Chalara fraxinea (Kowalski)
Significance: Ash dieback caused by Chalara fraxinea has been highlighted in previous NAPPO-PAS Pest Alerts (http://www.pestalert.org/viewNewsAlert.cfm?naid=69; http://www.pestalert.org/viewNewsAlert.cfm?naid=26; http://www.pestalert.org/viewNewsAlert.cfm?naid=86). The pathogen causes a serious dieback disease of European ash (Fraxinus excelsior), as well as other Fraxinus species, in many parts of Europe. The disease was first observed in parts of Eastern Europe in the mid-1990s, although the actual causal agent of the disease, Chalara fraxinea, was not identified until 2006 (Kowalski, 2006). Initially the teleomorph was reported to be a previously identified species Hymenoscyphus albidus, which was considered to be non-pathogenic, native and widespread in Europe (Kowalski and Holdenreider, 2009). However, further molecular work showed that a morphologically similar species, Hymenoscyphus pseudoalbidus, is the actual teleomorph (Queloz et al., 2010).
Ash dieback is characterized by a rapid dieback of the crown, associated with the presence of cankers or lesions girdling the affected plant parts, usually leading to tree death (Kowalski & Holdenrieder, 2009; DEFRA, 2012). It has seriously affected a high percentage of ash trees in continental Europe. It is estimated that as much as 90 per cent of ash trees in some areas are affected by the disease (Forestry Commission, 2012a). Some scientists believe it may be causing similar reductions to Fraxinus populations throughout Europe (Coghlan, 2012).
Issues of Concern: Ash dieback continues to spread rapidly across Europe, through natural spread via wind-blown ascospores, as well as through the movement of infected host materials, such as nursery stock. Some have also suggested that because the pathogen is able to produce conidia on infected wood, that wood and wood products might also be able to spread the pathogen (Husson et al., 2012). Restrictions on the movement of timber or firewood may help prevent spread (Forestry Commission, 2012b). Seeds of declining Fraxinus excelsior trees have also been shown to carry the pathogen at low levels, although what effect the pathogen has on seed germination and seedling emergence (i.e., seed-transmission) is currently not known (Cleary et al., 2012).
Distribution: The actual origin of the ash dieback pathogen remains unknown. Some have speculated that the disease may have an Asian origin (Coghlan, 2012). Since its recognition in Poland in 2006 reports of the fungus have spread rapidly across Europe. It is currently reported from the following countries: Austria, Belarus, Belgium, Croatia, Czech Republic, Denmark, Estonia, Finland, France, Germany, Hungary, Italy, Latvia, Lithuania, The Netherlands, Norway, Poland, Romania, Russia, Slovakia, Slovenia, Sweden, Switzerland (EPPO, 2010; Timmermann et al., 2011). Most recently the disease has been reported for the first time from the United Kingdom (EPPO, 2012), and is now considered widely distributed there (Forestry Commission, 2012c). It has also been detected in Ireland (IGNS, 2012).
Cleary, M.R., N. Arhipova, T. Gaitnieks, J. Stenlid, and R. Vasaitis, 2012. Natural infection of Fraxinus excelsior seeds by Chalara fraxinea. Forest Pathology (Short Communication): doi: 10.1111/efp.12012. 3 pp. Coghlan, A., 2012. Are Europe's ash trees finished?" New Scientist. 31 October 2012. http://www.newscientist.com/article/dn22449-are-europes-ash-trees-finished.html. DEFRA, 2012. Rapid Risk Assessment: Rapid assessment of the need for a detailed Pest Risk Analysis for Chalara fraxinea. http://www.fera.defra.gov.uk/plants/plantHealth/pestsDiseases/documents/chalaraFraxinea.pdf (accessed November 6th, 2012). EPPO, 2010. EPPO Reporting Service No.9:2010/151 - First report of Chalara fraxinea in the Netherlands. EPPO, Paris, France. EPPO, 2012. EPPO Reporting Service No.4:2012/080 - First report of Chalara fraxinea in the United Kingdom. Forestry Commission, 2012a. Chalara dieback - Key scientific facts (12 Nov., 2012). http://www.forestry.gov.uk/website/forestry.nsf/byunique/infd-8zss7u. Forestry Commission, 2012b. Chalara - Effects of new legislation on the timber and firewood trades. http://www.forestry.gov.uk/forestry/INFD-8ZYHKS. Forestry Commission, 2012c. Chalara dieback of ash (Chalara fraxinea) (14 Nov., 2012). http://www.forestry.gov.uk/chalara#Importrestrictions. Husson, C., O. Cael, J. P. Grandjean, L. M. Nageleisen and B. Marcais, 2012. Occurrence of Hymenoscyphus pseudoalbidus on infected ash logs. Plant Pathology: Doi: 10.1111/j.1365-3059.2011.02578.x. IGNS, 2012. Chalara disease found in young ash trees. News Release, Published on Friday 12th October 2012. MerrionStreet.ie (Irish Government News Service). http://www.merrionstreet.ie/index.php/2012/10/chalara-disease-found-in-young-ash-trees/?cat=12. 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. 2009. The teleomorph of Chalara fraxinea, the causal agent of ash dieback. Forest Pathology 39: 304-308. 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). Timmermann, V., I. Borja, A.M. Hietala, T. Kirisits and H. Solheim. 2011. Ash dieback: pathogen spread and diurnal patterns of ascospore dispersal, with special emphasis on Norway. IN EPPO Workshop on Chalara fraxinea, A Major Threat for Ash Trees in Europe, Oslo, Norway, 30 June-2 July 2010. Bulletin OEPP/EPPO Bulletin 41(1):14-20.