I’ve just been listening to a chilling account of the spread through Europe of ash dieback. It was a special on BBC Radio 4 and part of a feature on trees.It is suspected that ash dieback (Chalara fraxinea) found a foothold in Europe some 20 years ago, probably due to spore blowing in from parts of Asia. Vast areas of ash forest, including plantation and natural forests, have been denuded of these trees.The reporter, Dr Adam Hart, visited wooded areas in Poland where ash dieback was first identified and where ash trees are now a rare sight. Most have died or have been felled.
The disease has now spread to the UK (we reported on this recently on GardenDrum) where scientists are concerned the effects of the disease will be even more pronounced as ash makes up a greater percentage of woodlands and also hedge rows throughout the UK than in Europe.
In Poland, ash accounted for about one per cent of the trees in woodlands. In the UK this figure is more like five per cent.
It is interesting to listen to reports like these in the face of our own current battle with myrtle rust. Here in Australia and in the UK and Europe, scientists are saying that it is not possible to control or eliminate the new diseases, even though they respond to fungicides. Both ash dieback in the UK and myrtle rust in Australia are, it is felt, just too widespread and virulent.
In the UK the control measures for ash dieback mostly involve removing infected trees especially in coastal areas that may be exposed to wind-blown fungal spores from Europe.
Trees can be injected to cure or protect them from the disease. However there are concerns that the fungus itself could become immune to the fungicides. As well, many fungicides that are effective against the disease are not registered for use on amenity or woodland trees in the UK.
One hope for ash dieback is that resistant forms of ash might be discovered surviving in the wild. So far there have been mixed results from the trials of seedlings grown from apparently resistant ash trees.
Even if resistant forms are found and grown, it will be a slow path to recovery for ash trees. The devastation caused to elms throughout Europe and the UK by Dutch elm disease is witness to that. Although there are now resistant forms available, there are few elms to be seen growing in the UK or Europe.
Immune system response
Even if trees don’t have a genetic resistance, survival can be assisted with careful management was the take home message from the BBC4 report. Encouraging the good health of trees so they can trigger their natural production of defence mechanisms can aid their survival.
Trees have up to 10 different forms of defence that can be triggered in their fight against diseases like ash dieback. Natural defence mechanisms possessed by trees include producing slightly thicker leaves that rebuff spore invasion and triggering various immune systems including antibodies known as phytoalexins. They also possess secondary metabolites that are plant equivalents to our production of adrenalin.
These type of products are being trialled against bleeding cankers of horse chestnuts (another devastating tree disease in the UK) but have yet to be trialled on ash dieback. I guess it’s a case of let’s watch this space.
For more information on the disease visit the UK Forestry Commission’s website.