Abstract: Phoma stem canker (Leptosphaeria maculans) and light leaf spot (Pyrenopeziza brassicae) are the two most serious diseases of winter oilseed rape in the UK. Despite expenditure of more than £20M on fungicides each growing season, these two major diseases account for more than £120M of losses (at a price of £225 t-1 ). The distribution of each disease is affected by climate, with Phoma stem canker most severe in the warmer, drier south and east of the UK and light leaf spot most severe in the wetter, cooler west and north with epidemics being particularly severe in Scotland. Little work has been done to predict the impacts of climate change on plant disease epidemics. To investigate possible impacts, a weather-based disease forecasting model for Phoma stem canker was combined with a climate change model predicting UK temperature and rainfall under high and low CO2 emissions for the 2020s and 2050s. Multi-site data collected over a 15-year period from across the UK were used to develop and validate the model to forecast the severity of epidemics on oilseed rape. The model predicted that Phoma stem canker epidemics will increase in severity and the range of the disease will spread northwards into Scotland by the 2020s. However, using the same climate change scenarios, a weather-based light leaf spot forecast model predicted that light leaf spot will become less serious throughout the UK, especially in southern England.Crop protection and resistance to these two major UK pathogens make important contributions to climate change mitigation, since low-yielding diseased crops use more nitrogen fertilizer per ton of grain and require more crop-area to achieve the same national yield of oilseed rape. This work suggests that predictions of impacts of climate change on other plant diseases are needed to guide policy and practice in adapting to impacts of climate change on food security, environment and wildlife.