Originally the disease appears to have come to Europe from Eastern Asia (Baral and Bemmann, 2014). It was first identified in England in 2012 due to the importing of infected trees from the continent as well as from windborne ascospores (Orton et al., 2017). The increase in global trade is blamed for this and many other diseases of a similar nature. When you look at the figures which suggest that ash imports could have been as high as 3.5 million trees a year (Sansford, 2013), it is no surprise that the disease eventually made it to these shores.
From Forestry Commission reports, “Ash accounts for approximately 14% of total broadleaved standing volume in GB” (Sansford, 2013), but the impact when looking at the Yorkshire Dales National Park is much greater. “Ancient semi-natural woodland covers about 1% of the Yorkshire Dales National Park. About 80% of this woodland is made up of ash, making it the iconic tree of the Dales.” (Yorkshire Dales National Park Authority, 2018)
Ash disease has two forms:
- When the tree is in leaf (summer), its form is Chalara fraxinea. It fives in leaves and twigs and damages them by releasing viridiol which is toxic to the tree. It also produces sticky spores called conidia which can be spread around the tree by rain
- In autumn, it turns into Hymenoscyphus pseudoalbidus, a small white cup-fungus, which appear on fallen ash leave stems (midribs). These cups puff out ascospores (1/1000mm in size), one midrib can have around 20 cup-fungi, each producing around 1,500 ascospores an hour for about 2 weeks (Rackham, 2014) or around 100,000 ascospores each (BBC, 2018). These ascospores float on the air and are spread by the wind.
When I initially investigated Ash dieback, I thought it meant the end of all ash trees in the Yorkshire Dales, which would be a devastating loss. However, on further investigation it appears that this is not the case and it does not always kill the trees it infects. The disease starts in the leaves and spreads into the twigs and then can travel into the branch. However, the tree’s natural defence mechanism walls off the damaged areas and the infection appears not to continue in future year’s growth and new shoots form from buds below the damaged area (Rackham, 2014). In less vigorous trees the new shoots can become infected also and the whole tree may die. This is also certainly the case in young trees as is evident in the woods near my house.
Some trees appear to have a genetic resistance to ash dieback and in studies 5-10% of trees do better than others, even when next to dying trees. Professor James Brown is confident that ash will recover in Britain, but it will take a few hundred years for this to happen (BBC, 2018).
It was interesting doing the research into the disease, but what am I going to do with this new found knowledge? Well, apart from being less depressed by knowing some ash trees are likely to survive, I have taken a few things from this:
- Much of the detailed science in the available papers on the disease goes over my head, but many of the papers provide diagrams and images of the fungus and ascospores which have interesting forms and may provide inspiration for drawings or sculptures.
- Representing the two different states of the disease might be an interesting approach?
- The release of many thousands of ascospores might be interesting to represent – maybe with charcoal dust, or sparks from gunpowder?
- The likelihood of some trees surviving gives a glimmer of hope – this might be represented by using gold leaf?
Baral, H. and Bemmann, M. (2014). Hymenoscyphus fraxineusvs.Hymenoscyphus albidus– A comparative light microscopic study on the causal agent of European ash dieback and related foliicolous, stroma-forming species. Mycology, 5(4), pp.228-290.
BBC (2018). The New Forest. [podcast] Gardeners Question Time. Available at: https://www.bbc.co.uk/programmes/m0000sdq [Accessed 15 Apr. 2019].
Orton, E., Brasier, C., Bilham, L., Bansal, A., Webber, J. and Brown, J. (2017). Population structure of the ash dieback pathogen, Hymenoscyphus fraxineus, in relation to its mode of arrival in the UK. Plant Pathology, 67(2), pp.255-264.
Rackham, O. (2014). The ash tree. Toller Fratrum: Little Toller Books.
Sansford, C. (2013). Pest Risk Analysis for Hymenoscyphus pseudoalbidus for the UK and the Republic of Ireland. [online] Webarchive.nationalarchives.gov.uk. Available at: https://webarchive.nationalarchives.gov.uk/20140904094312/http://www.fera.defra.gov.uk/plants/plantHealth/pestsDiseases/documents/hymenoscyphusPseudoalbidusPRA.pdf [Accessed 15 Apr. 2019].
Yorkshire Dales National Park Authority. (2018). Ash dieback hits Park. [online] Available at: http://www.yorkshiredales.org.uk/living-and-working/other-services/press-office/news/recent/ash-dieback-hits-park [Accessed 15 Apr. 2019].