The British landscape was once full of elm trees. The introduction of a microscopic fungus in the 20th century changed all of that. Over the course of two epidemics, 60 million elm trees were lost, landscapes were irreversible changed, and ecosystems altered forever.
Since then, our forests and woodlands have faced an onslaught of new invasions. From Phytophthora ramorum, a ‘fungus-like’ organism that infects a number of trees including larch and some species of oak, to ash dieback, a disease first discovered in the UK in 2012 and causing such alarm that the government convened Cobra, the UK government emergency response committee usually reserved for terrorist threats and other such national emergencies.
It’s not just the UK that’s affected; this is a global issue and agricultural crops and natural environments around the world are facing similarly grave threats. Most commentators point to changing trends in world trade and travel and the consequent movement of plant material. In the US and Brazil, citrus industries have suffered from a spate of exotic diseases originating from South East Asia which have devastated production; in Florida citrus canker arrived in 1995 and $1 billion was spent trying to eradicate it. In 2005 the towel was thrown in on the eradication program and, that same year, a far more devastating disease - citrus greening - arrived and subsequently spread throughout the State. Global banana production is currently facing a severe threat from a strain of ‘Panama disease’ called Tropical Race 4. Global wheat production is being challenged by a new virulent strain, Ug99 of stem rust disease, so named because it was first discovered in Uganda in 1999, and has since spread across Africa and the Middle East. The list is goes on…
The latest threat to Europe - Xylella fastidiosa - is particularly concerning however. This bacterial pathogen has spread through Italy and poses a potential threat to the whole EU region. The pathogen has a long history in some parts of the world but it has never been found in Europe before. That is until 2013, when the disease - thought to have arrived accidentally on coffee plant’s from Costa Rica - was found on olive trees in Apulia. It’s spread by sap-feeding insects such as spittlebugs and invades a plant’s water distribution system, preventing essential water and nutrients from being transported around the plant.
But here’s the really worrying part: whereas many plant diseases are capable of infecting only a small number host species, Xylella has a vast list of potential hosts plants. Different sub-species of the pathogen can infect a range of species, from Britain’s native pedunculate oak, to citrus trees, and olive.
In an attempt to prevent the further spread of Xylella, the European Commission has implemented a series of emergency measures, including the removal of olive trees and other host plants in some locations. But the destruction of these ancient groves has caused public uproar. The battle against the disease, and what to do about it, rumbles on, most recently with an appeal from Italy against the EC measures in the European Court of Justice. Clearly, these diseases can have deep social, as well as environmental and economic ramifications
Yet amidst all this doom and gloom there is some light. The spread of most pathogens is driven by spatial processes. Advances in our ability to gather data and map plant disease gives us a powerful weapon against these invading threats. But it’s not an easy conflict. Unlike other types of invasive species, plant diseases are incredibly good at evading detection. These microscopic organisms can inhabit plants and pump out inoculum for weeks, months or even years before their hosts show even the first visual symptoms. So how do we spot them?
Mapping the risk
Disease risk maps are increasingly popular tools in trying to second-guess where and when new epidemics might pop up. For example, the US Department of Agriculture use risk-based mapping methods to layer a range of risk factors associated with different diseases. These range from meteorological drivers of disease spread such as wind and rainfall, to socio-economic factors including knowledge of which residential communities have good connectedness to areas of the world where a particular disease is already present and, hence, represent a threat of its accidental introduction.
Technologies such as remote sensing have also begun to play a pivotal role. For example, remote and proximal sensing instruments such as photointerpretation are being developed by researchers in Italy to recognise and classify tree canopies that show signs of leaf scorching and water stress symptoms typical of the disease caused by Xylella. This information can be mapped in web-based GIS platforms and linked with other data sources, such as mobile phone apps, for field reporting.
Smartphone technology has also been adopted in the UK against forest pest and diseases. The UK project ‘OPAL’ has a Tree health Survey app (www.opalexplorenature.org/tree-health-app) that enables members of the public to identify different diseases and report them to Forest Research through the Tree Alert service. All this information and opportunities to detect and map outbreaks provides vital real-time data on the spatial distribution of disease.
Better spatial information can be put to very good use. Plants don’t tell anyone when they get sick, so we have to actively search for new cases which takes an incredible amount of investment and public resource. Better targeting of surveillance programs is incredibly important to find new epidemics and remove them before they get out of control. For example, researchers from the University of Salford and Rothamsted Research in Harpenden are developing new quantitative approaches that use spatial information on epidemics to help inform smart surveillance strategies.
Spatial information can also be used to make more realistic computer models of spatial spread. Researchers at the Department of Plant Sciences at Cambridge University have developed state-of-the-art computer models of a range of invading diseases, such as ash dieback. Availability of good spatial information can enable real-time models that will help predict disease outbreaks and identify areas of high risk where government agencies can target management efforts.
Plant disease epidemics represent a growing and important threat to agricultural economies, food security and natural environments. It’s clear that, in a globalised society, opportunities for the spread of plant diseases are only going to increase. New ways of detecting and mapping their encroachment will play an increasingly important role in combating the menace.