Horse chestnut at risk

GROW:An increasing number of these stately, lofty trees are being lost to a highly damaging bacterial disease

IT MAY NOT be native to Ireland but the horse chestnut is still one of our loveliest trees and a stately, lofty presence in our parks and gardens, where its giant candlewick blossoms flag the start of summer and its glossy, chocolate-coloured conkers mark its end. But not, perhaps, for much longer.

As a result of the recent spread throughout Europe of the damaging bacterial disease identified as bleeding canker (Pseudomana syringae pv. aesculi), an increasing number of these trees are now being lost from the Irish landscape, with even young specimens succumbing.

In the Botanic Gardens, director Matthew Jebb estimates that they’re losing one tree every year to the disease, while in the Phoenix Park (the first place in Ireland where the disease was confirmed, in 2010), several horse chestnut trees along Chesterfield Avenue have had to be felled. A further two-dozen of the park’s infected trees have received tree surgery, while others are undergoing experimental treatments, all in the hope of arresting its spread.

So where exactly did this disease come from? In Britain (where almost half of its horse chestnut trees are now infected, and confirmed cases of the disease were first recorded as early as 2000-2001), studies suggest that it came from Europe. Most probably it evolved from an Indian strain of the disease – a more benign version that affects only the leaves of Indian horse chestnuts rather than its woody parts. Equally worrying, genome sequencing of the disease in the UK suggests that it probably originated from a single recent introduction into the country – in other words, on imported, infected plant tissue.

Ask how this damaging disease spread through Europe might have been curtailed and you’ll open up a can of worms that raises troubling questions about the nature of free trade and its potentially devastating consequences for national plant biosecurity. Prof Clive Brasier, the highly regarded British plant pathologist specialising in tree diseases, neatly summed up the dilemma by describing it as trade at any cost, arguing that the UK’s independent plant health controls have been sacrificed in the interests of EU membership. Pointing to the dramatic increases in the frequency of damaging plant pathogen importations over the past decade, he suggested that the UK is now in danger of losing much of its historic tree and forest heritage. His blunt conclusion was that the European plant security “door” is, quite simply, off its hinges.

By way of example, Brasier made the point that since 2011, at least five new “events” have been documented in Britain, each one of them potentially catastrophic in terms of tree health. Amongst them is a devastatingly aggressive fungal disease already affecting mainland Europe’s ash trees (Chalara fraxinea) and found in the UK for the first time earlier this year in a Buckinghamshire nursery, in trees that had been freshly imported from Holland.

Another is the distinctive, black-and-white Asian longhorn beetle, first accidentally introduced into the US and Italy some years ago, with the first UK populations confirmed in Kent this spring. This exotic insect lays its eggs inside the host tree/shrub, which can be any of a number of different species, including sycamore, elm, horse chestnut, willow, poplar, birch and some fruit trees. Wooden pallets and packing crates carrying imported goods sometimes serve as Trojan horses, with the beetle and its larvae hitching a ride on the timber, while the pest may also be imported unwittingly with living plant material.

Oak Processionary Moth (Thaumetpooea processionea), another serious tree pest whose caterpillars pose an added risk to human health, causing asthma and skin allergies, was recently discovered in London trees.

Although not yet present in Ireland, many are wondering if it’s only a matter of time before some of these devastating tree diseases or pests reach our shores.

While the international trade in plants has a history almost as old as that of mankind, increasing globalisation has created great challenges in terms of effective pest and disease control.

“It’s a brand new world out there,” says Matthew Jebb, and even in terms of the ever-increasing scale of internet purchasing, the implications are enormous.

“Being an island state just like Britain, we’re lucky in having a natural firewall – a barrier of water – that mainland Europe doesn’t have. But if we continue to reduce trade barriers as the EU wishes, that won’t be enough to protect us.”

Tips

Try to buy Irish-grown trees,propagated from seed when possible and sourced from a reputable garden centre/nursery. Always carefully inspect a plant for signs of disease or pest infestation before you purchase it. Avoid plants that show signs of mechanical damage (cuts to the bark, broken branches, torn roots) that might serve as an entry point for disease. Give long thought to preferred growing conditions – poorly planted/sited trees will be more prone to pests/ disease as result of stress. Nourish the health of existing trees by avoiding compaction of the root system and mulching with leaf mould. For more information on new tree diseases, see forestry.gov.uk/pestsanddiseases.

DIARY DATES

Thursday, October 18th, 8pm. Keith Wiley, designer of Wildside gardens in Devon and former curator of The Garden House, will be giving a talk at the National Botanic Gardens on "An Holistic Approach to Plants". Admission free. See irishgardenplantsociety.com