At this time of year, as they stand in full leaf, trees play a fundamental role in the English landscape. In the UK, we have a huge number of mature and veteran trees, more than other much larger European countries. The Ancient Tree Forum, via The Ancient Tree Hunt map, records these significant trees (http://www.ancient-tree-hunt.org.uk).
Horse Chestnuts (Aesculus hippocastanum), more commonly and affectionately known as Conker trees, have had a huge presence in the landscape since their introduction in the 1600’s.
It is easy to understand why they are loved for their conkers. However, the tree has so much more to offer including a characteristic size, shape and stature, beautiful candle shaped flowers and a huge leaf. These assets made the tree a clear winner, with the Victorians planting them as Avenue trees throughout the country.
Although they have naturalised to the UK, they have recently come under attack from both the leaf miner and a soil pathogen, Bleeding Canker.
Horse Chestnut Leaf Miner (Cameraria ohridella)
The browning of the leaves in July and August is a result of an infestation of Horse Chestnut leaf miner (Cameraria ohridella). The leaf miner is a small moth like creature that lays its eggs into the leaf of the chestnut. The egg hatches and the tiny grub then feeds on the inner tissue of the leaf, finally as it prepares to transform into the adult moth it eats its way out of the leaf. These small areas then brown and die.
Each feeding patch is very small most will be less that 1cm in diameter. However, the populations of the leaf minor will reach the thousands per tree. Therefore, hundred of larvae will be present on each leaf and the areas will link leading to complete leaf death.
The affects of the leaf minor are clear to be seen, it is considered to be a mainly aesthetic problem. Monitoring is still being undertaken on the originally affected trees, which were on Wimbledon Common in 2005, to date it has shown no reduction in vitality.
Treatment
There is no cover-all treatment for this. Although sprays and chemicals are available that control leaf miners, they are not effective due to re-infection from surrounding trees and are unlikely to create a lasting clear canopy without huge detriment to other insects.
Simple steps like clearing leaf litter from around the tree will have a benefit to the tree. If these are removed and composed the eggs from within the leaf litter cannot simply re-infest from the ground below the tree.
Horse Chestnut - Bleeding Canker (Pseudomonas syringae pv aesculi or Phytophthora)
There two different strains of bleeding canker, it is however the second strain Pseudomonas that is more aggressive and poses a threat to these trees. Current research conducted by Forest Research at Alice Holt shows that around 50% of UK Horse Chestnuts are affected by Bleeding canker.
The indication for bleeding Canker is a usually small, but growing black/ rust coloured spot on the truck. It is often seen low on the truck, as this is a ground based infection.
This small area represents a large area of dead cells in the vascular tissue of the tree; it will extend from the ground into the upper structure of the tree. At this point, you may see leaves browning and areas of dead bark.
Treatment
Again unfortunately there is no cure to this disease. However, there are a number of things that can be done to try to support the tree.
A tree does have a defence system that can limit or slow the pathogen. However, like us in times of stress, this will be less effective, allowing the disease to attack and spread. Therefore, if you can remove stresses and increase the soil condition around the tree it will show a marked improvement.
It should be noted that, if there are areas of dead bark in the tree or limbs, sections of the crown may become unsafe and you should seek the advice of an arboriculturalist, who will be able to advise about the best action for the safety of people and the benefit of the tree.
Ben Clutterbuck, Tree Care Manager
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