Understanding host resistance to improve control of light leaf spot on winter oilseed rape in the UK

Light leaf spot is the most economically important disease of oilseed rape (Brassica napus) in the UK. This disease can cause annual yield losses of winter oilseed rape worth > £100M, despite the use of fungicides. Light leaf spot is caused by the fungal pathogen Pyrenopeziza brassicae. With the loss of some effective fungicides, development of fungicide insensitivity in P. brassicae and predicted global warming, yield losses are likely to increase. Use of host resistance to control the disease is becoming ever more important. Previously, in England oilseed rape disease control focused on phoma stem canker, with little research on control of light leaf spot because light leaf spot was considered a disease problem only in Scotland. However, recently light leaf spot has become the major disease problem in England as well. Results of recent disease surveys on winter oilseed rape (CropMonitor) showed that the severity of light leaf spot epidemics has increased progressively across the UK. This recent widespread occurrence of light leaf spot on oilseed rape has made it a high priority disease on oilseed rape in the UK. This application is for a PhD student linked to the HKEP (Hertfordshire Knowledge Exchange Partnership) scheme with an industry partner making in-kind contributions. This project aims to improve understanding of host resistance for better control of light leaf spot on winter oilseed rape. To achieve the aim, there are three related objectives. (1) To investigate virulent races in pathogen (P. brassicae) populations for effective control of light leaf spot. Populations of P. brassicae will be sampled by taking air samples and crop samples. Timing & abundance of P. brassicae ascospore release will be assessed by species-specific quantitative PCR, and the frequencies of virulent/avirulent isolates of P. brassicae will be determined by inoculation of a differential set of cultivars. (2) To identify host (B. napus) resistance genes against P. brassicae. Based on previous work, the identification of major resistance (R) candidate genes for resistance against P. brassicae will be focused on the resistance derived from oilseed rape cv. Imola. Our recent work has mapped an R gene to a c. 42kbp region on the bottom of B. napus chromosome A1. Gene contents of this region in the Imola genome sequence, the published B. napus genome sequence and Brassica pan transcriptome will be analysed to identify the candidate genes. (3) To investigate mechanisms of host resistance against P. brassicae. Based on results from our preliminary experiments, four DH (doubled haploid) lines with different levels of quantitative resistance, cv. Imola with R gene mediated-resistance and one susceptible cv. Bristol will be used to understand the specific host-pathogen interactions at different stages of disease development by microscopic observation and quantification of defence-related gene expression. This work will provide important insights into molecular events in host-pathogen interactions and help understand the importance of different resistance phenotypes in-relation to management of light leaf spot.