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. Some of the money has spent 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 government 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 Europe 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 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 often no need to plant ash if there are mature trees in the vicinity. It will seed itself on most planting sites. Spot spraying around young trees in April with Glyphosate makes the perfect site for Ash regeneration.
Ash is the most widespread tree in lowland Britain with its own associated food chains and habitats. If there are no parent trees around for natural regeneration, then it should be planted as part of a broadleaved mixture in any new woodland planting scheme. Even if there are no government grants to pay for it.
The government planted large trial plots of pure ash in 2013 to check the virulence of Chalara. Over 90% of the trees died within 2 or 3 years. I planted Nursery Wood in 2008 with 15% ash in an intimate mixture with other broadleaved species. 50% of the ash showed signs of Chalara infection. I managed all the trees as described below and only one tree died. This was a tree on the edge of the wood that I had left unmanaged as a control. Other trees continued to get annual infections from which they recovered the following year. These trees are slowly being removed in a thinning program to leave a population of thriving, genetically tolerant trees. See the updates on Ash Trees in Nursery Wood.
Pruning. Chalara infections 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. The spores released from the ground will drift away horizontally.
Pollarding. Once the main stem of a young ash less than 10cm diameter is infected, the tree is soon girdled 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. If the main stem is more than 10cm in diameter, the ash sapling can often recover, putting on new growth to bypass the damaged stem tissue.