Pest pathways and tree disease: How can we save our forests?

What would you say is the greatest threat to the survival of our trees? Deforestation? Climate change? How about disease?

Many of us don't really think about trees getting sick or infected but pests and diseases are actually one of the biggest dangers to the future survival of some of our best-loved tree species, from the mighty oak to the ancient ash. The bad news is they're on the rise, but there are a few clever tools we can use to help protect our trees.

Standing like frail ghosts of the forest these tanoak trees in California were all killed by the fungus-like pathogen Phytophthora ramorum, known in the USA as "Sudden Oak Death". It has caused the death of millions of tanoak trees in the USA since the mid-1990s.

According to a recent review in the journal Science, the number of pest and disease outbreaks in trees are on the rise across the globe. This is largely due to the growing international trade in plants and plant material, but climate change also plays a role.

Often, what makes these pests and pathogens such a big problem is that when they are introduced to countries or areas beyond their natural range, native tree species there are unlikely to evolved any kind of resistance or defence mechanisms against them². Climate change only exacerbates this both by altering the range of infecting organisms, and by increasing abiotic (physical) stresses such as drought, flooding and severe gales, all of which weaken the tree making it even more susceptible to attack. Not only will climate change influence the health of the plants, but the increase in tree mortality due to infection could also accelerate climate change due to the loss of a carbon sink, and the resulting carbon emissions from the decomposition of organic matter³.

When an infected tree dies or has to be cut down you not only lose the timber product but a wide range of ecosystem services with it, such as carbon storage and water purification, benefits to human well-being, and biodiversity - something which is not so easily regained by simply replacing lost trees. Our priority therefore is not only to prevent new diseases being introduced, but to improve forest recovery from these outbreaks. As an example, let's take a look at one of the most serious tree diseases in the UK at the moment: Ash Dieback, caused by the fungus Chalara fraxinea.

Building resistance - genome secrets and citizen science

Not all trees are as susceptible to certain pathogens or insects as others, so one way in which we can look to combat rising outbreaks is to build up the number of resistant trees. Just a couple of months ago the UK ash tree genome was sequenced in the hopes of discovering the genetic basis of resistance to ash dieback. Unfortunately this is unlikely to come in the form of a single gene which codes for resistance, instead we are almost certainly looking at the cumulative effect of several different genes. However, whilst this poses a much more difficult task, it may also provide greater possibilities if different alleles from different tolerance-related genes could be brought together to create something which is more resistant than any current naturally occurring tree¹.

Scientists have also sequenced the DNA of the Chalara fungus itself, and looking at both sets of genetic data together will help to shed light on how the fungus spreads, how it kills trees and why some trees are able to survive it. Even better, the genetic data for both the tree and the fungus have been released on the internet so that scientists across the globe are able to make use of it.

And the public can help too. By playing the Facebook game "Fraxinus", anyone sitting at home can help scientists to genetically map variations in the ash tree and Chalara fungus by matching sequences of 21 nucelotides by rearranging patterns of coloured leaf shapes. The benefit being that people are much better at this than computers alone.

Battling bugs

Unfortunately, ash dieback is not the only threat to ash trees. The emerald ash borer has already killed tens of millions of ash trees in North America and is currently spreading across Europe from the outskirts of Moscow. Its effect has been so devastating in the US because trees there have never evolved resistance to the beetle, which was transported there from its native range in Asia. It is not yet clear whether the EAB will pose as much of a threat to European Ash, but its dispersal and cases of infection are being monitored.

One of the main reasons herbivorous insects become pests is because they have escaped the natural enemies of their native habitat. Introducing a biological control agent (natural predator) can help to control pest species but success is quite varied². As with the Chalara fungus, genome sequencing of pest insects, as done earlier this year with the Mountain Pine Beetle in Canada, could provide vital information about the insect which may hold the key to modeling and mitigating future outbreaks.

Patching up pathways

Prevention is always better than cure, so to prevent new outbreaks and reduce the spread of diseases and pests we need to identify the pathways by which they enter or move around a country. We humans actually contribute to this a lot more than we may think - not only internationally, via trade of plants or wooden packaging, but also on local and regional scales by transporting spores or insects on live plants, shoes, clothes, tools, machinery and tyres. Ash dieback came to the UK in February 2012 in an import of nursery stock plants. Since then a Plant Health Order has been issued which prohibits all imports of ash seeds, plants and trees, and all internal movement of ash seeds, plants and trees, helping to reduce further spread of the disease.

Early warning systems

If we can identify potential pests and pathogens before they even reach the trees they target we may be able to slow their arrival or at least better prepare for it. A new EU plant health project aims to highlight future threats by using a network of "sentinel plants" in arboreta and botanic gardens across the globe. Using a range of field-based and remote diagnostic technologies the International Plant Sentinel Network (IPSN) will monitor "outliers" of different species from around the world, many of which are being grown outside of their natural ranges. If any of these are shown to be susceptible to a particular pest or pathogen they can act as an "early warning system" giving an indication of potential diseases or invasive herbivores that could have an impact in the plant's native country in the future. The project is being funded through EUPHRESCO and will be co-ordinated by Botanic Gardens Conservation International, who's Plant Search Database contains more than 1 million records of plants which could act as sentinels in botanic gardens worldwide.

Think fast

As with many environmental or disease-related threats, time is of the essence, and the best thing that we can do is to act fast. Even if the science moves quickly it will never be quick enough in areas where the pest or pathogen in question has already taken hold. We will likely be living with the effects of such cases for years to come but continuing to encourage international scientific collaboration and strict trading restrictions will hopefully give our trees a fighting chance.

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