Horse chestnuts
What's wrong with my Horse Chestnut Tree?
Over the last few years autumn appears to have come quickly for one of our most common trees the Horse Chestnut. Leaves going brown and falling off in June and July has been very noticeable during 2008. There are three main causes for this and many trees have symptoms of all three!
Horse Chestnut Leaf Miner
This is a small moth whose larvae burrow inside the leave which creates as a very distinct light-brown patch called a 'mine' , hence the name of the moth. Some trees may only have a few mines but some may have more than a hundred making the leaf look very brown all over. Horse chestnut leaf miner Cameraria ohridella was first found established in the UK in the London Borough of Wimbledon in July 2002 and it is suggested that the first moths had arrived from the continent in either 2000 or 2001. From this initial area of infestation, the moth has spread rapidly, and it is now present across most of south-central England, East Anglia and the Midlands. The current distribution extends to Wells on the north Norfolk coast, and to Derby, Shrewsbury, Cardiff and Somerset, 190-230 km from the site of the original infestation.
Continued and extensive attack by the leaf miner is likely to put some trees under stress, which although itself may not be too detrimental, could reduce a trees resistance to more serious diseases. The moths emerge in the Spring after over-wintering as a chrysalis within the fallen leaves beneath the tree. The best treatment therefore, if practical, is to remove as many leaves as possible to reduce next years attack. Mild winters are also allowing them to survive and spread each year.
Guignardia leaf blotch
The most common disease is Guignardia leaf blotch (caused by the fungus Guignardia aesculi). The disease is recognisable by the reddish or dull brown, irregular blotches that are often concentrated at the tips and margins of infected leaflets. The blotches are often outlined by a conspicuous yellow band. Occasionally, Horse Chestnuts are seen with leaflets that are browned around the edges but without the yellow margin which is caused by another fungus. Trees that are affected are often attacked again the following year but there is no practical treatment.
Bleeding Canker
This is currently the most serious threat. Forest Research scientists estimate some 35,000 to 50,000 trees are affected with Bleeding canker (Pseudomonas syringae) and probably a few thousands have already been felled as a result of the disease. This is often but incorrectly referred to as being caused by one of the Phytophthora root diseases which, although has been found on Horse Chestnut is actually rarely found.
Attack is very visible with the 'bleeding' which ranges from a rusty-red colour through to black often seen coming from small cracks in the bark. A small crack on its own does not pose a threat to the tree but if these spread it will cause death to that part of the tree and often a tree may have several dead branches
Horse chestnuts ranging from 10-15 year old vigorous saplings to large mature amenity trees can all be affected by the disorder. In Bedfordshire trees planted within the past 50 years appear to be much more susceptible compared to the older Victorian plantings. Many are highly visible amenity trees in parks and public gardens, others form important features in avenues, historic gardens and landmarks. The Forestry Commission undertook a survey in 2007 to assess how widely trees are affected. It confirmed that bleeding canker on horse chestnuts is now very widespread with around 49% of all the trees assessed showing symptoms to some degree. The extent of the disease also varies in different parts of Britain. Currently there are no tested treatments available and when a tree is badly affected it usually has to be felled. Sadly because of this disease the planting of new Horse Chestnut trees is not being carried out. Any young trees grown from conkers may be best collected from disease-free trees, but this is no guarantee of future immunity.