A number of new diseases have emerged in forest ecosystems over the last century, such as chestnut blight, Dutch elm disease and white pine blister rust, mainly as a result of increased global trade of timber and plant material. One of the most damaging diseases, root rot (Heterobasidium annosum), is caused by a fungus and is responsible for annual losses to the European forest sector of €790 million. Loss of trees through disease can also upset the balance of entire ecosystems by threatening other resident insects, lichen, fungi and plants. On a larger scale, tree loss could also impact on the global carbon cycle by altering carbon sequestration processes.
The researchers, partly funded by the EU ISEFOR project1, explored existing barriers to effective legislation against the outbreak of diseases, most notably the difficulties scientists face in predicting the distribution and severity of new pathogens, some of which may increase with the effects of climate change. Part of the problem is a lack of common understanding about why a small minority of pathogens become a threat and others do not, and why some forests are more susceptible than others. Both native organisms and newly introduced species can become pathogenic if their development becomes invasive. Some may carry a ‘novel weapon’, against which a native population has not evolved sufficient defence. Others may be favoured by temporary changes in the forest ecosystem or by artificially stable conditions, such as those in single-species forests (monocultures). A pathogen that spreads efficiently by wind or water may also become widespread very quickly.
However, many other complex biological factors are involved. New species of pathogens can form by ‘hybridisation’ when different exotic pathogens come into contact, such as in plant nurseries, leading to new diseases. Changing climatic conditions may cause changes to both host and pathogen distributions. Genetic diversity can also influence susceptibility, making it very difficult to predict the likely outcome of a newly introduced pathogen.
Currently, the EU allows free trading of plant material (i.e. cut flowers, bulbs, seedlings, tubers) within its borders and from outside the EU as long as a ‘plant passport’ is presented2, meaning the plants have been examined and are free of all known pathogens. However, this does not help with identifying unknown diseases or when sanitary measures fail to detect the spread of pathogens. The researchers suggest a global network of ‘sentinel plantings’ to act as early warning systems for the transfer of unknown pathogens between countries. Also within EU, a monitoring system based in spore traps around timber import and export centres could enable a faster response in the case of an undetected known threat. Finally, internet based reporting systems that allow the general public to signal anomalies have proven to be very effective in Sweden and the US.
Epidemiological monitoring studies and genetic sequencing tools may help to strengthen our ability to make predictions about specific pathogens given future conditions, say the researchers. However, it is possible that stricter legislation in individual countries may be limited by the pressure to protect free trade within the EU.
If a new disease establishes itself, management recommendations need to be based on a thorough understanding of the biology of that individual pathogen, say the researchers. This presents a sizeable challenge, however, as some sub-groups of a species may be pathogenic while others may not, for example. For these reasons, clearer communication of current scientific knowledge is needed, particularly with respect to definitions and terminology, so that risks are clearer for the public and policy makers, and appropriate policy responses can be developed.
1. ISEFOR (Increasing Sustainability of European Forests: Modeling for Security Against Invasive Pests and Pathogens under Climate Change) is supported by the European Commission under the Seventh Framework Programme. See: www.isefor.com