California’s oak trees are dropping dead in their hundreds of thousands in an epidemic described as “unstoppable”. The emerald ash borer has wiped out ash trees across North America with losses amounting to billions of euro. Olive trees are being knocked out by a bacterium first found in Italy in 2013 but now spreading rapidly through plantations.
Here in Ireland, ash dieback disease, also known as Chalara, has spread to nearly every county since it was first discovered in a tree plantation in Co Leitrim in 2012.
And there are other insects, bacteria and fungi causing varying amounts of damage to food crops, ornamentals and trees around the world.
What has happened to cause this destruction and what does it mean for our shared future, given that plants feed us, shelter us, clothe us and provide energy to warm us?
The Red List
The International Union for the Conservation of Nature's "Red List" estimates that 21 per cent of plants are now threatened with extinction.
This stark assessment is backed up in a new report, State of the World's Plants, released in May by the Royal Botanic Gardens at Kew.
It provides a snapshot of the plants that populate the Earth and details a range of drivers that are pushing plants to the edge.
These range from global land cover change to runaway plant diseases, and unsurprisingly climate change is there, too.
Kew's report acknowledges that there is "little published evidence" linking plant extinctions directly with climate change. Even so, scientists know a lot about a plant's environmental tolerances and can model the likely impact of climate change. The upshot is that many plant species could be on "borrowed time", the report states.
“These models predict that suitable climate space will become so severely restricted for many species that widespread climate-related extinctions are expected,” according to the report.
Dr Gerry Douglas, a research officer in forestry at Teagasc, stresses there is as yet little evidence to say climate change has caused the emergence of pathogens but, even without such evidence, opportunistic organisms such as fungi and bacteria can exploit conditions and take hold.
Factors such as a short-term drought, a spring arriving too early or unexpectedly late or the loss of a top predator to control pathogens can align to trigger an outbreak.
“They can be catastrophic, but I would separate those from the transplantation of pathogens that were never present and take hold in a forest,” he says. This is where a fungus, insect, beetle or bacterium is introduced by accident, as happened with ash dieback here, triggered by a fungus originally from Asia. There the organism has little impact, but in Ireland ash trees have no natural resistance to it and become the perfect host, he says.
The Xylella fastidiosa bacterium that is decimating Italian olive trees is an import, as is the notorious emerald ash borer. This latter bacterium is of particular concern because it seems to spread quickly but can also damage other species, for example citrus trees, so it might have a wider impact.
The real question is: could a super pathogen emerge that could cause damage to all types of vascular and flowering plants, thus denuding the world of life-sustaining vegetation? Consider that more than 40 per cent of the world’s oxygen arises from the planet’s rainforests.
Damage seen from space
The fossil record going back millions of years is not definitive, but there are hints on this issue, says Prof Jennifer McElwain, professor of plant biology at University College Dublin's Earth Institute.
"We have events in the geological past at extinction boundaries where there is an overwhelming increase in pathogenic fungus spores that suggest mass degradation of plant life," she says.
These boundary events have happened a number of times, always accompanied by a matching destruction of existing plant and animal life.
The spores, visible in fossils from the time, are at low levels, but then, over a period, there would be a “fungal spike” of spores, she says.
The question is: were the spores from the kind of fungus that acted only on dead plant material, or could a super fungus have arisen that caused all the deaths? “Unfortunately we don’t know that, but it is a question we should be trying harder to answer,” she says.
Climate change might not be the primary cause of a plant dieback event, but it certainly can tee up such events by putting plants under environmental stress. A mild winter might allow a damaging beetle to survive in much higher numbers, and if there is even moderate drought, then stress along with the primary cause of damage could trigger massive landscape-scale plant death so great that it is easily seen from space, Prof McElwain says.
FIGHTING BACK: BOOSTING PLANT IMMUNITY
Globalisation has helped disease-causing agents to move all around the world, but even without this, there are more pathogens and more virulent pathogens emerging all the time, says Dr Barbara Doyle Prestwich of University College Cork's School of Earth and Environmental Sciences.
Plants themselves are not without a response, however. They can't move to escape an attack but they have a well-developed natural immune system that can fight a fungus or bacteria.
“We are trying to work out how to boost that immune system,” says Doyle Prestwich, who is also president of the International Association of Plant Biotechnology.
Her research group has travelled to South America, the original home of the potato, to look for varieties that, over generations, have developed resistance to the organism that causes blight.
She is also studying “microbial volatiles” – gases emitted by bacteria that move through the air and might be useful in controlling plant diseases. It might also be possible to introduce resistance into a plant using genetic modification, she says.
Ultimately which plants survive and which succumb to a pathogen could come down to evolution. A plant with traits that help it survive attacks by a fungus will survive, as will its progeny if they too carry a resistance gene.
Unfortunately it might take 100 generations to breed in disease resistance, with climate change working to ensure this will not happen. This opens the way for new plant species to arise.
