1CSIRO Tropical Agriculture, CRC for Tropical Plant Pathology, University of Queensland, Brisbane 4072, Australia; 2University of Rostock, D-18051 Rostock, Germany; 3IRRI, PO Box 933, Manila 1099, The Philippines

Setting the scene
Atmospheric composition and global climate is changing as a result of human activities driven by technological, economic, social and demographic changes. The concentration of radiatively active gases such as CO2 has increased by 30% since 1750, contributing to a global warming. Changes in climate are expected to continue, with projected global mean temperature rising between 1 and 3.5C by 2100 [1]. Under the current climate, biotic plant diseases cause significant economic losses resulting from reduced productivity and quality, cost of disease management and the penalty from growing less-profitable alternative crops. Changes in climate can influence the economic impact of plant diseases, affecting the 20 or so crops that stand between world population and starvation. Plant pathogens will be among the first organisms to exhibit effects of climate change due to their large population size and short generation time.

Climate change and plant diseases
Published information on the effects of CO2, ozone and UV-B on pathogens and diseases [2] can be used to explore impacts of changing atmospheric composition on plant diseases. Based largely on growth-cabinet experiments, changes in atmospheric composition have been shown to alter stages and/or rates of development in the life cycle and pathogenicity of causal organisms and to modify the physiology and resistance of the host plant. Evolution of pathogen populations may accelerate from enhanced UV-B radiation and/or increased fecundity in elevated CO2 to rapidly overcome host resistances. Predicting the impact of climate change, on the other hand, becomes speculative due to uncertainties in our ability to predict future climate change and a paucity of knowledge on the effect of climate change on individual plant diseases. As the gradual process of climate change interacts with other concomitant processes in the production system, it becomes difficult to isolate effects due to climate change alone. Only a handful of work has dealt with climate change impacts on plant diseases. Most probably, impacts will be felt in altered geographical distribution and increased or decreased crop loss, and efficacy of control measures due to changes in the physiology of host-pathogen interactions. Changes may occur in the type, amount and relative economic importance of pathogens and diseases. With host species migrating to new areas, new disease complexes may arise while some diseases will cease to be economically important. Crops, especially perennials, may continue to be grown in marginal climates for economic reasons, and chronic stress would lead to progressive deterioration in plant health, increasing susceptibility. Disease management may be influenced due to altered efficacy of biological and chemical control options.

Future prospects
Experimental studies, which are beginning to enhance understanding of climate change impacts on plant diseases, need to continue. However, experimental approaches will have limited value in impact assessment due to the large number of interacting factors, scenarios and mitigation strategies that need to be considered. Analytical frameworks or models are necessary to consider the effects of climate change on plant diseases in the functioning of natural and managed ecosystems. Plant protection professionals will need to look beyond the science of plant pathology to seek and invite concepts and ideas realizing that climate change may have positive, negative or neutral impact on plant diseases. Research should be as much about identifying new opportunities as preparing to minimize negative impacts. Impacts of other interacting factors, such as changes in production systems, cannot be ignored. Integrating crop and disease models should essentially be seen as the first step in assessing the impact of climate change at a scale that is more relevant as an environmental unit. To do this, climate-change prediction must be made location-specific from its current continental scale.

1. ICPP, 1996. Contribution of Working Group 1 to the second assessment report of the Intergovernmental Panel on Climate Change, pp. 289-357.
2. Manning WJ, Tiedemann AV, 1995. Environmental Pollution 88, 219-245.