Ash dieback, caused by 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, C. fraxinea was added to the EPPO Alert List. However, much data is lacking on its pathogenicity (other biotic and abiotic factors could be involved in ash dieback), biology, geographical distribution and economic impact. Therefore EPPO organized a workshop to bring together researchers and experts in forestry quarantine to discuss recent research progress and advice NPPOs how to slow down the spread of this disease.
At the kind invitation of the “Norsk Institutt for Skog og Landskap” (Norwegian Forest and Landscape Institute) the workshop was organized in Oslo. A total of 44 experts from 23 countries participated in this workshop. Warm thanks are due to Mr Bjørn Økland and his colleagues for the fine organization of the meeting and their warm welcome in Oslo.
The workshop discussed in particular the following main topics:
The workshop participants agreed, based on the current knowledge of this pest, several observations, conclusions and recommendations which are considered essential for NPPOs.
Technical visit (view more pictures of the Workshop>>)
The causal agent of dieback of European ash (Fraxinus excelsior) was first (2006) described as Chalara fraxinea. Study of the teleomorph of C. fraxinea revealed a species complex but that the strain causing ash dieback should be re-assigned as Hymenoscyphus pseudoalbidus (2009).
A new real-time PCR test for the identification of Chalara fraxinea was presented which has been recently accredited by the French accreditation body. The test is specific for C. fraxinea and does not cross react with the non-pathogenic Hymenosciphus albidus.
F. excelsior and F. angustifolia are susceptible but C. fraxinea has not been reported on F. ornus (although some susceptibility was seen in inoculation experiments). Regarding American and Asian Fraxinus spp. more information is required regarding their susceptibility to C. fraxinea.
Many uncertainties still remain regarding the life cycle, infection process and the influence of environmental conditions and further research is required to address these.
Reports of the spread and impact of C. fraxinea were presented from Austria, Czech Republic, Denmark, Estonia, Finland, Germany, Latvia, Lithuania, Norway, Slovakia, Slovenia and Sweden. A large endangered area of the EPPO region is however not infested yet (in particular the Western part of Europe).
The economic and environmental impact of this disease is very substantial. For instance in Denmark it is estimated that 60-90 % of ash stands are affected and will disappear. Comparable impacts were reported from Germany, including serious losses in nurseries growing ash plants. The ecological value of F. excelsior is very important e.g. as a valuable species on river banks and in cultural landscapes.
It is necessary to systematically collect data regarding the impact of C. fraxinea, including survey results regarding the spread of the disease and its presence in nurseries, number of diseased trees removed, costs of measures.
In infested areas C. fraxinea is not only found in forest and amenity plantations but also in nurseries (including on 1-year old seedlings), as reported by Austria. Severe impacts on young ash plants in nurseries were reported by Germany.
The importance of maintaining apparently tolerant specimens in infested stands was underlined as these may form a basis for the development of future tolerant populations. If trees in the urban environment are infested, the progress of the disease may be slowed down by removing leaves on the ground during autumn to lower ascospore production. It is observed that ash dieback symptoms are more serious in ash trees planted in wet and organic rich soils.
Although questions regarding spread mechanisms still remain, it is likely that natural spread is the most important factor on the continent. However, for endangered areas which are not infested yet the introduction (UK, Irl) or spread can be slowed down by preventing the spread associated with plants for planting. Plants for planting which are produced in nurseries outside infested areas from disinfected seeds and traded in spring (to avoid trade of asymptomatic plants) pose a lower risk.
Chalara fraxinea identity, hosts and life cycle
Hymenoscyphus species associated to European ash
A.M. Hietala & H. Solheim (NO)
Mechanisms of spread and epidemiology of Chalara fraxinea
Disease spread and tracking spores
H. Solheim, V. Timmerman & I. Børja (NO)
Seasonal activity and genetic variation of Chalara fraxinea
S. Johansson (SE)
Endangered area and impact
Impact of Chalara fraxinea ash dieback on Danish forests
I. M. Thomsen (DK)
Studies on ash dieback caused by Chalara fraxinea in Austria - VIDEO: Ascospore release (1.8 Mbytes)
T. Kirisits & T. L. Cech (AT)
Contemporary situation on ash dieback in Czech Republic
L. Jankovsky (CZ)
Ultrastructural modifications in ash tissues colonized by Chalara fraxinea
L. Montecchio, E. Dal Maso, L. Scattolin (IT)
Root rot and dieback of Fraxinus excelsior
R. Vasaitis (SE)
Rachises as key to ash decline due to Chalara fraxinea
N. Ogris (SI)
Survey and control measures
Circumstances for successful pathogen eradication
D. Jurc (SI)
New approaches for biocontrol of the ash dieback caused by Chalara fraxinea in Egypt
F.M. Salem (EG)
Chalara fraxinea, how to deal with it in the cultural landscape
J. Stenlid (SE)
- Influence of temperature and available water on growth of Chalara fraxinea. T. Hauptman (SI)
- Ash dieback in Czech Republic – pathogen Chalara fraxinea. V. Kostalova (CZ)
- Chalara fraxinea in Estonia. R. Drenkhan (EE)
- Ash (Fraxinus excelsior) decline in Lithuania and investigation of fungus Chalara fraxinea. A. Gustiene (LT)
- Common ash (Fraxinus excelsior) dieback in Latvia. N. Arhipova (LV)
- Chalara fraxinea on Fraxinus spp. in Norway (. V. Talgo (NO)
- Occurrence of ash dieback in Slovakia. R. Leontovic (SK)
- Chalara fraxinea: survey strategy in Wallonia. A. Chandelier (BE)