Tuesday, 30 July 2013

Ash dieback: the fungal villain

Tony Leech, NWT Trustee and County Fungus Recorder

As far as trees are concerned, there are good fungi and there are bad fungi. For an ash tree, Chalara fraxinea is totally evil. This is the fungus that causes ash dieback, a disease which is killing over 90% of the ash trees throughout much of Europe. It was first detected in Poland in 1992 but the fungus responsible was only identified in 2006. It is thought that it may have spread from Japan where it appears not to attack the native species of ash.

Early signs include leaves wilting and withering, with black streaks appearing on the young twigs and branches. But where is the fungus? Chalara fraxinea grows within the tissues of the ash, never flaunting itself as a recognisable fruit body. However, to get to another ash tree, and thus spread the disease, it must shape-shift into something much more recognisable as a fungus.
Hymenoscyphus pseudoalbidus, June 2013, photo by Neil Mahler
If two mating strains of Chalara come together in the same fallen leaf, their mycelia grow together and exchange nuclei. This is fungal sex – whether they enjoy it or not we don’t know! The resulting ‘joint’ organism now produces a fruit body in which spores are produced. Although much smaller than seeds, these spores have the same potential to develop into new organisms and are the agents of dispersal. In the case of Chalara dispersal is by wind - except that the fungus is no longer Chalara fraxinea – it has become Hymenoscyphus pseudoalbidus. This dual personality of many fungi has been known for years. Different scientific names are given to the asexual stage (in this case Chalara fraxinea) and the sexual stage (Hymenoscyphus pseudoalbidus). Often it can be difficult to confirm that they are the same species.

Last October, ash dieback was found for the first time in an established woodland in Britain, at NWT Lower Wood, Ashwellthorpe, a Norfolk Wildlife Trust reserve near Wymondham. Within the next few weeks it had been found at a number of Norfolk sites and elsewhere in the east of the country.

It was almost inevitable that the sexual stage would appear this summer and on 2 June the first specimen in Britain was found by Anne Edwards at Ashwellthorpe. Anne is a member of the group at the John Innes Centre in Norwich working on the DNA of H. pseudoalbidus. They have determined its base sequence to compare with that of the very similar, but non disease-causing, Hymenscyphus albidus to get clues about the mode of attack.

Very soon after the discovery at Ashwellthorpe, ash trees at NWT Foxley Wood, another NWT reserve, were found to have the disease. On Saturday 29 June, Neil Mahler and I went there looking for H. pseudoalbidus and found a number of specimens. Neil motorcycled back up from Leiston in Suffolk to deliver them to Anne first thing on Monday morning and by evening they had sequenced enough of the DNA to confirm its identity. The hunt is now on to find specimens of the non-pathogenic H. albidus.

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