We must breed new life into resistance to ash tree threat

The fungal threat to our ash trees has thrown up a number of issues which must be addressed

If you have lived in Ireland for any reasonable length of time, you will likely be familiar with the ash tree. A native species, we use it for fuel and products such as furniture and hurleys. And as a nation we have been busy planting thousands of hectares of the hardy and fast-growing tree over the past two decades or so.

But now there’s a threat. Ash die-back has made it to Ireland. The fungal disease has potentially catastrophic consequences for individual trees, and stopping its spread can involve felling and burning entire plantations.

So what is this fungus? What’s the potential damage? And what kinds of steps can we take to combat it?

The root of the problem is a fungus called Chalara fraxinea, and the signs of infection start showing up when leaves literally start dying back, explains Dr Gerry Douglas, a principal research officer at Teagasc in Kinsealy.

“Ash die-back was first confirmed in Europe in 2005 but the disease had been noticed for some years before that,” he says. “The leaves of ash trees were beginning to wilt during the summertime, then they would turn black, and the shoots would die from the tip back to a branch.”

Fungus outbreaks

The fungus is now in forests in various parts of continental Europe, there have been more than 220 confirmed findings in the UK and the fungus recently turned up in young ash trees in Co Leitrim.

“That outbreak of the disease was traced to an importation, a consignment of more than 30,000 trees that were planted in 11 different sites around the country,” says Dr Douglas. “Those entire plantations were destroyed and burned so they wouldn’t be a source of infection.”

In October, the Department of Agriculture, Food and the Marine announced new legal measures were now in place that “make it an offence to import ash plants and seed from areas within the EU that are known to have the disease” and regulations have also been introduced to cover imports of ash wood, which is a substantial source of material for making hurleys.

So what are we looking to protect? Ireland has just one native ash species – Fraxinus excelsior – explains Dr Douglas, and it’s important both for biodiversity and as a crop. Today, ash covers around 19,000 hectares, which amounts to about 3 per cent of our forest cover.

“On average we have planted about a million ash trees each year for the last 20 years,” he says. “And the main reason for planting ash trees is the fact that it’s a high value wood and it grows fast.”

Because our ash forests are young, it’s difficult to put a value on them, according to Dr Douglas, but he estimates that the State has invested more than €90 million to support the development of plantations over the past two decades.

“It’s a huge investment and a huge potential loss, because if the level of infection is very high not only will a proportion of the trees be killed, most of the other trees just won’t develop as they normally would.”

Because the fungus kills off a proportion of the leaves, it can stunt an individual tree’s ability to photosynthesise and grow. Younger trees are particularly vulnerable because they have less in reserve, explains Dr Douglas. And even when the diseased leaves fall off, the story isn’t always over – infected leaves can harbour the fungus and spores get released the following summer, when they can can re-infect the tree and even spread to others.

In addition, Ireland’s climate tends to be one that fungi generally like, he adds.

Halting the spread

One way to try and halt the fungus is to fell and burn infected trees, and to burn or bury the leaf litter that could be a source of spores.

Another positive step would be to implement a more rigorous tracking system for imported ash, according to Dr Douglas.

“So far any of the disease that has come in has been on imported plants,” he says. “At the moment there is documentation when you import plants to say where they come from, but plants are traded very extensively and it’s sometimes difficult to follow the paper trail. We really need a verifiable system using DNA technology so that we can trace where the trees come from if we import any plants. ”

And in the longer term, breeding programmes could offer a lifeline.

“Even if the outbreaks aren’t very bad in Ireland, we have to now consider breeding for resistance – we need to take this breathing space to gain a lead on the disease,” says Dr Douglas.

He is carrying out research to select and propagate trees with good “stem” qualities, and a small proportion of these trees may be resistant to ash dieback disease.

They could be identified and they could form the basis for new generations of high quality trees which also carry resistance, he explains: “That’s what we are planning at the moment.”