Horse chestnut leaf damage 'not linked' to canker risk

Some believed that leaf miner damage weakened horse chestnuts, leaving them vulnerable to disease

Horse chestnut trees infested with foliage-damaging leaf miner insects are not at greater risk from a bacterial disease, a study shows.

Forest Research scientists carried out the study to examine the impact of the bugs and whether it left trees susceptible to bleeding canker disease.

Horse chestnuts in the UK, until 2002, were considered disease-free and widely planted in parks and gardens.

The findings appear in the Agriculture and Forest Entomology journal.

Co-author Nigel Straw, project leader at Forest Research's Centre for Forestry and Climate Change, said one of the key questions the study was able to answer was whether there was any interaction between the two pathogens.

"There is a fear that if the insect damages the foliage and weakens the tree sufficiently, it may reduce the trees' resistance to disease, therefore they could suffer more from the bleeding canker disease and are more likely to die," he told BBC News.

"What we found was that the leaf miner seems to have no impact on horse chestnuts, even though it can make the foliage look very unsightly," Dr Straw added.

"The damage comes too late in the season to have any major effect on the tree."

The horse chestnut leaf miner (Cameraria ohridella) is a moth in its larval stage, which feeds on the leaf tissue of the European white-flowered horse chestnut (Aesculus hippocastanum).

Unsightly but harmless

A prevailing view had been that trees with foliage heavily damaged by leaf miner predation would have a reduced capacity to convert sunlight into the energy it needed for new growth in the coming year.

As a result, if a tree suffered a sustained attack over a number of years, it would be under stress and weakened, leaving it susceptible to other pathogens.

The leaf miner was first recorded in the UK in south-west London in 2002 and quickly spread to many parts of England and Wales.

Writing in their paper, Dr Straw and his colleague, Dr David Williams, observed that apart from the aesthetic impact of damaged foliage, the study of 193 European white-flowered horse chestnut trees and 46 red horse chestnut trees (Aesculus carnea J. Zeyh) over a 10-year period recorded no adverse effects in terms of tree growth or crown density.

"Those individual A. hippocastanum that suffered the highest rates of damage might have been expected to show some decrease in growth and canopy condition over time but even in these trees, there was no detectable response," they explained.

They added that red horse chestnut trees were "rarely attacked and only when growing close to heavily infested [white-flowered horse chestnut trees]".

However, it was not the case when it came to bleeding canker disease, which seemed to spread more rapidly through the A. carnea population.

They said data showed that the disease was responsible for the death or removal of 27% of the UK's red-flowered horse chestnut trees and 11% of the nation's white-flowered horse chestnuts.

Bleeding canker disease (BCD) has been recorded in the UK for decades, but recently there has been a surge in the number of cases being recorded throughout many European countries, including the UK.

'Serious threat'

Experts have identified that most of the new cases were being caused by a bacterium, Pseudomonas syringae pathovar aesculi, which has previously only been known to infect Indian horse chestnuts.

They say early symptoms tend to be limited to bleeding lesions of dark, sticky liquid from patches of bark on infected trees' stems or branches.

Forestry Commission plant health scientists say that if the disease is severe and the areas of bark which are killed are extensive, large trees can undoubtedly be killed. However, younger trees (10-30 years old) are at greater risk and can succumb to the disease in just a few years.

Drs Straw and Williams described BCD as a "much more serious threat" because the "rate at which the disease has spread through the horse chestnut populations has shown no sign of slowing down in recent years, and it is still too early to say how many trees will be killed eventually and how many might survive".

Contrary to the opinion that trees with foliage damage caused by leaf miners would be more susceptible to BCD and help accelerate its spread, Dr Straw said the decade-long study did not support such a view.

"Trees that have heavily damage foliage by the leaf miner do not get the (bleeding canker) disease more often and they do not suffer from it any more severely so we cannot see that the impact of the leaf miner is causing sufficient stress in the trees to make them more susceptible to the disease," he explained.

"The two organisms seem to be operating independently."

Dr Straw said Forest Research scientists had been concerned by attempts to treat horse chestnut trees with insecticides in order to curb the spread of leaf miners.

"This was based on the argument that by controlling the moth you would help stop the trees from becoming infected with bleeding canker disease," he added.

"What we have shown is that there is no point doing that. There is no point controlling the moth because it will have no influence on whether the trees pick up the bleeding canker disease or not.

"What tree management needs to do is focus on the threat posed by the bleeding canker disease."

Dr Straw said that Forest Research was continuing to monitor the trees in the study area to see if the long-term project offered any clues on how far and how fast BCD would spread through the UK's horse chestnuts and whether there were any individual trees that showed a natural resistance to the pathogen.