Predicting timing of release of ascospores of Leptosphaeria spp. to improve control of phoma stem canker on oilseed rape in the UK

Phoma stem canker is an economically damaging disease of oilseed rape crops, causing an annual loss estimated at more than £87M in the UK at a price of £390/t. It is caused by two related pathogens: Leptosphaeria maculans and Leptosphaeria biglobosa, with L. maculans being more damaging to the stem bases of infected crops. The rate of maturation of pseudothecia on previous infected crop residues is dependent on wetness-adjusted daily temperature and therefore dates of release of significant number of ascospores from pseudothecia are affected by weather conditions each year. Accurate estimates of timing of ascospore release are essential for guiding timing of sprays on autumn-sown crops to manage severe phoma stem canker epidemics the next spring. Effective control of this disease is critical to protect the potential yield and therefore to achieve high, stable oilseed rape yields. Daily observations of numbers of ascospores of Leptosphaeria species, and records of daily mean temperature and rainfall at four separate sites from 2010/2011, 2011/2012 and 2012/2013 cropping seasons were used to validate existing weather-based models developed to predict the date of the first major release of ascospores after harvest of the previous crops. The usefulness and merits of these weather-based models were then evaluated for control of phoma stem canker epidemics in relation to annual uncertainties in the controlling weather variables both in the past and under future climate change. These assessments were considered in relation to methods that were proposed and are currently used to guide fungicide control of phoma stem canker disease in the UK.