Horse chestnut trees under attack

One of Britain’s most majestic trees, the horse chestnut (Aesculus hippocastanum), is under attack on two fronts. A recently arrived moth is laying leaf miner larva which is causing serious damage to the trees leaves and a bacterium is infecting the tree bark causing it to bleed. Of the two threats the biggest concern currently is in regard to the leaf miners. The leaf miner threat was featured on a recent edition of Autumnwatch Unsprung.

Bleeding canker in conker trees

Trees of this age can be killed by the disease within about 3 to 5 years

Bleeding of trunks and branches was first discovered back in the 1970′s in Britain, but it’s only been in the last few years that the number of trees affected has increased dramatically. Initially the cause of the bleeding was mis-diagnosed as a fungal infection. Later studies showed through that the cause is a bacterium Pseudomonas syringae. The occurrence of the disease is widespread across the UK and is also a growing problem on mainland Europe.

The bacterium can affect horse chestnut trees of all ages but is particularly damaging to younger trees – between 10 and 30 years old. Trees of this age can be killed by the disease within about 3 to 5 years because the circumference of the tree trunk is smaller and therefore more susceptible to being damaged. The symptoms of the disease is easily seen in the trees as staining caused by bleeding of sap along the main trunk and occasionally on the main branches.

In a survey by the Forestry Commission in 2007 it was found that 47% of all horse chestnut tree examined displayed some signs of infection of the disease. Across the country it appears that urban amenity trees and rural trees are equally likely to be affected though there are some regional variations such as in the West Midlands where most infected trees were in the urban environment. It’s believed that up to 50,000 trees are now infected with the bacterium.

Leaf miners in horse chestnut trees

The latest threat to the conker tree is the moth Cameraria ohridella which lays it’s eggs on horse chestnut leaves once hatched the larva (leaf miner) feed on the inside of leaves leaving tunnels and void spaces inside the internal structure of the leaf. This means the leaf is no longer able to function correctly and dies off. some tree lose their leaves as early as July due to infection by leaf miners which reduces their visual value.

The leaf miner was first discovered in Wimbledon in July 2002 which suggests that the moth first come to Britain during 2000 or 2001. It is currently spreading at a rate of between 40 and 60 kilometers a year and has now spread across much of England and also into Wales. It’s thought that the main method of spread is through the flight of adult moths but it’s also thought that the disease has also been spread through the transport of infected leaves that become attached to cars and other vehicles. Transport of leaves by vehicles is thought to explains how the leaf miner has appeared in cities miles away from naturally infected areas.

Trees can become heavily infected by the leaf miners and there are recorded instances of up to 700 leaf miners being found in a single leaf. Once an area becomes initially infected the occurrence of leaf miners rapidly increase and within 2 or 3 years the density of the larvae is sufficient to cause serious damage to the trees and reduce the visual value of the tree. As the tree becomes more infected the leaves will discolour and fall from the tree months earlier than they should. While there does not appear to be evidence of death related to the leaf miner it does lead to leaf loss as early as July. There have been some instances of trees which have lost their leaves very early in the season having a second flowering in the autumn.

So far it appears that even after a number of years of heavy infection by the moth larva the horse chestnut trees will recover the following year but will obviously again loss their leaves early. It appears that once a tree gets infected then remains infected at the same high level of density.

Dealing with the fallen leaves are an important way of controlling the infestation. The moth pupae overwinter in the fallen leaves to reemerge the following season. Burning the leaves is the most effective way of killing the pupae. Adding the leaves to high temperature commercial composting units can also kill the next generation but the temperatures that are reached in domestic compost heaps are not sufficiently high enough to kill the moth. If burning is not an option then it’s advised to bury the leaves under 10cm of soil and to leave the area undisturbed until at least May to stop the adult moths from re-infesting the tree.

Horse chestnut tree survey

photo credit: silverfox09