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Chalara ash dieback - Key scientific facts

The Government's Chief Scientific Adviser established an expert group in 2012 to advise on the scientific evidence and approach to Chalara dieback of ash.


Chalara dieback of ash is a disease of ash trees caused by a fungus of Asian origin called Hymenoscyphus fraxineus (H. fraxineus; formerly called Chalara fraxinea) [1] . 

The disease causes leaf loss and crown dieback in affected trees, and it can lead to tree death. Ash trees suffering from symptoms likely to be caused by H. fraxineus are found across Europe. These have included forest trees, trees in urban areas, and young trees in nurseries.  H. fraxineus is a quarantine pest under national emergency measures.

Confidence ratings as used below:

High – supported by experimental evidence and repeated observations which have been published in peer-reviewed scientific literature.

– supported by long-term observation or multiple observations from reputable sources which come to the same conclusion, or by comparative analysis of related species.

– supported by a few observations or many observations from reputable sources whicht are not suggesting a consistent pattern, or an accumulation of anecdotal evidence which does suggest a consistent pattern.

Chalara dieback of ash

1. Chalara dieback of ash is a disease of ash trees caused by the fungus H. fraxineus (Kowalski, 2006).  The disease causes loss of leaves, dieback of the crown of the tree, and can lead to tree death (Kowalski and Holdenrieder, 2009).  (High confidence)

2. H. fraxineus has infected many species of ash, but with differing intensities (Forest Research, 2012).  Because some ash species show very few symptoms after infection, they might act as undetected carriers. There is evidence of low susceptibility to disease in some Asian ash trees (Drenkhan and Hanso, 2010). (Moderate confidence)

3. Common ash (Fraxinus excelsior) is the most severely affected species.  Young trees are particularly vulnerable to H. fraxineus, and succumb to disease rapidly.  (Kowalski, 2006; Forest Research, 2012).  (High confidence)

4. Chalara dieback has seriously affected a high percentage of ash trees in continental Europe (Forest Research, 2012; Bakys et al., 2009; Engesser et al., 2009; Halmschlager and Kirisits, 2008; Ioos et al., 2009; Kowalski and Holdenrieder, 2008; Lygis et al., 2005; Ogris et al., 2010; Szabo, 2009; Talgo et al., 2009), most notably in Scandinavia (including Denmark, where an estimated 90 per cent of ash trees have been infected in some areas) and Baltic States. (High confidence)

5. There is no evidence that H. fraxineus can spread to tree species other than as,h or that it is harmful to human or animal health. (High confidence)

How infection happens

6. Infection occurs via spores from fruit bodies on leaf litter. Spore production (in the fruit bodies) occurs on infected fallen ash leaves and shoot material in the growing season after infection; trees are likely to need a high dose of spores to become infected (Timmermann et al., 2011). (High confidence)

7. H. fraxineus infection starts primarily with spores from the fruit bodies settling on the leaves of healthy ash trees, and is progressive over time, with dieback and stem lesions usually manifesting in the next growing season. Leaf symptoms can be detected within two months of infection (experience from Denmark). (Moderate confidence)

8. H. fraxineus causes infection from June to October, but mainly in July and August (Timmermann et al., 2011; Kirisits and Cech, 2009; Kowalski and Holdenreider, 2009).  Moist conditions favour production of the fruiting bodies. (High confidence)

How infection is likely to spread

9. Spores are produced on H. fraxineus fruit bodies formed on fallen leaves and shoots the year following infection. Natural spread is by wind-blown spores (ascospores) from these fruiting bodies (Kowalski, 2006; Kirisits et al.2009; Kowalski and Holdenrieder, 2009; Queloz et al., 2010). (High confidence)

10. Wind-blown spores cause the disease to spread up to 20-30km per year (Solheim, 2009; Solheim, et al., 2011). On occasions, spores may disperse much further on the wind. Longer-distance spread occurs via infected plants or potentially via wood products (Husson et al, 2012; EPPO, 2010; Prokrym and Neeley, 2009). (High confidence on wind dispersal; moderate confidence on untreated wood products).

11. There is low probability of dispersal on clothing and footwear, or via animals and birds. (High confidence). Transmission by routes other than wind and planting material are likely to pose a comparatively low risk, but the risk cannot be ruled out.

12. H. fraxineus is found in seeds (Cleary M., et al. 2012), and this is reflected in the legislation, which restricts the movement of seeds as well as plants.

13. There is a lower risk of H. fraxineus spreading over the winter because there is a ban on ash plant and seed imports into the UK, restrictions on plant and seed movements within the UK, and spore production does not resume until June. (High confidence)

The consequences of infection for ash tree health

14. Trees cannot recover from infection, but larger trees can survive infection for a considerable time, and some might not die. (Current experience from Denmark). (High confidence)

15. The impact of H. fraxineus infection depends on tree age, location, weather conditions and the co-presence of honey fungus (Armillaria) or other secondary pathogenic / opportunistic organisms. Trees in forests are more susceptible because of the greater prevalence of honey fungus. Ash trees being grown for the hardwood timber market are usually felled before they are killed by honey fungus.

  • Trees under 10 years of age are likely to die from H. fraxineus infection in 2-10 years.
  • Trees older than 10 years and less than 40 years old will die in 3-5 years if they are also infected with honey fungus, and likely more rapidly if they are already debilitated.
  • For mature trees more than 40 years old, there is no direct evidence of tree deaths just from H. fraxineus to date, but there is little comprehensive survey data from Europe on which to base firm conclusions.
    (Moderate confidence)

Further evidence would be helpful to strengthen the development of management options to minimise the environmental, economic and social impacts of H. fraxineus, including:

  • improving detection of H. fraxineus;
  • improving knowledge of the aetiology, pathology and epidemiology of H. fraxineus;
  • understanding the nature and scale of the environmental, economic and social impacts of the disease;
  • asessing how to mitigate the risks of Chalara dieback; and
  • asessing ways to adapt to the presence of Chalara dieback.


Bakys R, Vasaitis R, Barklund P, Ihrmark K and Stenlid J (2009). Investigations concerning the role of Chalara fraxinea in declining Fraxinus excelsior. Plant Pathology 58, 284-292.

Cleary M, Arhipova N, Gaitnieks, T Stenlid J and Vasaitis, R (2012) Natural infection of Fraxinus excelsior seeds by Chalara fraxinea Forest Pathology: 1-8

Engesser R, Queloz V, Meier F, Kowalski T and Holdenrieder O (2009). Das Triebsterben der Esche in der Schweiz. Wald u. Holz 6, 24–27.

EPPO (2010). Workshop on Chalara fraxinea, Oslo, Norway, 30 June to 2 July 2010.

Forest Research (2012). Rapid assessment of the need for a detailed Pest Risk Analysis for Chalara fraxinea

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

Husson C, Caël O, Grandjean J-P, Nageleisen L-M and Marçais B (2012). Occurrence of Hymenoscyphus pseudoalbidus on infected ash logs. Plant Pathology 61: 889-895.

Ioos R, Kowalski T, Husson C and Holdenrieder O (2009). Rapid in planta detection of Chalara fraxinea by a real-time PCR assay using a dual-labeled probe. European Journal of Plant Patholology 125, 329-335.

Kirisits T and Cech TL (2009). Zurücksterben der Esche in Österreich: Ursachen, Verlauf, Auswirkungen und mögliche Forstschutz- und Erhaltungsmaßnahmen.

Kirisits T, Matlakova M, Mottinger-Kroupa S, Cech TL and Halmschlager E. (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, Eg? irdir, Turkey, 11–16 May 2009. (Ed. by Dogmus-Lehtija T.) SDU Faculty of Forestry Journal, ISSN: 1302-7085, Serial: A, Special Issue: pp. 97–119.

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

Kowalski T and Holdenrieder O (2008). A new fungal disease of ash in Europe. Schweiz. Z. Forstwes 159, 45–50.

Kowalski T and Holdenrieder O (2009). Pathogenicity of Chalara fraxinea. Forest Pathology 39, 1–7.

Lygis V, Vasiliauskas R, Larson K-H and Stenlid J (2005). Wood-inhabiting fungi in stems of Fraxinus excelsior in declining ash stands of northern Lithuania, with particular reference to Armillaria cepistipes. Scandinavian Journal of Forest Research 20, 337-346.

Ogris N, Hauptman T, Floreancig V, Marsich F and Montecchio L (2010). First report of Chalara fraxinea on common ash in Italy. Plant Disease 94(1): 133. DOI: 10.1094/PDIS-94-1-0133A.

Queloz V, Grünig CR, Berndt R, Kowalski T, Sieber TN and Holdenrieder O (2010). Cryptic speciation in Hymenoscyphus albidus. Forest Pathology. doi: 10.1111/j.1439-0329.2010.00645.x.

Prokrym DR and Neeley AD (2009). NPAG Report. Chalara fraxinea (T. Kowalski): Ash dieback. Ascomycetes / Incertae sedis. ET Approval Date: 03/13/2009. New Pest Advisory Group (NPAG) Plant Epidemiology and Risk Analysis Laboratory Center for Plant Health Science & Technology.

Szabo I (2009). First report of Chalara fraxinea affecting common ash in Hungary. Plant Pathology 58, 797.

Solheim H (2009). Bekymringsfull økning i askeskuddsjuka: Trær ser ut til å dø. Skogeieren 96 (7-8): 24-25.

Solheim H, Timmermann V, Børja I & Hietala AM (2011). Yggdrasils helsetilstand - Askeskuddsjuke er på frammarsj. Skogeieren 96 (1): 34-36.

Talgo V, Sletten A, Brurberg MB, Solheim H, and Stensvand A (2009). Chalara fraxinea isolated from diseased ash in Norway. Plant Disease 93(5): 548. DOI: 10.1094/PDIS-93-5-0548A.

Timmermann V, Børja I, Hietaka AM, Kirisits T and Solheim H (2011).  Ash dieback: pathogen spread and diurnal patterns of ascospore dispersal, with special emphasis on Norway. EPPO Bulletin, 41: 14-20. doi: 10.1111/j.1365-2338.2010.02429.x

[Version 2 - updated 09.11.12 to reflect emerging knowledge – point 4 and point 10 have been modified.]


[1]  Chalara fraxinea was the name of asexual form (anamorph) of Hymenoscyphus pseudoalbidus, responsible for the ash dieback epidemic in Europe (Kowalski, 2006; Queloz et al., 2010).  For ease of reference, Chalara fraxinea was used as the common term for both forms of the organism before the name Hymenoscyphus fraxineus was adopted for both forms.

Last updated: 17th November 2017