Altered distribution and life cycles of major pathogens in Europe
Fitt, Bruce D.L.
Tiedemann, A. V.
Climate is a major determinant of crop productivity. Besides its direct impact on plant growth it has a major effect on the prevalence and incidence of plant diseases. The sensitivity of pathogens to climate is pronounced. While the local climate determines the general pattern of prevailing pathogen populations, specific weather conditions are important drivers in distinct phases of the life cycles of pathogens such as dormancy and survival, asexual or sexual propagation, and infection and colonization of host plants. Knowledge about the impact of weather variables on pathogens and diseases is an important part in predictive crop protection strategies. There is considerable data available from the past 30 to 40 years on climate-disease relationships, which have been used to develop weather-based forecasting models with the aim of predicting epidemic severity to maximise economic use of pesticides. A large number of such models have been constructed for specific diseases to help farmers and advisers in making their decisions about crop protection to save unnecessary sprays. Such weather-based disease forecast models can also be used to simulate epidemic severity and/or the geographic spread of a particular pathogen under future climatic change scenarios. However, there are some important constraints in making climate change-disease projections, the first resulting from the large variability and uncertainty of current climate prediction models themselves. Further complicating factors arise from the fact that climate not only affects pathogen or pest populations directly but also induces changes in the crop production systems and cropping techniques (soil tillage, irrigation, sowing dates, cultivars, crop species) that indirectly alter the prevalence of pathogens or pests. It is difficult to separate direct from indirect climate effects. Climate change effects can be exemplified with major pathogens of oilseed rape. Greater mean temperatures may be associated with spread of phoma stem canker further north and altered temporal pattern of the fungal life cycle and disease stages in the UK. There is also some indication that sclerotinia stem rot, after mild winter conditions, may perform a pre-seasonal sclerotial stage. In addition, root infection has occurred more frequently with yet unknown relation to recent climate shifts. Yield losses from ascospore infections at early flowering stages may increase compared to late infections. Soil-borne diseases will specifically be affected by altered temperature profiles in the soils. After mild winters, V. longisporum caused greater yield losses in Germany. Clubroot has recently become a serious threat in Germany, but its relationship to recent climatic changes is still unknown. Research is needed to improve and combine climate and disease prediction models to provide a realistic forecast of disease risks associated with climate change. As climate models are likely to continue to lack sufficient accuracy, thorough surveillance of disease epidemics will be more crucial than ever before, in order to detect changes and adaptation in pathogen (and pest) populations and to establish the appropriate crop protection systems early enough