Plant disease risk is strongly influenced by environmental conditions [1]. While some animal hosts may provide their pathogens with a consistent range of body temperatures, plant pathogens are generally much more exposed to the elements. Plant disease will tend to respond to climate change, though a number of interactions take place among host, pathogen, potential vectors. In some cases, the actions of land managers may also complicate interpretation of climate change effects. In this chapter, we present a brief introduction to plant disease and a synthesis of research in plant pathology related to climate change. We discuss the types of evidence for climate change impacts ('climate change fingerprints') that might be observed in plant disease systems and evaluate what evidence of climate change fingerprints currently exists.

The battle against plant disease is not a new one, and plant disease management is essential for our continued ability to feed a growing human population. The Great Irish Hunger is one striking example of the impact of plant disease: in 1845 more than a quarter million Irish people starved as the result of an epidemic of potato late blight [2]. Plant diseases continue to cause serious problems in global food production. Currently more than 800 million people do not have adequate food and at least 10% of global food production

Climate Change: Observed Impacts on Planet Earth

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is lost to plant disease [3]. Not only does plant disease affect human food production, it also impacts natural systems [4]. Introduced diseases such as chestnut blight in the Eastern US, and more recently the increasing occurrence of sudden oak death, have resulted in the rapid decline of dominant tree species and triggered major impacts on forest systems [5].

Plant pathogen groups include fungi, prokaryotes (bacteria and mycoplas-mas), oomycetes, viruses and viroids, nematodes, parasitic plants and protozoa. The very different life histories of this diverse group of organisms and their different interactions with host plants produce a wide range of responses to environmental and climatic drivers. For example, viruses may be present in hosts while symptom expression is dependent on temperature [6]; thus, even the difficulty of detection of these pathogens varies with climate. Fungal pathogens are often strongly dependent on humidity or dew for plant infection [7], so changes in these environmental factors are likely to shift disease risk. Genetic variation in pathogen populations often makes plant disease management more complicated when pathogens overcome host disease resistance [3]. Pathogen species may quickly develop resistance to pesticides or adapt to overcome plant disease resistance, and may also adapt to environmental changes, where the rate of adaptation depends on the type of pathogen [8]. Pathogen populations may explode when weather conditions are favourable for disease development [9,10]. The potentially rapid onset of disease makes it difficult to anticipate the best timing of management measures, especially in areas with high levels of interannual variability in climatic conditions.

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