Will the pests and pathogens conquer the conker?


ANOTHER LIFE:WITHIN DAYS of the salty Atlantic storm that seared the west coast at the end of May, I was fielding anxious questions from Donegal and elsewhere: would the trees find the energy to put out a whole new set of leaves? Looking at all the scorched seaward branches on our own acre, it seemed safer to sound doubtful, while hoping to be wrong.

Wrong, I was. Within weeks the big ash was turning green again, and one by one the others followed: oak, beech, lime, sycamore, all pushing out fresh leaves beyond the withered ones. Even the young horse chestnut, down by the hedge, draped in mourning where its leaves survived at all, has conjured tender miniatures of the big, sturdy fans unfolded in spring.

Aesculus hippocastanum,the horse chestnut from the warmer corners of eastern Europe – Turkey, Macedonia, Albania – doesn’t really belong on a Mayo coastal hillside. Somebody, probably me, dropped a handful of conkers into a hole and forgot them. Now we have a five-stemmed tree that lit its first flower-candle in the weeks before the storm. It must be one of the most westerly specimens in Europe, and so might even survive the twin plagues that are decimating the species.

Carried as conkers by travellers from Constantinople, the tree was growing in Vienna by 1576 and France and Britain in a few decades more.

Ireland’s Lord de Vesci, never far behind fashion, planted it at Abbeyleix, where, by 1791, Samuel Hayes could admire a horse chestnut with “a spreading canopy over a circle of 72 feet diameter”. Since then this magisterially decorative tree has come to grace parks, estates and market towns across the island. Even more, perhaps, than those of the elm, each tree death will be witnessed with much public sorrow.

Horse chestnuts are prone to several pest and pathogen problems, intensified, perhaps, by a limited genetic base. One of the most common has been a fungus, Guignardia, that blotches the leaves with brown. But this has now been overtaken by two more serious afflictions, one from an insect leaf miner and the other from a bacterium. Both can turn the tree brown and leafless, conspiring in its terminal decline.

So far, in Ireland, it is the bacterium that does the damage, notoriously a “bleeding canker” that oozes liquid from patches of dying bark on trunk or branches. The symptom used to be blamed on a fungus-like Phytophthoraorganism (the same family as potato blight).

But its sudden increase in these islands and western Europe has been traced instead to a bacterium, Pseudomonas syringae pathovar aesculi, known previously only from the leaves of horse chestnuts native to the northwest Himalayas.

Identified in Ireland last year, it will, as feared by Dr Matthew Jebb, director of the National Botanic Gardens, cost Ireland some of its best stately avenues. Dublin’s Phoenix Park has already lost 20 horse chestnuts to the pathogen, so contagious that lopping off branches can spread it and a dying tree has to be cordoned off and buried when dead, rather than being burned. “We must sit on our hands,” says Dr Jebb, “and let nature take its course.”

More positively, he points out that some of the fast-growing trees can stay healthy even beside infected specimens. Unlike hedgerow elms, they do not share root systems – a factor that helped spread Dutch elm disease. But our white-flowered horse chestnuts now face another new plague, the leaf-mining, little caterpillars of the invasive micro-moth Cameraria ohridella.Hatching in huge numbers, these tunnel the leaves and turn them brown by mid-August.

While quick to rouse public dismay, this need be far from fatal.

Recurring from year to year, it limits the tree’s photosynthesis and produces smaller conkers. But its potential interaction with the lethal bleeding canker is of real concern. The tiny, chestnut-striped, whiskery-tailed moth – new to science since its discovery in Macedonia in the 1980s – has managed to invade almost all of Europe, probably travelling with cars and lorries. In Britain, now widely infected, its first record in south Devon was in a holiday resort car park, and in Germany it has been nicknamed “ Trampermotte” (hitch-hiking moth).

In Berlin, the city council has limited some of the damage by gathering up the autumn leaves (in which the larvae can overwinter) and encouraging citizens to follow suit in their gardens. Burying the leaves, or stuffing them into the heart of a hot compost heap, can kill the larvae. The hunt is on for a parasitic wasp that is active at the right time to attack them. There is also a UK project, called Conker Tree Science,using the help of schools to record the spread of the moth and also its parasitic enemies.

So far, the pest has been reported in Ireland from Dublin, Waterford and Northern Ireland, but its further invasion seems, alas, inevitable.

Eye on nature

I noticed a bird in our garden that looked like a robin with a red breast but it had the head markings of a thrush.Rosalie Prendergast, Dalkey, Co Dublin

It was an immature robin in the process of getting its adult plumage.

We saw a comma butterfly at Ferrycarrig, Wexford, on July 9th, and again on July 24th in our garden in Ballybrack. Have they been here long?Virginia Chipperfield, Ballybrack, Co Dublin

They were first recorded on this island in 1997 and are seen mainly in the southeast.

One of the great tits visiting my peanut feeder has an orange patch on the front of its head.Declan Quigley, Wicklow

It has been feeding on insects in the pollen-filled flower heads of New Zealand flax.

On July 24 I witnessed a swarm of ants that nest in our compost bin fly high into the sky. At this time of year they’ve done this for the past two years.Martin Coffey, Tallaght, Co Dublin

It is the annual mating flight of the ants and happens in times of high humidity. The males die and the queens return, go underground and lay eggs for the next 20 years.

Michael Viney welcomes observations at Thallabawn, Carrowniskey PO, Westport, Co Mayo. Email: viney@anu.ie. Please include a postal address