection Organization (OEPP/EPPO, 1990) has recommended that all countries should prohibit the importation of plants for planting, cut branches and isolated bark of Populus from the Americas. If wood is imported from the Americas, the consignment must have been debarked or kiln dried. However, M. populorum has been isolated from wood, so debarking may not be adequate to prevent the introduction and possible survival of M. populorum in wood, and this requires further investigation.

Cultural Control and Sanitary Methods

Burying leaves in the soil by ploughing should reduce primary inoculum but will not eliminate the pathogen from infested locations because M. populorum can survive in cankers on trees.

Host-Plant Resistance

The use of resistant or tolerant poplar clones is the only practical management strategy for commercial production areas.

Biological Control

The inhibition of leaf spot development by strains of Streptomyces and by the fungus Phaeotheca dimorphospora has been studied (Yang et al., 1994; Gyenis et al., 2003), but efficacy and economic benefits in commercial poplar production have not been demonstrated.

Chemical Control

Multiple fungicide applications have reduced the incidence of canker on susceptible poplar clones in nursery stool beds (Ostry, 1987). Both hot-water and chemical treatments have been tested for surface disinfestation of artificially infested cuttings (Waterman and Aldrich, 1952; 1954). Hot-water treatment produced lethal effects on cuttings. Some chemicals reduced the frequency of isolation of fungi from buds and lenticels, but whether M. populorum was eliminated completely was unclear and additional research was recommended.

References

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Anon., 1960. Index of Plant Diseases in the United States. Crops Research Division, Agricultural Research Service, Agriculture Handbook No. 165. Washington, USA: United States Department of Agriculture.

Anon., 1970. California Fungi (Exsiccati set), Nos. 1-1325. Herbarium, University of California, Berkeley, California, USA.

Ares A; Gutierrez L, 1996. Selection of poplar clones for the Lower Valley of the Colorado River, Argentina. Forestry (Oxford), 69(1):75-82; 33 ref.

Bier JE, 1939. Septoria canker of introduced and native hybrid poplars. Canadian Journal of Research, 17:195-204.

Bowersox TW; Merrill W, 1976. Stand density and height increment affect incidence of Septoria canker in hybrid Poplar. Plant Disease Reporter, 60(10):835-837

CABI/EPPO, 1998. Distribution maps of quarantine pests for Europe (edited by Smith IM, Charles LMF). Wallingford, UK: CAB International, xviii + 768 pp.

Callan BE, 1998. Diseases of Populus in British Columbia: a diagnostic manual. Diseases of ^italic~Populus^roman~ in British Columbia: a diagnostic manual., ix + 157 pp.

Conners IL, 1967. An annotated index of plant diseases in Canada and fungi recorded on plants in Alaska, Canada and Greenland. Research Branch, Canada Department of Agriculture, Publ. 1251.

EPPO, 1990. Specific quarantine requirements. EPPO Technical Documents, No. 1008. Paris, France: European and Mediterranean Plant Protection Organization.

EPPO, 2002. EPPO A1 Quarantine List 2002-09. http://www.eppo.org/QUARANTINE/lists.html#a1.

EPPO, 2002. Pest specific phytosanitary measures. http://www.eppo.org/standards/sqr.html.

EPPO, 2014. PQR database. Paris, France: European and Mediterranean Plant Protection Organization. http://www.eppo.int/DATABASES/pqr/pqr.htm

Eriksson OE; Baral H-O; Currah RS; Hansen K; Kurtzman CP; Rambold; G; Laess¢e T, eds. , 2003. Outline of Ascomycota - 2003. Myconet, 9:1-89.

Farr DF; Bills GF; Chamuris GP; Rossman AY, 1989. Fungi on Plants and Plant Products in the United States. St. Paul, Minnesota, USA: APS Press, 1252 pp.

Filer TH Jr, 1975. Septoria leafspot and canker on cottonwood. In: Peterson GW, Smith RS, eds. Forest Nursery Diseases in the United States. USDA Forest Service, Agriculture Handbook, 470, 101-102.

Filer TH; McCracken FI; Mohn CA; Randall WK, 1971. Septoria canker on nursery stock of Populus deltoides. Plant Disease Reporter, 55:460-463.

Ginns JH, 1986. Compendium of plant disease and decay fungi in Canada, 1960-1980. Ottawa, Canada; Canadian Government Publishing Centre1813:416..

Gyenis L; Anderson NA; Ostry ME, 2003. Biological control of Septoria leafspot disease of hybrid poplar in the field. Plant Disease, 87:809-813.

Hanlin RT, 1963. A revision of the Ascomycetes of Georgia. Georgia Agricultural Experiment Station, Mimeo Series n.s., 175:1-65.

Hansen EA; Ostry ME; Johnson WD; Tolsted DN; Netzer DA; Berguson WE; Hall RB, 1994. Field performance of Populus in short-rotation intensive culture plantations in the north-central US. USDA Forest Service Research Paper NC-320.

Kirk PM; Cannon PF; David JC; Stalpers JA, 2001. Ainsworth and Bisby's dictionary of the fungi: 9th edition. Ainsworth and Bisby's dictionary of the fungi: 9th edition, Ed.9:xi + 655 pp.

Krupinsky JM, 1989. Variability in Septoria musiva in aggressiveness. Phytopathology, 79(4):413-416

Lo MH; Abrahamson LP; White EH; Manion PD, 1995. Early measures of basal area and canker disease predict growth potential of some hybrid poplar clones. Canadian Journal of Forest Research, 25(7):1113-1118

Luley CJ; Tiffany LH; McNabb HS Jr, 1987. In vitro production of Mycosphaerella populorum ascomata. Mycologia, 79(4):654-658

Maxwell DL; Kruger EL; Stanosz GR, 1997. Effects of water stress on colonization of poplar stems and excised leaf disks by Septoria musiva. Phytopathology, 87(4):381-388; 35 ref.

Mio LLMde; Amorim L, 2000. Diseases of poplars. (Doenças do álamo.) Floresta [Seminário de atualidades em proteção florestal: incênios, pragas e doencas, 06 a 08 de novembro de 2000, Curitiba, PR.], 30(1/2):139-153.

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

Ostry ME, 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

Ostry ME; McNabb HS Jr, 1985. Susceptibility of Populus species and hybrids to disease in the north central United States. Plant Disease, 69(9):755-757

Ostry ME; Wilson LF; McNabb HS Jr, 1989. Impact and control of Septoria musiva on hybrid poplars. General Technical Report - North Central Forest Experiment Station, USDA Forest Service, No. NC-133:5 pp.

Peck CH, 1884. Report of the state botanist. New York State Museum Report 35, 125-164.

Preston DA, 1945. Host index of Oklahoma plant diseases. Oklahoma Agricultural College, Agricultural Experiment Station Technical Bulletin (suppl.), T-21:1-39.

Romo Lozano Y; Moreno Rico O; Romero Cova S, 1992. Incidence and severity of quaking aspen (Populus tremuloides) aerial diseases in Aguascalientes, Ags. Revista Mexicana de Fitopatologia, 10(1):38-43

Sarasola AA, 1944. Dos septoriosis de las alamedas Argentinas. Rev. Argentina de Agron., 11:20-43.

Shaw CG, 1973. Host fungus index for the Pacific Northwest. I. Hosts. Bulletin, Washington Agricultural Experimental Station, No. 765.

Sprague R, 1955. A checklist of fungi of glacier bay, Alaska. Res. Stud. State Coll. Wash., 23:202-224.

Stanosz GR; Stanosz JC; Rousseau RJ, 2002. Hybrid poplar stem cankers caused by Mycosphaerella populorum in Kentucky, USA. Plant Pathology, 51(3):384; 4 ref.

Stanosz JC; Stanosz GR, 2002. A medium to enhance identification of Septoria musiva from poplar cankers. Forest Pathology, 32(3):145-152; 21 ref.

Thompson GE, 1941. Leaf-spot diseases of poplars caused by Septoria musiva and S. populicola. Phytopathology, 31:241-254.

Waterman AM, 1954. Septoria canker of poplars in the United States. USDA Circular 947.

Waterman AM; Aldrich KF, 1952. Surface sterilization of poplar cuttings. Plant Disease Reporter, 36:203-207.

Waterman AM; Aldrich KF, 1954. Additional information on the surface sterilization of poplar cuttings. Plant Disease Reporter, 38:96-100.

Weiland JE; Stanosz JC; Stanosz GR, 2004. Prediction of long-term canker disease damage from responses of juvenile poplar clones to inoculation with Septoria musiva. Plant Disease (in press).

Yang D; Bernier L; Dessureault M, 1994. Biological control of Septoria leaf spot of poplar by Phaeotheca dimorphospora. Plant Disease, 78(8):821-825

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

Distribution Maps

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