Katherine Hayden: What to do about Hymenoscyphus fraxineus

A young Common Ash Tree with wilting leaves. Picture: PA
A young Common Ash Tree with wilting leaves. Picture: PA
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IT’S what causes ash dieback, and similar pathogens are now wiping out our junipers. We can’t stop them without changing our whole way of life, writes Katherine Hayden

In September 2015, recent outbreaks of plant disease hit home at the Royal Botanic Garden Edinburgh. An iconic tree, a Juniperus rigida which had stood in the Rock Garden for 77 years fell ill. It developed a characteristic coppery brown foliage, soon confirmed by Forest Research to have been caused by the introduced pathogen Phytophthora austrocedri.

Juniper in Scotland has been threatened by P. austrocedri since at least 2007 – a particular problem for our native Juniperus communis, best known for its berries, and indispensable for gin. Plantation forests of Japanese larches have been killed by a different species of Phytophthora, P. ramorum, first observed in the UK in 2002. And there is, of course, the well-publicised threat to ash trees from ash dieback, caused by the fungus Hymenoscyphus fraxineus. This pathogen landed in the UK following a march across Europe, and has begun killing ash trees in Scotland.

If it seems these stories are heard more frequently than before, it’s because they are. Global trade in plants and plant products has meant the inadvertent movement of plants’ invisible associates –the suite of microscopic friends and foes that make their living in leaves, bark, wood, roots and soil. Organisms that are inconspicuous in one environment – checked by natural enemies or hosts that have adapted defences over millennia of co-evolution – can become cause major problems in a new location, where they lack natural enemies or face a defenceless host.

Plant diseases are not all bad -- they play an important role in natural communities. They help to generate diversity, by thinning out too-common species, creating gaps in forests for other species to fill in, or by pushing plants to develop new defences and strategies in a co-evolutionary arms race that can even result in compounds useful in human medicine.

However the balance can shift when landscapes are modified – as in large single-species plantations – or when microbes are transported to a new environment and to naïve hosts. Forest trees can be especially vulnerable as their long generation times make them slow to adapt, giving new pathogens the advantage in any arms race. And this matters – forest trees are important themselves and also support ecosystems by providing food and habitat for a multitude of other plants, animals, and fungi.

So what can be done? There is clearly an inevitable trade-off between the availability and demand for goods on the one hand, and restrictions on plant movements to prevent the introduction of pathogens on the other. The two most important pathways for importing pathogens and pests lie in live plants and in wood products, including packing materials like crates and pallets. These dangers are not limited to pests and pathogens that are already known to be threats. The solutions to this complex problem must come from across sectors, public to politicians, scientists to businesses, and will involve import restrictions and surveillance, effective quarantine and monitoring of material in trade, and consumer- driven demand to reduce unnecessary risk from unnecessary imports.

Individual action can help. Don’t bring plants and especially soil from abroad as soil around even healthy plants can harbour resting spores of disease-causing organisms, and check where your nursery plants come from. Likewise with firewood, which can carry disease-causing pathogens and insects – “buy it where you burn it,” and ask for local wood. Follow the advice of the Forestry Commission Scotland and “Keep it Clean”: clean dirt off of shoes, bike tyres, and dog paws before and after a forest visit, to avoid spreading plant disease.

All these efforts can slow the pace, but new introductions and outbreaks will inevitably occur. When they do, the best hope to controlling forest disease lies in coordinated surveillance and early intervention, underpinned by a strong and relevant body of scientific evidence, and coupled with well managed and biodiverse landscapes. Research on ash dieback has allowed the rapid identification of genes indicating disease tolerance in ash trees, even as citizen-science efforts are underway to find more tolerant trees.

These findings may accelerate the restoration of ash trees in landscapes. And, as pathogens are always evolving, and are thus a moving target, a key focus for research is that of resilient landscapes – where diversity of species and genes provides insurance against any one particular threat. Understanding and conserving the full diversity of our wild plants can maximise their ability to hold their own against the next big threat.

• Katherine Hayden is a mycologist at the Royal Botanic Garden Edinburgh, www.rbge.org.uk