Autumn comes early as new threat puts the horse-chestnut at risk

Goddards is a beautiful red-brick house built near the Knavesmire racecourse, in York, by Noel Goddard Terry, of the chocolate-making dynasty. Between its Tadcaster Road entrance and the house is an eye-catching archway followed by a splendid avenue of horse-chestnuts.

The trees were planted by the noted garden designer, George Dillistone, and have greeted visitors to Goddards ever since the house was built in 1926.

But this year, autumn came early to the trees. In springtime, no sooner had the leaves appeared than many changed colour and fell to the ground. The premature colouring and reduced leaf cover made the trees look "a bit sad when they should be all lush and beautiful", according to the National Trust, which now uses the house as its Yorkshire headquarters.

The cause of the leaf-fall was a tiny gnat-like moth known as the leaf miner, which lays its eggs on horse-chestnut leaves, producing caterpillars with such voracious appetites that they can soon cover an entire tree with dead or dying leaves.

The moth is a relatively new migrant to Britain from the Balkans – it was first found in the Wimbledon area of London in 2002 – and its proliferation in Europe is thought to have been aided by the higher temperatures brought by climate change.

In England, its spread northwards has been slow, but the summer of 2010 has seen the highest infestation of leaf miners yet in Yorkshire.

The damage to leaves reduces the trees' ability to photosynthesise. This progressively weakens the tree, leading, in some cases, to the loss of limbs and the eventual need to be felled.

Britain's horse-chestnuts are already suffering from another disease known as bleeding canker caused by a Phytophora fungus which is thought to have come to British garden centres from the US and is related to the fungus which caused Ireland's potato famine in the 1840s.

The main visible symptom on affected trees is a tar-like "bleeding" from trunks and branches.

At Goddards, the trees were already afflicted by yet another fungus, called leaf blotch, but it is the appearance of damage by the leaf miner that has left the avenue of horse chestnuts in such bad condition they may eventually have to be felled.

The trust's gardens and parks adviser, Raoul Curtis-Machin, says: "Over time, we will assess things. The worst-case scenario is that we make a decision about replacing them with something else. I'd be tempted to think along the lines of maybe a cherry avenue, or possibly something like liquid amber, which gives nice autumn colour."

The gardener at Goddards, Alison Green, believes spraying the trees with insecticide would be impractical because of their size and raise health and safety issues. Also, there would be nothing to stop the trees being reinfested from untreated horse-chestnuts on neighbouring properties.

The only action she can take against leaf miners at the moment is to gather up all the leaves and bag them as compost, allowing the high temperature to kill the larvae which might otherwise overwinter on the ground and attack the trees next spring.

"It is a beautiful, distinctive avenue of trees but if we lose one of them there will be pressure to fell them all because they are so symmetrical."

Leaf miner damage is very noticeable elsewhere in York, according to the city's parks and open spaces manager, Brian Williams.

"Many horse-chestnuts started to look autumnal months ago because of the damage, but there doesn't seem to be much we can do about it. These trees are among the biggest and most numerous we have, so it would be extremely difficult to spray them with anything."

Not far from Goddards, several horse-chestnuts have already been felled on the western side of Knavesmire because of the so-called bleeding canker infection.

More than a century ago, they were planted in a row, so the gaps are noticeable.

More bleeding canker-afflicted trees have been identified in Rowntree Park as well as on the banks of the River Ouse. Their condition is being monitored and if they get too weak, they will be felled before they start to fall apart and become unsafe.

There are similar stories across much of Yorkshire. At Yorkshire Water's Tophill Nature Reserve, near Driffield, the warden, Richard Hampshire, has, for the first time, seen damage by leaf miner moths afflict horse-chestnuts which were already suffering from bleeding canker. The trees are expected to be felled early next year.

Gloomy though the picture is for horse-chestnuts – perhaps second to the oak as the nation's favourite large tree and now a popular choice for memorial trees – there is some hope on the horizon.

A Yorkshire tree consultant is field-testing a potential cure for bleeding canker which has the side-effect of also getting rid of leaf miner moths.

Halifax-based Jonathan Cocking and his team are running experiments on horse-chestnut trees at 40 different sites across the UK, including trees at Elvington Primary School, in the Vale of York.

It involves injecting into each tree's sap system a chemical called Allicin, which is produced by crushing garlic. It is already well-known as an anti-bacterial and anti-viral product. But so far it has been hard to make because each garlic bulb contains a tiny amount of the chemical. Also, its infection-fighting properties can degrade within half-an-hour.

However, in partnership with a Dutch company, Cocking has found ways of both producing larger quantities and keeping its key properties stable for up to a year.

"The solution is expensive to produce, that's the main problem at the moment," he says. "At the sort of concentration required, Allicin costs about 47 a litre, which is similar to a good malt whisky, and it takes around three litres to treat one tree.

"But it effectively kills the bleeding canker, which is seen to dry up. Making the tree's leaves unattractive to leaf moths is a totally unforeseen side-effect.

"We imagine that when we are in a position to make hundreds of litres of the stuff, then the price will come down, and although it will still be an expensive product, a lot of much-loved horse-chestnut trees will

be saved."

CW 9/10/10