Ash dieback in Europe and possible implication of Chalara fraxinea: addition to the EPPO Alert List
During the last decade, a new dieback of ash trees (Fraxinus excelsior) has been observed in several European countries. Affected trees are showing shoot dieback, bark necrotic lesions, stem cankers, wilting and premature leaf fall, and may die. Ash dieback was first observed in Poland and Lithuania in the early 1990s, in the mid-1990s in Latvia and Estonia, and then in Germany, Denmark, Sweden, Austria and the Åland archipelago of Finland (but not on the mainland). In Poland, a new fungus species was found associated with ash dieback and named Chalara fraxinea sp. nov. (Kowalski, 2006). So far, no teleomorph of this species has been observed in cultures or in nature but it is suggested that it may be closely related to species having a Ceratocystis perfect stage. C. fraxinea could be isolated from diseased shoots at the onset of the disease and also from dead roots of living trees. More recently, C. fraxinea has also been isolated from diseased ash trees in Germany (Schumacher et al., 2007). Although C. fraxinea is suspected to be the main cause of ash dieback, many other fungi were isolated from diseased ash trees in European countries and other abiotic factors (e.g. frost, drought) may also be involved in the disease. Despite the fact that studies are clearly needed to better understand the role of C. fraxinea in ash dieback, the EPPO Secretariat considered that it could usefully be added to the EPPO Alert List because the disease may be a threat to ash trees growing in European forests, parks, and nurseries.
Chalara fraxinea (Ash dieback)
Why: A disease suspected to be caused by a newly described fungus species, Chalara fraxinea, has increasingly been observed in European countries on ash trees (Fraxinus excelsior) in the last ten years. Because ash dieback may represent a serious threat to forest, amenity and nursery ash trees, the EPPO Secretariat decided that C. fraxinea should be added to the EPPO Alert List. However, it is acknowledged that much data is lacking on its pathogenicity (other biotic and abiotic factors could be involved in ash dieback), biology, geographical distribution and economic impact.
Where:
EPPO region: Germany, Poland (C. fraxinea has been identified in these two countries). On the basis of symptoms, the disease has also been observed in: Austria, Denmark, Estonia, Finland (Åland archipelago but absent on the mainland), Latvia, Lithuania, Sweden.
On which plants: Fraxinus excelsior (European ash). No data is available on the susceptibility of other Fraxinus species.
Damage: Initially, small necrotic spots (without exudate) appear on stems and branches. These necrotic lesions then enlarge resulting in wilting, dieback of branches and particularly in the death of the top of the crown. The disease is often chronic but can be lethal. Ash dieback has been observed not only on forest trees but also in urban areas (parks and gardens) and in nurseries.
Pictures of the disease can be viewed on the Internet: http://www.plantesygdomme.dk/Asketoptoerre/thumbnails.html
Dissemination: Data is lacking on the biology of C. fraxinea. It was isolated from diseased twigs and branches, as well as in dead roots of living ash trees.
Pathway: Although data is lacking on the biology of the fungus, it seems likely that plants for planting and wood of F. excelsior could be pathways for spreading the disease over long distances.
Possible risks: Fraxinus are widely grown across the EPPO region both for forestry and amenity purposes. Although data is still lacking on the exact role of C. fraxinea in ash dieback, EPPO member countries should be warned that ash dieback is emerging in Europe and that there may be a risk in moving diseased F. excelsior plants across the region without any precaution. Further studies are obviously needed on the etiology of ash dieback, its geographical distribution and economic impact.
EPPO RS 2007/179
Panel review date - Entry date 2007-09
Sources
Kowalski T (2006) Chalara fraxinea sp. nov. associated with dieback of ash (Fraxinus excelsior) in Poland. Forest Pathology 36(4), 264-270.
Schumacher J, Wulf A, Leonhard S (2007) [First record of Chalara fraxinea T. Kowalski sp. nov. in Germany – a new agent of ash decline.] Nachrichtenblatt des Deutschen Pflanzenchutzdienstes 59(6), 121-123 (in German).
INTERNET (last retrieved in 2007-09)
Forest & Landscape Denmark. Ash dieback in Denmark. http://en.sl.life.ku.dk/forskning/fagdatacenterskov/skovsundhed/skader/asketoptoerre.aspx
NAPPO – Pest Alert System. Chalara fraxinea Kowalski - Intensive dieback of European ash in Poland associated with a newly described fungal species, Chalara fraxinea. http://www.pestalert.org/viewNewsAlert.cfm?naid=26
Nordic Forest Research Cooperation Committee. Ash decline in Nordic and Baltic countries. http://www.metla.fi/org/pathcar/ash-decline.htm
Research and Training Centre for Forests, Natural Hazards and Landscape (BFW)
Actual situation of dieback of ash in Austria by TL Cech and U Hoyer-Tomiczek. http://bfw.ac.at/400/pdf/fsaktuell_40_3.pdf
Ash dieback and premature leaf shedding in Austria by TL Cech. http://bfw.ac.at/400/pdf/fsaktuell_37_8.pdf