THIS week we shall study a pathogen which in recent years has been causing widespread ill health to Horse Chestnut trees throughout the UK and Europe.
The Horse Chestnut (Aesculus hippocastanum) are found almost anywhere in the UK from urban parks and gardens to the rural countryside and woodlands, but yet it is not a native to our shores. It has flourished since it was introduced to Britain from the Balkans in south east Europe during the 16th Century.
Horse Chestnuts have been prone to infection from pathogens from the genus Phytopthora for several decades, but infection rates which caused ill health remained at a relatively low level. However, for the last few years incidences of ill health in Horse Chestnuts have observed to be increasing at an alarming rate. Scientific research carried out on affected trees repeatedly confirmed the presence of a pathovar known as ‘Pseudomonas syringae pv aesculi’.
A pathovar can be defined as a bacterial strain or set of strains with the same or similar characteristics that are differentiated at infra-sub-specific level from the other strains of the same species or sub-species on the basis of distinctive pathogenicity to one or more plant hosts.
This pathovar is known to be able to infect the Indian Horse Chestnut (Aesculus indica) which is native to the north west Himalayan region of the Indian sub-continent. It is likely that this pathovar gained entry into Europe on infected planting stock.
The initial symptoms of an infection by Pseudomonas syringae pv. aesculi may be revealed by the presence of small ‘bleeding lesions’ on the stem of the tree. At the start of the growing season these lesions exude a transparent, dark coloured liquid. As the growing season progresses the exudation changes to a rusty brown colour and also changes from transparent to opaque. When exudation activity decreases with the onset of autumn it dries to leave a dark and brittle crust. Further inspection around the lesions and under the outer bark will reveal an inner bark or cambium which is a dark brown colour, instead of the normal white-cream colour. A cambium which is dark brown in colour would suggest that it is dead and therefore unable to carry out the important physiological functions which keep the tree alive. As the infection progresses and the patches of dead cambium coalesce around the entire girth of the stem, symptoms of ill health may appear in the crown. Typically this may be observed through yellowing of the foliage, premature leaf fall and possibly death of the tree in severe cases of infection.
Not all trees which become infected will necessarily succumb to the disease, and some trees may fight the infection for up to four years. The fate of a tree depends on many factors, the most important of which may be the intensity of the infection. Sometimes only part of the crown may die, yet the tree may still recover.
When a tree dies in part or in whole a health and safety issue may arise. Horse Chestnut scaffold limbs are susceptible to sudden failure as the wood dries out. This means that when trees become infected by Pseudomonas syringae pv. aesculi they should be inspected by a competent professional in order to determine which stage the disease is at within the tree. Recommendations for appropriate remedial work to prevent a failure occurring would then be provided.
Since the pathovar first began infecting Horse Chestnuts in Northern Ireland observations appear that the incidence of the disease may be reducing. The exact cause of why is unknown, but the occurrence of prolonged cold winters over the past two years may be a contributory factor, however, for as long as mild winters continue this may only be temporary.
For further Arboricultural Advice on a specific matter contact: Brian Malcomson MICFor, Tech Cert (ArborA), Scottish Woodlands Ltd.
Tel: 028 276 38026. Email: firstname.lastname@example.org. Website: www.scottishwoodlands.co.uk.