In case you missed the recent media storm, the fungus Chalara fraxinea has been found on British shores. But what is Chalara fraxinea – and why is it such a problem?
Chalara fraxinea is a fungus that essentially digests and eats ash tree leaves, causing crown dieback and leaf loss. It has the potential to devastate the UK ash tree population, similar to the Dutch elm disease outbreak in the 1970s. The demise of ash trees in Britain would be a huge blow to our ecosystem and biodiversity, especially in a time when climate change is high on the agenda.
Chalara fraxinea is actually the asexual form of a fungus called Hymenoscyphus psuedoalbidus, and is closely related to the saprotrophic Hymenoscyphus albidus. The fungus being saprotrophic means it feeds on the dead leaves of ash trees by secreting enzymes onto the leaf that then break the leaf down into smaller molecules; by breaking the food into smaller molecules. The fungi can then absorb the digested nutrients without the use of internal organs. Because Hymenoscyphus albidus uses dead ash tree leaves for nutrition it is not considered dangerous to the ash tree population, but Chalara fraxinea feeds and grows on live ash trees causing crown dieback.
How did it get here?
Many ash trees being sold in Britain have been nurtured abroad, even if the seed originates from Britain. The globalisation of the plant trade is one way the disease is believed to have been spread, although another prominent theory is that the spores have been naturally carried on the wind. Chalara fraxinea was first sighted in Poland in 1992, and with the emergence of ash dieback in Britain the majority of Europe now suffers with the problem.
If an ash tree has been infected by Chalara fraxinea the symptoms present themselves as dark patches on the leaves, and then small lens-shaped lesions or necrotic spots appear on the bark and stems. The necrotic lesions then enlarge into cankers causing wilting, dieback of shoots and branches, causing the top of the crown to die. The wood turns a brownish grey under the bark lesions which usually extends longitudinally past the area of bark necrosis. One of the easiest ways to spot a tree affected by Chalara fraxinea is the lack of branches at the crown of the tree, but retaining them around the middle. Determining whether the dark lesions on the tree’s leaves are a result of Chlara fraxinea can be difficult at this time of year, and one of the only ways to tell if the tree is diseased is using DNA testing rather than natural leaf fall causing the leaves to change colour.
A study conducted by Lars-G?ran Stener into the ash dieback disease in Sweden indicates that the disease is strongly influenced by the trees genotype. All of the trees in the population exhibited symptoms of ash dieback, but some of the trees exhibited reduced susceptibility to Chalara fraxinea. This resistance was retained over a period of six years under heavy infection pressure, making the idea of breeding ash trees that are resistant to ash dieback a real possibility.
Breeding for resistance
Dr Robin Sen, an expert in soil microbial ecology and biotechnology at Manchester Metropolitan University, has suggested that “breeding for resistance would be possible, but it will take time as stable resistance will need to be rigorously screened”. This process could take years, and the fungus would also have to be under selection to maintain pathogenicity. By isolating the genes that cause the resistance to the fungi, and genetically modifying clones of the resistant trees, the very next generation of ash trees would have the genetic resistance.
If any of you green fingered gardeners see an ash tree that you think may have been affected by Chalara fraxinea then don’t do anything hasty; cutting down an infected ash tree would distress the fungi and cause it to release extra spores. If you are concerned about an ash tree in your area, call the Forestry Commission Plant Health Service on 0131 314 6414
Lars-Göran Stener (2012): Clonal differences in susceptibility to the dieback of Fraxinus excelsior in southern Sweden, Scandinavian Journal of Forest Research, DOI:10.1080/02827581.2012.735699
Simon Howarth is a graduate from Leeds University’s Faculty of Biological Sciences with a keen interest in gardening and naturalism. You can read his post on responsible pest management here http://greenbuildingelements.com/2012/10/05/guest-post-responsible-management-of-garden-pests/.