Investigation of effects of weather conditions on timing of ascospore release for effective control of light leaf spot on winter oilseed rape in the UK

Diseases of arable crops are major threats to food production in the agricultural industry. Light leaf spot is the most economically important disease of oilseed rape (Brassica napus) in the UK, including East Anglia. This disease caused annual yield losses of winter oilseed rape that increased from <£20M in 2005 to > £100M in 2018, despite the use of fungicides costing >£20M. Light leaf spot is caused by the fungal pathogen Pyrenopeziza brassicae. With the loss of some effective fungicides and development of fungicide insensitivity in P. brassicae populations, yield losses are likely to increase. Despite investigations of different aspects of the epidemiology of light leaf spot and efforts to control the disease, results of disease surveys on winter oilseed rape (CropMonitor) showed that the incidence and severity of light leaf spot and subsequent yield loss have increased in the UK in recent years. There is an urgent need to improve control of this disease, by re-assessing the components of epidemics with respect to the production of primary inoculum – ascospores. Current control of this disease using fungicides is based on forecasting (i.e. risk of light leaf spot symptoms) or observation of symptoms. However, there is a long symptomless period after initial infection by ascospores. Fungicide applications are often ineffective because they are applied when symptoms are visible, which is too late (e.g. disease severity is too great) for effective control. Therefore, effective fungicide application for control of light leaf spot should be applied before visible symptoms appear. The timing of fungicide applications should be based on the timing of release of P. brassicae ascospores not the timing of visible symptoms. This project aims to understand effects of environmental factors on timing of P. brassicae ascospore release for effective control of light leaf spot on oilseed rape in the UK. To achieve the aim, there are three related objectives. (1) To determine the timing of P. brassicae ascospore release in different seasons. (2) To identify key factors related to P. brassicae ascospore release for building models to predict the time of P. brassicae ascospore release. (3) To investigate the relationship between the timing of P. brassicae ascospore release and severity of light leaf spot on different cultivars at different stages of crop development. (4) To transfer new knowledge into new disease control practice. New knowledge obtained from this project about effects of environmental factors on timing of P. brassicae ascospore (source of initial inoculum) release will be used to build models for predicting timing of ascospore release, which can be used to guide targeted fungicide applications to avoid unnecessary fungicide sprays. Therefore, results from this project will help to improve control of light leaf spot on oilseed rape so that yields are maintained to benefit farmers and contribute to national food security. The environment will also benefit from reduced use of fungicides.