Scientific Name: Chalara fraxinea

Describer: Kowalski

Common Name: Ash dieback

Title: Update on ash dieback (Chalara fraxinea) in Europe: New confirmed hosts and description of the perfect state

Summary:

Significance: In previous Alerts (http://www.pestalert.org/viewNewsAlert.cfm?naid=69, http://www.pestalert.org/viewNewsAlert.cfm?naid=26), Chalara fraxinea was associated with a serious dieback disease of European ash (Fraxinus excelsior). While this disease has been observed in parts of Europe since the mid-1990s, the causal agent was not identified until 2006 (Kowalski, 2006). To date the pathogen has not been reported from outside Europe.

Issues of Concern: Initially, European ash (Fraxinus excelsior) was the only known host for Chalara fraxinea. In 2008 the fungus was also found on F. excelsior ‘Pendula’ (weeping ash) and Fraxinus angustifolia subsp. danubialis (narrow-leaved ash) (Kirisits et al., 2008). Field observations in 2009 showed that Fraxinus ornus was also a host, although this species was less susceptible than the other European ash species (Kirisits et al., 2009). New studies in Estonia have found several North American Fraxinusspecies to be susceptible: Fraxinus nigra (black ash), F. pennsylvanica (green ash) and F. americana (white ash), as well as an Asian ash species, F. mandschurica (Manchurian ash) (Drenkhan and Hanso, 2010). Black ash trees were particularly affected with symptoms including wilting of leaves, dieback and necrotic lesions of shoots and twigs, and death of canopy. Green ash trees were moderately affected (with symptoms similar to black ash, but with less evidence of dead shoots within the canopy). White and Manchurian ash trees were the least affected with symptoms including wilting of leaves, but only minor shoot and twig dieback and bark necrosis.

Initial taxonomic studies concerning Chalara fraxinea established that its perfect state was the ascomycete Hymenoscyphus albidus (Gillet) W. Phillips, a fungus that has been known from Europe since 1851. However, in a recent molecular taxonomic study of Hymenoscyphus albidus, there was significant evidence for the existence of two morphologically very similar taxa, H. albidus, and a new species, Hymenoscyphus pseudoalbidus (Queloz et al., 2010). Furthermore, studies now suggest that H. albidus is likely a non-pathogenic species, whereas H. pseudoalbidus is the virulent species responsible for the current ash dieback epidemic in Europe (Queloz et al., 2010). Hymenoscyphus pseudoalbidus was present in Switzerland for at least 30 years prior to the ash dieback epidemic, which commenced there in 2007. It is possible the fungus is an introduced species, or it has become increasingly pathogenic through mutation.

Distribution: Ash dieback continues to spread in Europe, presumably through natural spread via wind-blown ascospores, or through the movement of infected host materials. To date the pathogen is known to occur in the following countries: Austria; Czech Republic; Denmark; Estonia; Finland; France; Germany; Hungary; Italy; Latvia; Lithuania; Netherlands; Norway; Poland; Russia (Kaliningrad); Slovakia; Slovenia; Sweden; Switzerland (Drenkhan and Hanso, 2010; EPPO, 2010a; EPPO, 2010b; McKinney et al., 2010). 

References:

Drenkhan R, and Hanso M, 2010. New host species for Chalara fraxinea. New Disease Reports 22, 16. [doi:10.5197/j.2044-0588.2010.022.016] 

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. 

EPPO, 2010a. EPPO Alert on Chalara fraxinea. European and Mediterranean Plant Protection Organization, Paris, France. http://www.eppo.org/QUARANTINE/Alert_List/fungi/Chalara_fraxinea.htm (last updated Feb., 2010). 

EPPO, 2010b. EPPO Workshop on Chalara fraxinea (Oslo, Norway, 2010-06-30/07-02). European and Mediterranean Plant Protection Organization, Paris, France. http://archives.eppo.org/MEETINGS/2010_conferences/chalara_oslo.htm 

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.

Kirisits, T., M. Matlakova, S. Mottinger-Kroupa, T.L. Cech, and E. Halmschlager. 2009. The current situation of ash dieback caused by Chalara fraxinea in Austria. IN: Proceedings of the Conference of IUFRO Working Party 7.02.02, Egirdir, Turkey, 11-16 May, 2009. Ed. by T. Dogmuslehtijarvi. SDU Faculty of forestry Journal, ISSN:1302-7085, Serial: A, Special Issue: pp. 97-119. (cited in Queloz et al., 2010).

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

McKinney L.V., L.R. Nielsen, J.K. Hansen, and E.D. Kjær, 2010. Presence of natural genetic resistance in Fraxinus excelsior (Oleraceae) to Chalara fraxinea (Ascomycota): an emerging infectious disease. Heredity [Epub ahead of print], Sept., 8th, 2010.

Queloz, V., C. R. Grünig, R. Berndt, T. Kowalski, T.N. Sieber and O. Holdenrieder, 2010. Cryptic speciation in Hymenoscyphus albidus. Forest Pathology doi: 10.1111/j.1439-0329.2010.00645.x (published on-line 30 March, 2010).