What is wrong with our Horse Chestnuts?
Trees, like people, can suffer from many ailments. Some can be terminal, others may disfigure, and some will be short term.
In recent years, Horse Chestnuts (Aesculus sp.) have been suffering from a number of different problems that have given their owners great cause for concern.
Some of the problems are well known, and not restricted to Horse Chestnuts, such as Honey Fungus (Armillaria sp.). We must also remember that trees, like people, grow old and will eventually die. There are, however, three particular problems that have recently become more noticeable:
Leaf Blotch (Guinardia aesculi): this is a fungus, which causes large chestnut-red or dull brown, irregular blotches to be seen on the leaves, from mid summer. These are often concentrated at the tips and margins of the leaves, and are often outlined by a conspicuous yellow band. Severely browned leaves are rolled upwards longitudinally, and whole leaves fall prematurely.
The effects may be considered unsightly, but as the damage occurs after most growth has taken place, it has little effect on the general health of the tree. It is often mistaken for early autumn onset, but autumn colour changes are fairly uniform, not irregularly blotchy. It is commonly found on most varieties of Horse Chestnut in this country.
There are no specific controls available, but it is believed that the fungus over-winters on fallen leaves, and develops in the spring to infect new leaves. Raking up, and disposing of fallen leaves may, therefore, be helpful.
Bleeding Canker (Phytopthera cactorum and P. citricola): these are fungi most commonly seen on Horse Chestnut, Birch, and occasionally Lime. There appears, however, to be no reason why they could not be seen on a number of other species of tree.
The fungi are confined to the bark of the tree, and grow through it to kill the phloem and cambium layers beneath. It does not seem to require an injury to the bark for the fungi to become established. The fungi also cause Phytopthera root disease, so it is probable that they are resident in the soil, or on the roots of other plants in the area.
Infection is recognised by scattered drops of rusty-red, yellow-brown, or almost black, gummy liquid ooze from patches of bark on the stem or branches of the tree (this does not contain the fungus). Eventually, limbs may die, but the weeping is so conspicuous that it is this that will draw attention to the problem.
An attempt to control the infection can be made by cutting out all dead and dying bark, and then removing a strip of healthy bark 2″ wide from the periphery of the wound. The exposed tissue should then be treated with an approved wound paint containing a fungicide effective against Phytopthera. (Special care is required to ensure that the requirements of the Pesticide Regulations are adhered to). All removed bark should be burnt.
There are currently no known methods of prevention.
Horse Chestnut Leaf Miner (Cameraria ohridella): this is a moth that can cause severe damage to the foliage of the Horse Chestnut on an annual basis. The larvae of the moth mine within the leaves destroying most of the leaf tissues (there can be up to 700 on a single leaf), and severely damaged leaves shrivel and turn brown early often defoliating the tree before normal leaf-fall in the autumn. The spread is by the flight of adult moths, and through the passive transportation of the moth or infested leaves on vehicles, trains etc. The insect over-winters as a pupa in fallen leaves.
As with Leaf Blotch, the most effective way to apply any control measures is to rake up fallen leaves and dispose of them. The commercial composting or burning of the leaves destroys the pupae, thus reducing the moth population the following spring. Garden composting is not adequate, as the heat generated is insufficient to kill the pupae. Small heaps of leaves can be buried under a layer of soil to prevent adult emergence the following spring.
The use of insecticides to control the moth is not feasible, as they would also kill many beneficial insects.
Despite these problems, there is no cause for panic. If the majority of owners introduce sensible maintenance measures, we can hopefully keep the problems at a low level.
Landscape Services Manager
Braintree District Council