Arboriculture: Damage limitation

Is horse chestnut's susceptibility to pests and disease making it a liability

Could the long-term future of the horse chestnut (Aesculus hippocastanum) as an amenity tree in Britain be at risk? London Tree Officers Association chairman Andy Tipping thinks so.

"We are still being told that leaf miner is a cosmetic disorder, but I think it's more serious than that," he says. "Together with bleeding canker, it means horse chestnuts are dying rapidly. It's becoming a national problem. Their use as amenity trees is being compromised, and if it turns out to be terminal, it will change the landscape hugely."

The twin problems of horse chestnut leaf miner and horse chestnut bleeding canker have spread rapidly in recent years following initial outbreaks in the South East and they are likely to become endemic across the country. As yet there is no widely approved treatment for either.

Recognising the signs

The London Borough of Merton has had longer to assess the problem than most, as the pathogen made its first UK appearance within its boundaries, on Wimbledon Common in 2002.

But according to arboricultural manager David Lofthouse: "Leaf miners don't worry me so much because leaves are only temporary mechanisms. Bleeding canker is far more serious - we have got lesions, dead strips and some trees are coming apart, though we haven't felled many yet."

The current sight of prematurely brown and wrinkled leaves on the borough's trees has not caused too much local upset, he says. "People know about it because there's been quite a lot in the press, so they are quite understanding. They realise there's nothing we can do other than prune and remove trees that are copping it."

Lofthouse says this may compromise its future as an amenity tree, but adds: "I don't tend to plant many anyway — it's already a common tree and can be invasive. I don't want a whole lot springing up in an oak wood. Aesculus indica would be a possible alternative though."

Cambridgeshire's Barcham Trees is a major supplier of containerised trees to the amenity market. The company has noticed an effect on orders of horse chestnut, says sales executive David Johnson. "There certainly has been a reduction and we have scaled down production compared with three or four years ago. They are still being specified in a few cases, but not, I don't think, by any London boroughs."

Barcham recently hosted seminars by Dr Glynn Percival, plant physiologist at the University of Reading's Bartlett Tree Research Laboratory. He says: "Over the past few years I've been evaluating the effect of two insecticides on horse chestnut leaf miner — Imidacloprid and Abamectin. Both have been shown elsewhere to give excellent control of this pest and my work initially with containerised horse chestnuts under controlled conditions supported these results."

Earlier this year, Imidacloprid was given specific off label approval by the Pesticides Safety Directorate (PSD) for the control of amenity tree pests.

Percival says: "The data I presented at the seminar was the results of root drenches of Imidacloprid applied around mature horse chestnut trees. Because Imidacloprid is so expensive I also evaluated the effect of applying this insecticide as a microcapsule trunk injection. In all cases, control was around 80 per cent."

Percival intends to apply to the PSD for approval for the trunk-injection method with Imidacloprid and with Abamectin, already widely used in the US.

"We should know by Easter 2009 if approval has been granted," he says. "If so, control options will exist for leaf miner that can be applied as either root drenches or trunk injections."

Johnson is optimistic about the treatment's potential. "We are cautious about recommending Aesculus, but this might be a lifeline," he says.

However, Lofthouse is more sceptical. "Even if it does work, how do you ensure that it affects only the leaf miners and not the things that eat them?" he says. "And then there's the cost issue. I haven't the resources to treat one tree, never mind 5,000."

Trees Project technical director Dr Marcus Bellett-Travers also has reservations: "I'm not generally a fan of injecting trees as it tends to damage them."

Bellett-Travers argues that the problem is overstated anyway and that neither pathogen tends to threaten trees' long-term health. "People think things are worse than they are," he says.

"I get asked to look at a lot of trees and often you struggle to find anything wrong with them. With horse chestnuts, there's a lot of growth cracking and splitting anyway, which may have been there for years before people noticed (the effects of leaf miner and bleeding canker)."

Extent of the problem

Bleeding canker is often found on trees that are already weakened by stressful growing conditions and it is these, rather than the canker itself, which can give rise to thinning canopy or shed limbs, Bellett-Travers says.

"It's an emotive subject. It can damage the value of house prices, which is obviously a sensitive subject right now. But a lot of people are looking at felling them based on the cosmetic manifestations of two pests. In these cases the only alternative is replanting, which means no trees at all for some time. Most urban trees look quite shabby anyway. And it takes so much time and effort to get a tree established. The data shows that you'll lose 70 per cent of trees in the first 15 years."

Bellett-Travers adds that contractors should take care not to unwittingly worsen the problem. As bleeding canker is waterborne, it can be spread via contractors' clothing and equipment, though it can also be spread via aerosol-type sprays thrown up by road traffic.

He suggests that there is little reason to plant horse chestnuts. "Why plant something that you know won't look very good?"

Fellow consultant Andrew Cowan says of the leaf miner problem: "Trees that are under stress achieve low functional growth and are more exposed to other ailments, which could include Phytophthora.

"The worry is that trees won't produce sufficient carbohydrates. But leaf miner impacts late in the season and this year it has taken trees longer to suffer. They still had a good period at the beginning of the season to generate carbohydrates."

However, he points out that the most favoured control of leaf miner — sweep up and burn the fallen leaves which contain the dormant moth pupae - is often physically impossible. "Contractors aren't collecting leaves in September, they're still mowing," he says. "And the leaves spread into neighbouring properties. We are never going to get on top of the sanitation."

Forest Research, the research agency of the Forestry Commission, also advises tree managers not to over-react to the pathogens. According to principal pathologist Dr Joan Webber: "Bleeding canker is not just cosmetic — it's more damaging than that. But cutting trees down is not needed at all."

While the long-term development of the disease is unclear, evidence from monitoring from the first UK episodes, and the experience over a longer time in Europe, suggests a plateauing after the initial flush.

"There's already quite a dataset and it shows that no real long-term damage since the 1980s," says Webber. "But tree managers should at least survey their trees so they have a baseline from which they can tell if they're getting better, worse or staying the same.

"Some trees are susceptible and some have died and will die. Others show signs of recovery or at least remission and some are very resistant. Individual trees in the middle of affected areas have remained unaffected. These trees could be propagated."

MONITORING THE PROBLEM

Dr Marcus Bellett-Travers says the thermal imaging techniques pioneered by his consultancy - Trees Project - show that damage to horse chestnuts from bleeding canker is often overstated.

"We've been doing thermal imaging for five years," he says. "With good water connections, the heat moves around easily around the tree.

"A barrier changes the flux and it becomes sub-optimal, which has an effect on surface temperature. How close to the surface and how big it is affects the size and temperature difference."

Cankers are at the surface and have a distinct, localised temperature pattern — unlike Phytophthora, which has a spreading effect on the vascular system, Bellett-Travers explains. "I've been looking at bleeding cankers for six years and an incredibly small number of trees have been damaged."


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