Abstract: Temperature and rainfall influence the timing of release of ascospores of Leptosphaeria spp., which are the initial source of inoculum causing the disease phoma stem canker on oilseed rape. Phoma stem canker of oilseed rape is controlled chemically by the application of fungicide seed treatments or foliar fungicide sprays. A forecast system for phoma stem canker has been developed (http://www.rothamsted.ac.uk/phoma-leaf-spot-forecast/phoma-forecast) for guiding timing of fungicide spays. This forecast provides information on the date in autumn when 10% of oilseed rape plants in a crop are expected to show symptoms of phoma leaf spot, which is generally the threshold for a fungicide application against phoma stem canker. The forecast uses temperature and rainfall data to simulate the development of the pathogen on crop debris and subsequent infection of young plants. The model described by Huang et al. (2007) was used to predict the first dates when 10% of the maximum ascospore release occurred at three sites in the UK in the 2010/11, 2011/12 and 2012/13 cropping seasons and at one site in the 2013/14 cropping season. The environmental factors at each site in each cropping season that may have influenced these differences are discussed. Moreover, this study examined the relationships between accumulated rainfall (mm) for the period from the date of 25% of maximum ascospore release until the date of maximum ascospore release and the date of maximum ascospore release. There is evidence that this parameter could be used to model the date of maximum of ascospore release to guide the timing of fungicide treatments.