Bleeding canker threatens horse chestnut

I can’t walk past a horse chestnut tree without stopping to examine it.

I look for signs of disease, for the bleeding canker of horse chestnut. It’s a new disease and it’s virulent in western Europe. Thirty percent of horse chestnuts in the Netherlands, Germany, France and Belgium are likely infected. Fifty percent in Britain. I can’t find statistics for Ireland, but it’s bad here as well. The OPW is felling infected trees in Dublin’s Phoenix Park.

The first signs are a dark, tarry or rusty gum that exudes from the bark on the trunk or on infected limbs. The infection spreads and girdles the tree, which dies. This can take a while with a large tree and it’s the crown that dies back first. Occasionally, there is a spontaneous remission and the tree recovers. It was thought to be a fungal disease but it’s now known to be caused by a bacterium. There is no known cure, though possibilities are under trial. Horse chestnuts are magnificent trees, particularly when they’re in bloom, and it’s sad to think that if the disease becomes rampant the game of conkers may become extinct.

The other day I got sadder news. On two occasions, I’ve travelled on the Canal du Midi in western France. It’s a wonderful waterway, the largest and oldest in Europe and a UNESCO World Heritage site. One of the things that make it wonderful is that most of it is lined on both sides by great, 200-year-old plane trees. They provide shade and protect the banks from erosion, but they also make a huge contribution to the beauty of the waterway. They’ve started cutting them down.

A disease, a different one, has attacked them. This time it is a fungal disease and the trees can’t be saved. There are 42,000 plane trees along the canal. Just 2,000 have been felled and it seems inevitable that they’ll all have to go. They will be replaced by young trees that are resistant to the disease. But it will be 200 years before the waterway regains its former glory.

Another fungal disease is having an effect along Ireland’s inland waterways. Alder is a tree that likes to grow beside water, particularly along the banks of streams, rivers and canals. Yet another fungal disease has been attacking them, reducing many to gaunt, dead skeletons. There’s something ominous and ugly about dead trees.

Then, I think about the massive wych elms that grew in limier parts of the country when I was a child. They were the dominant tree in these areas and a good specimen was a very impressive sight. Now Dutch Elm disease has brought them close to extinction.

We know a bit about which trees grew where and in what quantities through the ages, thanks to evidence from bogs and the pollen record. There are indications that tree diseases have struck before. There was a previous crash in elm numbers 3,000 years ago, for example. But it seems to me that the number of disease outbreaks in my lifetime is far greater than during any period in the past.

There’s an explanation for this. It’s the increase in the movement of people and goods around the world. There’s a theory that the bacterium that’s attacking the plane trees arrived in France in the wooden ammunition boxes of American GIs during World War Two. The alder fungus is a hybrid, the result of two fungi from different parts of the world meeting and cross-breeding. Globalisation has some unfortunate consequences.

* dick.warner@examiner.ie

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