Despite the lush summer leaves, our trees are under attack

Enough to make the leaves fall off even in summertime. Gareth Fuller/PA

Looking at the countryside now in the middle of summer, it is hard to believe that trees are under threat from an array of diseases and pests. Warm and wet conditions with plenty of sunshine have led to an verdant explosion of plant growth. After news of the spread of ash dieback across the UK many people may be surprised to see ash trees in Britain coming into leaf at all, but tree diseases are not necessarily as quick as media hyperbole might suggest. It can take years for a mature tree to succumb to disease, but once the pathogenic fungus, bacteria or virus takes hold then while the end may be slow, it’s also virtually inevitable.

At the recent Cheltenham Science Festival, I was joined by three leading UK experts in tree disease: Professor Steve Woodward from the University of Aberdeen, Dr Ruth Mitchell from the James Hutton Institute and Dr Richard Buggs from Queen Mary University of London. Ash dieback was at the forefront of the audience’s concerns, with many reporting seemingly healthy ash trees in their back gardens.

Ash dieback is a suite of disease symptoms caused by a fungus, and trying to piece together the life cycle, history and ecology of this fungus has not been easy. As Steve Woodward explained: “What we now know is that the fungus causing ash dieback jumped species to infect European ash. It occurs naturally in a species of ash native to Asia where, like a similar fungus in European ash, it causes no problems. The global trade in trees and greater planting of ornamental and commercial species (ash counts as both) with little regard for quarantine probably helped bring these species together, allowing the fungus to infect European ash.”

As is common in cases where species move to new hosts, the new host had no defences against the fungus. It spread rapidly across Europe to the UK, killing most of the ash trees it infected. The situation is so bad in Northern Europe that trees with some natural tolerance to the fungus – a great example of natural selection in action – are rare enough to become known as individuals, for example Tree 35 in Denmark.

But is this a big problem for the UK: are ash trees an important component of our biodiversity? I put the question to Ruth Mitchell, who was clear on the wider implications of ash dieback. “Losing ash trees will really matter. It is one of our most common trees and there are at least 45 species such as moths, beetles and butterflies that only use ash, another 62 species that predominately use them, and a total of nearly 1,000 species that can be found using ash. That is a lot of species directly affected, and many more will be indirectly affected through various complex interactions.”

Worryingly, it’s only since ash dieback hit Britain that this research has got underway, and the species associated with other trees are only poorly known.

The emerald ash borer, among the more glamorous of tree pests. USDA, CC BY-SA

Ash dieback may be the current pin-up of the tree disease world, but there are plenty of other diseases threatening our trees. Acute oak decline, chestnut blight and various Phytophthora fungi responsible for brown rot are among the microorganisms causing disease. There are also Asian longhorn beetles, horse chestnut leaf miner and oak processionary moth to contend with in terms of insects causing problems. Ash doesn’t escape insect pests either, with the Emerald Ash Borer spreading west from Russia.

With so many threats either already here or on the horizon, is there anything we can do? As Richard Buggs explained: “It is probably already too late for ash in the UK but we can use genetic research to develop a strong genetically-diverse base of tolerant trees to re-plant Britain in the future.” The answers to tree disease aren’t just in the science. “There is a huge movement of plants across the world, and when plants move you also move diseases, pests and goodness knows what else living in the soil they’re transported in. We need better biosecurity controls over this movement,” says Steve Woodward.

The overwhelming conclusion of the panel was one that will be familiar to ecologists everywhere: the situation is complicated and we don’t know anywhere near enough about the diseases or their implications. One thing is clear though. We shouldn’t let the relatively slow pace of tree diseases (which take years rather than weeks to take hold) dictate our research efforts to combat them.

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