Chalara dieback of ash – an overview of the disease and current research

Chalara dieback of ash – an overview of the disease and current research Steve Hendry Forest Research Northern Research Station

Chalara dieback of ash Photograph courtesy of Iben Margrete Thomsen, University of Copenhagen

Chalara dieback of ash

Chalara dieback of ash • Cause not established until 2006 • Life cycle not clearly elucidated until 2010 and gaps in our knowledge remain • A new disease caused by a fungus (Chalara fraxinea / Hymenoscyphus pseudoalbidus) • Invasive species, origin likely to be eastern Asia • Local spread: wind borne spores • Long distance: plant trade(?) • Widespread in continental Europe • Mainly affects common ash (Fraxinus excelsior) but also other ash species

Chalara dieback of ash • After their initial formation, lesions do not tend to extend further in subsequent years. • i.e. dieback of trees is not a result of progressive development of existing Chalara cankers in the crowns of trees. • Dieback results from continued episodes of annual infection initiated by ascospores liberated from fruiting bodies of Hymenoscyphus pseudoalbidus produced on the leaf litter. • Disease severity therefore tends to be greatest where accumulation of infected litter can occur.

Chalara dieback of ash • Asexual fruiting bodies of the fungus (Chalara fraxinea) are rarely found in nature and they produce droplets of sticky conidia which do not appear to play any role in the infection process

Chalara dieback of ash • So how could disease spread from new plantings where infections are confined to dead shoots and necrotic lesions on stems? • Hymenoscyphus pseudoalbidus fruiting bodies can occasionally be produced on infected stem material

Chalara dieback of ash • This explains what may appear to be contradictory advice on dealing with diseased trees of different ages: removal of young trees vs leaving mature trees untouched. • In new plantings, sporulation on shoots and stems is the only mechanism by which the fungus is likely to spread. If control of the disease is called for, removal of infected ash is the main means of addressing that risk**. • In ‘wider environment’ cases, infection of the trees must have been initiated as a result of spore infections of foliage and that infected foliage will be present on the site – the relatively low risk of fruiting on shoots is far outweighed by the predictable fruiting on the leaf litter. So removal or pruning of infected trees will not remove the disease risk. It would remove our ability to detect potentially resistant genotypes of ash.

Chalara dieback of ash • Total of 74 sites • 2 nurseries • 64 new plantings • 8 wider environment

Chalara dieback of ash • Distribution of wider environment cases suggests that cross-channel spread of ascospores could have initiated disease. • Genetic diversity of the fungus in Europe is probably too low to allow any light to be shed on this by a comparison of genotypes from GB and Europe. • Modelling of historical meteorological conditions suitable for such spread, combined with evidence of when the disease reached SE England may provide more circumstantial evidence but not a definitive answer to this question.

Chalara dieback of ash • Preliminary study of diseased trees at 2 sites in East Anglia indicates that disease has certainly been present there since 2008 / 2009. • Given that these cases were not spotted earlier, where could wider environment infections actually be compared with our current maps of disease incidence? • Spring 2013 will provide a better opportunity to determine where infection of mature trees has already occurred.

Chalara dieback of ash • When the pathogen Hymenoscyphus pseudoalbidus becomes established within an area, the native leaf-colonising species Hymenoscyphus albidus tends to disappear • Over much of continental Europe, the species now appears to have become extinct • Possible to use H. albidus as a bioindicator of where H. pseudoalbidus is absent? • Survey carried out in Scotland in late summer / autumn 2010 by Danish pathologist Iben Thomsen.

Initial reports suggested that debilitation of trees resulted in reduced resistance to weak pathogens (particularly some Armillaria species).i.e. Damage by C. fraxinea predisposed trees to attack by other agents but did not directly kill themHowever • In heavily infected stands, basal stem lesions initiated directly by the pathogen have now been detected in a number of different countries

2008 2007 Chalara dieback of ash 2010 2009

Chalara dieback of ash • A small proportion of F. excelsior do display less damage than others when exposed to the pathogen

Chalara dieback of ash • Estimates of the proportion of “resistant” genotypes varies ranging from as low as 1% to as high as 5%. • Basis of this resistance / tolerance is uncertain • Possible association with early leaf shedding? • Caution required in screening – resistance to infection by foliar pathogens can break down

Chalara dieback of ash • Recent research in Scandinavia indicates that Chalara can be found in seeds collected from diseased stands. The process by which this has occurred and the risk of disease transmission via this route is currently under study. • Not known whether the pathogen could form cryptic infections in young plants. • Not known whether killed seeds could act as “surrogate” leaf stalks and give rise directly to fruiting bodies of Hymenoscyphus pseudoalbidus.

Chalara dieback of ash • Practical guidance on management of ash trees, leaf litter & general biosecurity measures at http://www.forestry.gov.uk/forestry/infd-92gefh • Ongoing work to assess the risk of spread of the pathogen associated with new planting sites. • Work in progress to plant out a range of GB ash provenances in SE England to accelerate the search for potentially resistant trees. • FR is an active participant in research consortia to sequence the genomes of both ash, and Chalara fraxinea in order to gain a better understanding both of potential mechanisms of resistance in the host and of pathogenicity in the fungus.

Chalara dieback of ash • Thanks to: • European colleagues for sharing of information & illustrative material on Chalara fraxinea - in particular Drs Iben Thomsen, Thomas Kirisits & Berthold Metzler • FR pathologists involved with advisory & research work relating to C. fraxinea – Joan Webber, David Rose and Gavin Hunter

Chalara dieback of ash – an overview of the disease and current research