Managing Chalara; Ash Die Back Disease
Chalara is the popular name for the fungal disease caused by Hymenoscyphus fraxineus, this is the name that I will use in this article.
In summer 2012 Chalara was first identified on Ash, Fraxinus excelsior, in Great Britain. The news received widespread publicity in the national media. The Prime Minister called a Cobra meeting in Whitehall which resulted in £2.4 million of public funding to combat the disease.
The Forestry Commission has spent some of the money on raising awareness in the general public, mapping the spread of the disease and establishing trial plots of young ash trees from known seed sources.
Most of the money has gone on scientific research, investigating the life cycle of the causal fungus and searching for genetic resistance in the native Ash population with the hope that resistant trees can be bred to repopulate the British countryside and woodlands.
The Forestry Commission website on Chalara states:
The best hope for the long-term future of Britain’s ash trees lies in identifying the genetic factors which enable some ash trees to tolerate or resist infection, and using these to breed new generations of tolerant ash trees for the future.
I believe that this represents a fundamental misunderstanding of the position of Ash in Britain today.
There is an estimate of 140 million Ash trees in Great Britain. If young trees are taken into account it must be in the billions. The important point is that much less than 1% of these were planted.
Ash grows naturally in the most intensively farmed areas of Britain as hedgerow trees and in field corners. It grows on roadsides, on railway tracksides and in gardens that had been allowed to run wild many years ago. Ash grows profusely in Ancient Semi Natural Woodlands, particularly where the woodlands have been over exploited for timber in the past to the benefit of coppiced Ash stools.
Whereas Oak trees have been planted for their timber for hundreds of years, Ash has not been planted because there was no need, it grew everywhere. It is only in the past 30 years or so that Ash has been planted in the British countryside as a component of new broadleaved woodlands or as a hedgerow tree in new hedges.
It is difficult to understand in our managed countryside that such dominant and ubiquitous features as Ash trees are not the product of man’s work, but are a totally natural part of the landscape that has been growing and reproducing very successfully with no help or hindrance from us.
The disease known as Chalara is also part of the natural environment. After being first identified in Poland in 1992 it quickly spread westwards across Europe. It was first discovered in Britain in imported Ash trees in summer 2012, but it was soon found in several unplanted woodlands in Norfolk, Suffolk and Kent where it had probably already been established for several years.
The geographical location of these early sites demonstrates that Chalara blew into Britain on wind born spores from the continent with no help from man and the nursery trade.
When a natural and widespread part of the British flora is attacked by a natural fungal disease it is difficult to see why we should be getting involved at all, particularly as the native Ash population seems so well equipped to deal with the problem by itself.
Male and female Ash flowers generally occur on different trees so ensuring cross pollination. Female Ash trees produce large quantities of genetically variable seed that readily germinates on any bare or grassy ground within 100 metres downwind of the parent.
These patches of natural regeneration are where the scientists should be looking for their resistant trees, not in their field trial plots. If resistant trees are found in patches of natural regeneration, there is no need to check back through records to find the location of the parent tree, it is right beside you.
Any future planting of Ash in Britain is likely to be as a small component of mixed native broadleaves. In these situations there is generally no need to plant Ash because it will seed itself on most planting sites. Spot spraying around young trees in April with Glyphosate makes the perfect site for Ash regeneration.
So not only is there no need for us to spend money searching for resistant/tolerant Ash, there is no need for us to plant them either.
I believe that the FC should be focusing its efforts to combat Chalara on those sites where Ash has been planted. That is young mixed broadleaved plantations under 30 years of age, usually established with the help of planting grants. Ash saplings growing in such woodland situations are also the most likely to suffer from an infection of Chalara.
In heavily infected young woodlands where Ash comprises 30% or more there may be no alternative but to replace Ash with a different species. But where Chalara infection is only light there are several management techniques that can be used to keep it under control. There are also several areas where more scientific research is needed:
Pruning. Chalara infection often starts on short spur shoots a few feet above ground level from where it quickly travels to the main stem. Removal of these spur shoots periodically by pruning can reduce the risk of infection of the main stem, which must be a priority. This will result in progressive crown raising which in turn will reduce the risk of infection from spores on the ground.
Pollarding. Once the main stem of an ash sapling is infected the tree is soon girdled by lesions and the plant above the girdling wilts and then dies through lack of water. At this stage the tree can be pollarded at a point well below the infection and a side shoot can then be encouraged to take over as the leader.
Timing and sanitation. What is the best time to prune and pollard as described above? Too early in the summer and pruned spur shoots will shoot again. Too late and you risk Chalara reaching the main stem. What is the danger of accidentally transferring Chalara from one tree to another on dirty secateurs? What is the best disinfectant to use bearing in mind that this will be smeared onto a cut stem that needs to heal rapidly?
Chalara life cycle. How does Chalara spread through an Ash tree once it has girdled the main stem? Does it continue to spread downwards year by year until the whole tree dies, or can the tree form a barrier to stop further spread? Chalara spores have been found on the fallen leaves and rachis, but are spores also produced from the lesioned stem of an infected tree?
It should be possible to answer these questions by careful observation of infected trees in young plantations over a number of years. With this information we can formulate a strategy for managing Chalara that does not rely on felling and replanting with a different species.
Hugh Dorrington June 2016
Revised July 2016