''Hymenoscyphus fraxineus'' () is an
ascomycete fungus
A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from t ...
that causes ash
dieback, a chronic fungal disease of
ash trees
''Fraxinus'' (), commonly called ash, is a genus of flowering plants in the olive and lilac family, Oleaceae. It contains 45–65 species of usually medium to large trees, mostly deciduous, though a number of subtropical species are e ...
in Europe characterised by leaf loss and crown dieback in infected trees. The fungus was first scientifically described in 2006 under the name ''Chalara fraxinea''. Four years later it was discovered that ''Chalara fraxinea'' is the asexual (
anamorphic) stage of a fungus that was subsequently named ''Hymenoscyphus pseudoalbidus'' and then renamed as ''Hymenoscyphus fraxineus''.
Trees reported dying in
Poland
Poland, officially the Republic of Poland, is a country in Central Europe. It is divided into 16 administrative provinces called voivodeships, covering an area of . Poland has a population of over 38 million and is the fifth-most populou ...
in 1992 are now believed to have been infected with this pathogen. It is now widespread in Europe, with up to 85% mortality rates recorded in plantations and 69% in woodlands.
It is closely related to a native fungus ''
Hymenoscyphus albidus'', which is harmless to European ash trees.
According to a 2016 report published in the
Journal of Ecology
The ''Journal of Ecology'' is a bimonthly peer-reviewed scientific journal covering all aspects of the ecology of plants. It was established in 1913 and is published by Wiley-Blackwell on behalf of the British Ecological Society.
The journal pub ...
a combination of ''H. fraxineus'' and
emerald ash borer
The emerald ash borer (''Agrilus planipennis''), also known by the acronym EAB, is a green buprestid or jewel beetle native to north-eastern Asia that feeds on ash species. Females lay eggs in bark crevices on ash trees, and larvae feed undern ...
attacks could wipe out European ash trees.
Genetics
The fungus ''Hymenoscyphus fraxineus'' was first identified and described in 2006 under the name ''Chalara fraxinea''.
In 2009, based on
morphological and
DNA sequence comparisons, ''Chalara fraxinea'' was suggested to be the asexual stage (
anamorph) of the
ascomycete fungus ''
Hymenoscyphus albidus''.
However, ''Hymenoscyphus albidus'' has been known from Europe since 1851 and is not regarded as pathogenic.
In 2010, through
molecular genetic
Molecular genetics is a sub-field of biology that addresses how differences in the structures or expression of DNA molecules manifests as variation among organisms. Molecular genetics often applies an "investigative approach" to determine the ...
methods, the sexual stage (
teleomorph
In mycology, the terms teleomorph, anamorph, and holomorph apply to portions of the life cycles of fungi in the phyla Ascomycota and Basidiomycota:
*Teleomorph: the sexual reproductive stage (morph), typically a fruiting body.
*Anamorph: an asex ...
) of the fungus was recognized as a new species and named ''Hymenoscyphus pseudoalbidus''.
Four years later it was determined that "under the rules for the naming of fungi with
pleomorphic life-cycles", the correct name should be ''Hymenoscyphus fraxineus''.
''Hymenoscyphus fraxineus'' is "morphologically virtually identical" to ''Hymenoscyphus albidus'', but there are substantial genetic differences between the two species.
![Eschentriebsterben Kocherwald](https://upload.wikimedia.org/wikipedia/commons/6/6e/Eschentriebsterben_Kocherwald.jpg)
''Hymenoscyphus fraxineus'' has two phases to its life-cycle: sexual and asexual.
The asexual stage (
anamorph) grows in affected trees attacking the bark and encircling twigs and branches.
The sexual, reproductive stage, (
teleomorph
In mycology, the terms teleomorph, anamorph, and holomorph apply to portions of the life cycles of fungi in the phyla Ascomycota and Basidiomycota:
*Teleomorph: the sexual reproductive stage (morph), typically a fruiting body.
*Anamorph: an asex ...
) grows during summer on ash
petioles in the previous year's
fallen leaves Fallen Leaves may refer to:
* "Fallen Leaves" (song), a 2006 song by Billy Talent
* ''Fallen Leaves'' (film), a 2023 Finnish film
See also
* The Fallen Leaves
The Fallen Leaves are a British garage rock group that formed in Richmond, Londo ...
.
The
ascospore
An ascus (; ) is the sexual spore-bearing cell produced in ascomycete fungi. Each ascus usually contains eight ascospores (or octad), produced by meiosis followed, in most species, by a mitotic cell division. However, asci in some genera or ...
s are produced in
asci and are transmitted by wind; this might explain the rapid spread of the fungus.
The origins of the disease are uncertain,
but researchers are investigating the theory that the fungus originated in Asia, where ash trees are immune to the disease.
Genetic analysis of the fungus ''Lambertella albida'' which grows harmlessly on petioles of the Manchurian ash (''
Fraxinus mandschurica
''Fraxinus mandshurica'', the Manchurian ash, is a species of ''Fraxinus'' native to northeastern Asia in northern China (Gansu, Hebei, Heilongjiang, Henan, Hubei, Jilin, Liaoning, Shaanxi, Shanxi), Korea, Japan and southeastern Russia (Sakhalin ...
'') in Japan, has shown that it is likely to be the same species as ''Hymenoscyphus fraxineus''.
Teams from The
Sainsbury Laboratory
The Sainsbury Laboratory (TSL) is a research institute located at the Norwich Research Park (NRP) in Norwich, Norfolk, England, that carries out fundamental biological research and technology development on aspects of plant disease, plant dis ...
(TSL) and the
John Innes Centre
The John Innes Centre (JIC), located in Norwich, Norfolk, England, is an independent centre for research and training in plant and microbial science founded in 1910. It is a registered charity (No 223852) grant-aided by the Biotechnology and ...
in
Norwich
Norwich () is a cathedral city and district of Norfolk, England, of which it is the county town. Norwich is by the River Wensum, about north-east of London, north of Ipswich and east of Peterborough. As the seat of the See of Norwich, with ...
sequenced the genome of the fungus in December 2012. The sequence has been published on the website OpenAshDieBack and offers clues to how the fungus infects trees. The study has uncovered toxin genes and other genes that may be responsible for the virulence of the fungus. In the long term researchers aim to find the genes that confer resistance to the pathogen on some ash trees.
Ash dieback
![Eschentriebsterben Symptome 2](https://upload.wikimedia.org/wikipedia/commons/4/4c/Eschentriebsterben_Symptome_2.jpg)
Trees now believed to have been infected with this pathogen were reported dying in large numbers in
Poland
Poland, officially the Republic of Poland, is a country in Central Europe. It is divided into 16 administrative provinces called voivodeships, covering an area of . Poland has a population of over 38 million and is the fifth-most populou ...
in 1992,
[ and by the mid 1990s it was also found in Lithuania, Latvia and ]Estonia
Estonia, formally the Republic of Estonia, is a country by the Baltic Sea in Northern Europe. It is bordered to the north by the Gulf of Finland across from Finland, to the west by the sea across from Sweden, to the south by Latvia, a ...
. However, it wasn't until 2006 that the fungus's asexual stage, ''Chalara fraxinea'', was first described by scientists, and 2010 before its sexual stage was described.[ By 2008 the disease was also discovered in Scandinavia, the Czech Republic, Slovenia, Germany, Austria and Switzerland.] By 2012 it had spread to Belgium, France, Hungary, Italy, Luxembourg, the Netherlands, Romania, Russia, Britain and Ireland. By 2016, it was already identified in central Norway, the northernmost distribution areas of ash tree.
Up to 85% mortality rates due to ''H. fraxineus'' have been recorded in plantations and 69% in woodlands. The disease has caused a large-scale decline of ash trees across Poland, and the experience there suggests that in the long term "15 to 20 per cent of trees do not die, and show no symptoms." In 2012, the disease was said to be peaking in Sweden and Denmark, and in a post-decline (or chronic) phase in Latvia and Lithuania. The disease was first observed in Denmark in 2002, and had spread to the whole country by 2005. In 2009 it was estimated that 50 per cent of Denmark's ash trees were damaged by crown-dieback, and a 2010 estimate stated that 60–90% of ash trees in Denmark were affected and may eventually disappear. The disease was first reported in Sweden in 2003. A survey conducted in Götaland
Götaland (; also '' Geatland'', '' Gothia'', ''Gothland'', ''Gothenland'' or ''Gautland'') is one of three lands of Sweden and comprises ten provinces. Geographically it is located in the south of Sweden, bounded to the north by Svealand, wit ...
in 2009 found that more than 50% of the trees had noticeable thinning and 25% were severely injured.
A Danish study found that substantial genetic variation between ash trees affected their level of susceptibility. However, the proportion of trees with a high level of natural resistance seemed to be very low, probably less than 5%. A Lithuanian trial based on the planting of trees derived from both Lithuanian and foreign populations of European ash
''Fraxinus excelsior'', known as the ash, or European ash or common ash to distinguish it from other types of ash, is a flowering plant species in the olive family Oleaceae. It is native throughout mainland Europe east to the Caucasus and Albo ...
found 10% of trees survived in all progeny trials to the age of eight years.
So far the fungus has mainly affected the European ash (''Fraxinus excelsior
''Fraxinus excelsior'', known as the ash, or European ash or common ash to distinguish it from other types of ash, is a flowering plant species in the olive family Oleaceae. It is native throughout mainland Europe east to the Caucasus and Albor ...
'') and its cultivar
A cultivar is a type of cultivated plant that people have selected for desired traits and when propagated retain those traits. Methods used to propagate cultivars include: division, root and stem cuttings, offsets, grafting, tissue culture ...
s, but it is also known to attack the narrow-leafed ash (''Fraxinus angustifolia
''Fraxinus angustifolia'', the narrow-leaved ash, is a species of ''Fraxinus'' native to central and southern Europe, northwest Africa, and southwest Asia.Flora Europaea''Fraxinus angustifolia''/ref>Rushforth, K. (1999). ''Trees of Britain and E ...
''). The manna ash ('' Fraxinus ornus'') is also a known host, although it is less susceptible than the other European ash species. Experiments in Estonia have shown that several North American ash species are susceptible, especially the black ash (''Fraxinus nigra
''Fraxinus nigra'', the black ash, is a species of ash native to much of eastern Canada and the northeastern United States, from western Newfoundland west to southeastern Manitoba, and south to Illinois and northern Virginia. Formerly abundant, ...
''), and to a lesser extent the green ash (''Fraxinus pennsylvanica
''Fraxinus pennsylvanica'', the green ash or red ash, is a species of ash native to eastern and central North America, from Nova Scotia west to southeastern Alberta and eastern Colorado, south to northern Florida, and southwest to Oklahoma an ...
''). The white ash (''Fraxinus americana
''Fraxinus americana'', the white ash or American ash, is a species of ''ash tree'' native to eastern and central North America.
The species is native to mesophytic hardwood forests from Nova Scotia west to Minnesota, south to northern Florida, ...
'') and the Asian species known as Manchurian ash (''Fraxinus mandschurica
''Fraxinus mandshurica'', the Manchurian ash, is a species of ''Fraxinus'' native to northeastern Asia in northern China (Gansu, Hebei, Heilongjiang, Henan, Hubei, Jilin, Liaoning, Shaanxi, Shanxi), Korea, Japan and southeastern Russia (Sakhalin ...
'') showed only minor symptoms in the study.
Symptoms and colonisation strategies
Initially, small necrotic spots (without exudate) appear on stems and branches. These necrotic lesions then enlarge in stretched, perennial cankers on the branches, wilting
Wilting is the loss of rigidity of non-woody parts of plants. This occurs when the turgor pressure in non- lignified plant cells falls towards zero, as a result of diminished water in the cells. Wilting also serves to reduce water loss, as it ...
, premature shedding of leaves and particularly in the death of the top of the crown
A crown is a traditional form of head adornment, or hat, worn by monarchs as a symbol of their power and dignity. A crown is often, by extension, a symbol of the monarch's government or items endorsed by it. The word itself is used, partic ...
. Below the bark, necrotic lesions frequently extend to the xylem
Xylem is one of the two types of transport tissue in vascular plants, the other being phloem. The basic function of xylem is to transport water from roots to stems and leaves, but it also transports nutrients. The word ''xylem'' is derived from ...
, especially in the axial and paratracheal ray tissue. The mycelium can pass through the simple pits, perforating the middle lamella but damage to either the plasmalemma
The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment (th ...
or cell walls was not observed. The disease is often chronic but can be lethal. It is particularly destructive of young ash plants, killing them within one growing season of symptoms becoming visible. Older trees can survive initial attacks, but tend to succumb eventually after several seasons of infection.
Management
There are currently no effective strategies for managing the disease, and most countries which have tried to control its spread have failed. The removal of trees in infected areas has little effect as the fungus lives and grows on leaf litter on the forest floor. Research at the Swedish University of Agricultural Sciences
The Swedish University of Agricultural Sciences, or Swedish Agricultural University (Swedish: ''Sveriges lantbruksuniversitet'') (SLU) is a university in Sweden. Although its head office is located in Ultuna, Uppsala, the university has several c ...
suggests that the deliberate destruction of trees in an infected area can be counterproductive as it destroys the few resistant trees alongside the dying ones. One approach to managing the disease may be to take branches from resistant trees and graft
Graft or grafting may refer to:
*Graft (politics), a form of political corruption
* Graft, Netherlands, a village in the municipality of Graft-De Rijp
Science and technology
*Graft (surgery), a surgical procedure
*Grafting, the joining of plant t ...
them to rootstock
A rootstock is part of a plant, often an underground part, from which new above-ground growth can be produced. It could also be described as a stem with a well developed root system, to which a bud from another plant is grafted. It can refer to a ...
to produce seeds of resistant trees in a controlled environment. A Lithuanian trial searching for disease-resistance resulted in the selection of fifty disease-resistant trees for the establishment of breeding populations of European ash in different provinces of Lithuania. A breeding programme for resistant trees is a viable strategy but the process of restoring the ash tree population across Europe with resistant trees is likely to take decades.
Ash dieback in the United Kingdom
The fungus was first found in Britain during February 2012 at sites that had received saplings from nurseries in the previous five years. A ban on imports of ash from other European countries was imposed in October 2012 after infected trees were found in established woodland. On 29 October Environment minister David Heath confirmed that 100,000 nursery trees and saplings had been deliberately destroyed. The government also banned ash imports but experts described their efforts as "too little too late". The UK Government emergency committee COBR
The Cabinet Office Briefing Rooms (COBR) are meeting rooms in the Cabinet Office in London. These rooms are used for committees which co-ordinate the actions of government bodies in response to national or regional crises, or during overseas e ...
met on 2 November to discuss the crisis. A survey of Scottish trees started in November 2012. A 2020 study suggested that certain landscapes with hedgerows and woods made up of different types of tree resisted the disease better than areas mainly populated with ash trees.
Government and Forestry Commission guidance
The Forestry Commission
The Forestry Commission is a non-ministerial government department responsible for the management of publicly owned forests and the regulation of both public and private forestry in England.
The Forestry Commission was previously also respon ...
has produced guidance and requested people report possible cases.
Comparisons have been made to the outbreak of Dutch elm disease
Dutch elm disease (DED) is caused by a member of the sac fungi (Ascomycota) affecting elm trees, and is spread by elm bark beetles. Although believed to be originally native to Asia, the disease was accidentally introduced into Americas, America ...
in the 1960s and 1970s. In 2012 it was estimated that up to 99% of the 90 million ash trees in the UK would be killed by the disease.
On 9 November 2012 the United Kingdom Government unveiled its strategy. Environment Secretary Owen Paterson
Owen William Paterson (born 24 June 1956) is a British former politician who served as Secretary of State for Northern Ireland from 2010 to 2012 and Secretary of State for Environment, Food and Rural Affairs from 2012 to 2014 under Prime Minist ...
announced that it was acknowledged that the disease was here to stay in the UK and that the focus would be on slowing its spread. Young and newly planted trees with the disease would be destroyed; however, mature trees would not be removed because of the implications for wildlife that depends on the trees for their natural habitat. The strategy unveiled by Paterson included:
* Reducing the rate of spread of the disease
* Developing resistance to the disease in the native UK ash tree population
* Encouraging the public and landowners to help monitor trees for signs of ash dieback.
In March 2013 Owen Paterson announced that the United Kingdom Government would plant a quarter of a million ash trees in an attempt to find strains that are resistant to the fungus.
In February 2016 the BBC program "Countryfile
''Countryfile'' is a British television programme which airs weekly on BBC One and reports on rural, agricultural, and environmental issues in the United Kingdom.
The programme is currently presented by John Craven, Adam Henson, Matt Baker ...
" presented an anecdotal report of enhanced resistance to ash dieback following soil treatment by injecting "Biochar
Biochar is the lightweight black residue, made of carbon and
ashes, remaining after the pyrolysis of biomass. Biochar is defined by the International Biochar Initiative as "the solid material obtained from the thermochemical conversion of ...
" - a type of charcoal. Twenty trees had remained free of disease over 3 years during a severe infestation of the surrounding trees.
In December 2016, writing in Nature
Nature, in the broadest sense, is the physical world or universe. "Nature" can refer to the phenomena of the physical world, and also to life in general. The study of nature is a large, if not the only, part of science. Although humans are ...
, Dr Richard Buggs reported that the common ash (''Fraxinus excelsior'') had been genetically sequenced for the first time and UK specimens appeared more resistant than Danish ones.
In August 2018 Defra DEFRA may refer to:
* Deficit Reduction Act of 1984, United States law
* Department for Environment, Food and Rural Affairs, United Kingdom government department
{{Disambiguation ...
and the Forestry Commission announced that at Westonbirt Arboretum
Westonbirt, The National Arboretum is an arboretum in Gloucestershire, England, about southwest of the town of Tetbury. Managed by Forestry England, it is perhaps the most important and widely known arboretum in the United Kingdom.
Planted ...
the fungus had been found infecting three new hosts: ''Phillyrea
''Phillyrea'' is a genus of two species of flowering plants in the family Oleaceae, native to the Mediterranean region, and naturalized in the Canary Islands and Madeira.
They are evergreen shrubs or small trees growing to 3–9 m tall, related ...
'' (mock privet), ''Phillyrea angustifolia
''Phillyrea angustifolia'', the narrow-leaved mock privet,RHS Plants. Available at: https://www.rhs.org.uk/Plants/24488/i-Phillyrea-angustifolia-i/Details ccessed 22/02/21/ref> is a species of flowering plant in the olive family Oleaceae, native ...
'' (narrow-leaved mock privet) and ''Chionanthus virginicus
''Chionanthus virginicus'' (white fringetree) is a tree native to the savannas and lowlands of the southeastern United States, from New Jersey south to Florida, and west to Oklahoma and Texas.USDA Woody Plant Seed Manual''Chionanthus virginicus' ...
'' (white fringetree). These were the first findings on hosts other than ''Fraxinus'' anywhere in the world. All three new hosts are in the same taxonomic family as ash, the Oleaceae
Oleaceae, also known as the olive family, is a taxonomic family of flowering shrubs, trees, and a few lianas in the order Lamiales, It presently comprises 28 genera, one of which is recently extinct.Peter S. Green. 2004. "Oleaceae". pages 2 ...
. The trees were all in the vicinity of infected European ash. In response to the findings on the new hosts, Nicola Spence
Nicola Jane Spence (born 22 February 1961) is the Chief Plant Health Officer and Deputy Director for plant and bee health at the Department for Environment, Food and Rural Affairs.
Early life and education
Spence was educated at The Mount ...
, the UK Chief Plant Health Office, said that, "Landscapers, gardeners and tree practitioners should be vigilant for signs of ash dieback on these new host species, and report suspicious findings through Tree Alert".
In June 2019, Defra published a report summarising the current state of knowledge of ash dieback, and priority areas for future research. In 2019 and 2020, the UK government and Future Trees Trust planted 3,000 ash trees in Hampshire
Hampshire (, ; abbreviated to Hants) is a ceremonial and non-metropolitan county in western South East England on the coast of the English Channel. Home to two major English cities on its south coast, Southampton and Portsmouth, Hampshire ...
to establish the Ash Archive. All the trees came from shoots of trees that demonstrated resistance to the fungus. The Ash Archive will form the basis of a breeding program.
Ash dieback in Ireland
On 12 October 2012 the Department of Agriculture, Food and the Marine
The Department of Agriculture, Food and the Marine ( ga, An Roinn Talmhaíochta, Bia agus Mara) is a department of the Government of Ireland. According to the department, its mission is to "lead the sustainable development of a competitive, con ...
confirmed the first recorded instance of the fungus in Ireland, at a plantation in County Leitrim
County Leitrim ( ; gle, Contae Liatroma) is a county in Ireland. It is in the province of Connacht and is part of the Northern and Western Region. It is named after the village of Leitrim. Leitrim County Council is the local authority for the ...
. Legislation was introduced in both Northern Ireland and the Republic of Ireland on 26 October banning the importation and movement of ash plants from infected parts of Europe. By 23 September 2013, a survey conducted by the Irish Government revealed that the disease had been identified at ninety-six sites across the Republic of Ireland. As of August 2021, Teagasc Teagasc (, meaning "Instruction") is the State-sponsored bodies of the Republic of Ireland, semi-state authority in the Republic of Ireland responsible for research and development, training and advisory services in the agri-food sector.
The offici ...
warns that: "The disease is now prevalent throughout most of the island of Ireland and is likely to cause the death of the majority of the ash trees over the next two decades."
The first cases in Northern Ireland
Northern Ireland ( ga, Tuaisceart Éireann ; sco, label= Ulster-Scots, Norlin Airlann) is a part of the United Kingdom, situated in the north-east of the island of Ireland, that is variously described as a country, province or region. Nort ...
were confirmed at five sites in counties Down and Antrim on 16 November 2012. By 4 December 2012 the disease had been confirmed at sixteen sites in counties Down, Antrim, Tyrone and Londonderry.
Wider ecological knockon effects
Due to the importance of ''F. excelsior'' as a host, Jönsson and Thor 2012 find that ''H. fraxineus'' is subjecting rare/threatened lichens to an unusually high (0.38) coextinction risk probability ''vis-a-vis'' the host tree in the wooded meadows of Gotland, Sweden. Studies detected no signs of ash mortality compensation by the surviving trees, particularly in mixed forests, indicating a mid-term habitat loss and niche replacement of ash.
See also
* Forest pathology
Forest pathology is the research of both biotic and abiotic maladies affecting the health of a forest ecosystem, primarily fungal pathogens and their insect vectors. It is a subfield of forestry and plant pathology.
Forest pathology is part ...
References
External links
Ash dieback: Spotter's guide and maps
BBC
Symptoms guide to Ash dieback disease
Forestry Commission
''Chalara'' dieback of ash (''Chalara fraxinea'')
Forestry Commission
Information pertaining to ash-dieback (''Chalara fraxinea'') work at Fera
Food and Environment Research Agency
FRAXBACK
Federating scientists for a comprehensive understanding of ''Fraxinus'' dieback in Europe
Open Ash Dieback
*
{{Taxonbar, from=Q20899360
Fungi described in 2011
Helotiaceae
Fungal tree pathogens and diseases
Fungi of Europe