Effects of rising air and soil temperatures on the life cycle of important pathogens in oilseed rape (Brassica napus L.) in Lower Saxony

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Climate change is expected to cause a mean annual temperature increase in Germany of 2°C by 2050 and up to 4°C by 2100, which will have effects on both crop and fungal pathogen development. Within the research framework “KLIFF - Climate Impact and Adaptation Research in Lower Saxony”, potential effects of higher air and soil temperatures on the life cycle of economically important fungal pathogens of oilseed rape (Brassica napus L.) were investigated both theoretically and experimentally. Within the theoretical approach, published knowledge about temperature effects on certain life cycle stages of Leptosphaeria maculans (Phoma lingam), Sclerotinia sclerotiorum and Verticillium longisporum, such as survival, sporulation, infection and further disease development, was collected. Cardinal temperatures of each life cycle stage were compared with current regional climate projections (REMO model) based on emission scenario A1B for the periods 2001 to 2030 and 2071 to 2100 in three different oilseed rape growing regions in Lower Saxony, using a baseline historical series of meteorological data collected from 1971 to 2000. This analysis suggests that higher temperatures may lead to shifts in future prevalence of these pathogens. While Verticillium longisporum and Sclerotinia sclerotiorum could be particularly favored during their early development stages, potentially leading to higher disease severity, Phoma lingam may only benefit in currently colder oilseed rape growing regions. In order to study effects of rising air and soil temperatures on the soil- and debris-borne life cycle stages of the three pathogens, experiments utilizing climate chambers and a newly constructed soil warming facility were carried out. Warming treatments reflected both regional temperature differences as well as mid- and long-term warming scenarios for Lower Saxony. Investigations included (1) infection of oilseed rape by Verticillium longisporum and further disease progress, (2) carpogenic germination of Sclerotinia sclerotiorum in spring and (3) phoma leaf spot and crown canker development in autumn and spring (field only). Results of climate chamber and field experiments were compared on a thermal time scale based on degree-days in spring. A mean temperature increase of 2°C may lead to advancement of oilseed rape flowering, but also apothecia production of S. sclerotiorum by 5 to 7 days. Hence, the coincidence between ascospore release from apothecia and end of oilseed rape flowering, leading to optimal infection conditions, may still occur under future warming, just earlier in the year. Warming may also advance the beginning of exponential growth of V. longisporum within the plant by 7 to 9 days, potentially leading to stronger colonization of susceptible cultivars and inoculum accumulation in the soil, which may result in higher yield losses in the future. Development of phoma symptoms may not necessarily be favored by higher temperatures. These assumptions, however, are only based on projected temperature increases and restricted by a lack of reliable, high-resolution future climate scenarios, also for other important weather parameters influencing pathogen development, such as soil moisture, relative humidity and precipitation. Assumptions are furthermore complicated by the prospective adaptations of farming techniques and crop genotypes to climate change.