Scientific Name: Chalara fraxinea

Describer: Kowalski

Common Name: Ash dieback

Title: Update on ash dieback (Chalara fraxinea) - Spreading in Europe

Summary:

Significance: Chalara fraxinea was recently identified as the causal agent of the intensive dieback of European ash (Fraxinus excelsior) occurring throughout Northern Europe (Kowalski, 2006). Symptoms begin at the top of the tree with small necrotic spots on stems and branches. These spots enlarge, resulting in wilting and dieback of branches, death of the top of the crown, and eventually death of the trees (Dobrowolska et al., 2008; EPPO, 2008; Halmschlager and Kirisits, 2008; Kowalski, 2006). Ash dieback affects trees of all ages, but is particularly severe on young saplings (Halmschlager and Kirisits, 2008). Symptoms of infection are often similar to those seen with infestation by the emerald ash borer (Agrilus planipennis) that is currently attacking North American ash trees. By 2002, approximately 60 percent of all ash stands in Lithuania had been killed by ash dieback (Vasaitis and Lygis, 2008). Eighty percent of ash stands in Poland are known to be affected (Lingren, 2008).

In 2006, the newly described fungus was causing intensive dieback of European ash in Poland (http://www.pestalert.org/viewNewsAlert.cfm?naid=26), and now C. fraxinea has spread throughout northern Europe (see distribution list). The teleomorph (sexual stage of the fungus) has recently been shown to be Hymenoscyphus albidus (Helotiaceae : Helotiales), a saprophyte apparently native and widespread in Europe (Kowalski and Holdenrieder, 2009b).

Issues of Concern: It is not known if Chalara fraxinea is more widely distributed in parts of Europe (or elsewhere) or if it poses a threat to other ash species, such as our 16 native North American species. Additional information is also needed regarding the host and distribution information for the teleomorph, H. albidus.

While it has been concluded that Chalara fraxinea is the primary causal agent of ash dieback, its exact role is not clear, as it is commonly found in declining ash trees that are also colonized by other potentially pathogenic fungi (Kowalski and Holdenrieder, 2009a). The reasons behind the recent emergence of ash dieback from the widespread teleomorph are not clear, though it may be attributed to changes in environmental conditions or the behavior of the pathogen (Kowalski and Holdenrieder, 2009b).

Currently, there are no effective control measures for Chalara fraxinea.

Pathways: The teleomorph, Hymenoscyphus albidus, is wind-transmitted and is likely to be more important for dispersal than the sticky conidia of C. fraxinea (Kowalski and Holdenrieder, 2009b). Chalara fraxinea may be able to disperse aerially, in soil, water, plants for planting, seeds, or wood (EPPO, 2008; Kile, 1993). Insect vectors are also important in moving various Chalara species (Nag Raj and Kendrick, 1993; Webber and Brasier, 2001), though none have been identified for C. fraxinea.

Hosts: European ash (Fraxinus excelsior) is the only known host for Chalara fraxinea. Single observations have also occurred on Fraxinus angustifolia subsp. danubialis(narrow-leaved ash) and Fraxinus excelsior ‘Pendula’ (weeping ash) (Kirisits et al., 2008). Susceptibility of other species to Chalara fraxinea is unknown.

Distribution: Austria; Denmark; Czech Republic; Finland; Germany; Hungary; Lithuania; Norway; Poland; Slovenia; Sweden (EPPO, 2008; Jankovsky et al., 2008; Ogris et al., 2009; Thomsen, 2008). Based on symptoms observed in European ash, Chalara fraxinea is also suspected to occur in Estonia, Latvia, and Switzerland (EPPO, 2008). 

References:

Dobrowolska, D., S. Hein, A. Oosterbaan, J. P. Skovsgaard, and S. P. Wagner. 2008. Ecology and growth of European ash (Fraxinus excelsior L.). ValBro. 35 pp. 

EPPO. 2008. Chalara fraxinea: Ash dieback. European and Mediterranean Plant Protection Organization (EPPO). Last accessed January 6, 2009, http://www.eppo.org/QUARANTINE/Alert_List/fungi/Chalara_fraxinea.htm. 

Halmschlager, E., and T. Kirisits. 2008. First report of the ash dieback pathogen Chalara fraxinea on Fraxinus excelsior in Austria. New Disease Reports 17. 

Jankovsky, L., D. Palocikova, and M. Dvorak. 2008. Alien Diseases of Woody Plants in the Czech Republic. Forstschutz Aktuell 44:32-34. 

Kile, G. A. 1993. Plant Diseases Caused by Species of Ceratocystis sensu stricto and Chalara. Pages 173-183 in M. J. Wingfield, K. A. Seifert, and J. F. Webber, (eds.). Ceratocystis and Ophiostoma: Taxonomy, Ecology, and Pathogenicity. The American Phytopathological Society, St. Paul, Minnesota. 

Kirisits, T., M. Matlakova, S. Mottinger-Kroupa, and E. Halmschlager. 2008. Involvement of Chalara fraxinea in Ash Dieback in Austria. Forstschutz Aktuell 44:16-18. 

Kowalski, T. 2006. Chalara fraxinea sp. nov. associated with dieback of ash (Fraxinus excelsior) in Poland. Forest Pathology 36:264–270. 

Kowalski, T., and O. Holdenrieder. 2009a. Pathogenicity of Chalara fraxinea. Forest Pathology 39:1-7. 

Kowalski, T., and O. Holdenrieder. 2009b. The teleomorph of Chalara fraxinea, the causal agent of ash dieback. Forest Pathology:1-5. 

Lingren, D. 2008. Low budget breeding and seed production with special attention to a seed orchard proposal of ash (Fraxinus). Swedish University of Agricultural Sciences, Department of Forest Genetics and Plant Physiology, Umeå, Sweden. Last accessed January 7, 2009, http://www-genfys.slu.se/staff/dagl/TREEBREEDEX/Ash_Seed_Production.htm.  

Nag Raj, T. R., and W. B. Kendrick. 1993. The Anamorph as Generic Determinant in the Holomorph: The Chalara Connection in the Ascomycetes, with Special Reference to the Ophiostomatoid Fungi. Pages 61-70 in M. J. Wingfield, K. A. Seifert, and J. F. Webber, (eds.). Ceratocystis and Ophiostoma: Taxonomy, Ecology, and Pathogenicity. The American Phytopathological Society, St. Paul, Minnesota. 

Ogris, N., T. Hauptman, and D. Jurc. 2009. Chalara fraxinea causing common ash dieback newly reported in Slovenia. New Disease Reports 19. 

Thomsen, I. M. 2008. Ash Dieback in Denmark. University of Copenhagen. Last accessed January 6, 2009, http://en.sl.life.ku.dk/Forskning/FagdatacenterSkov/SkovSundhed/Skader/Asketoptoerre.aspx.   

Vasaitis, R., and V. Lygis. 2008. Emerging forest diseases in south-eastern Baltic Sea region. Pages 14-15 in Network of Climate Change Risks on Forests (FoRisk): SNS Workshop, Umea, Sweden. 

Webber, J., and C. Brasier. 2001. Sapstain and Vascular Pathogens. Risks of Exotic Forest Pests and their Impact on Trade, An International Online Workshop to reduce movement of forest pests with a minimal impact on trade. April 16-29, 2001.