Understanding interactions between Leptosphaeria maculans and L. biglobosa for improving control of phoma stem canker on oilseed rape in the UK

Phoma stem canker is an economically important disease of oilseed rape in the UK, including East Anglia. This disease can cause annual yield losses of winter oilseed rape worth > £80M, despite the use of fungicides. It is caused by two fungal pathogens Leptosphaeria maculans (Lm) and L. biglobosa (Lb). Despite the efforts to control diseases using cultivar resistance and fungicides, the average UK farm yield has not increased over the last 10 years. There is a need to improve disease control to increase yield, which will make oilseed rape a more profitable crop for the UK’s farmers. Although phoma stem canker is caused by Lm and Lb, Lm is more damaging than Lb since Lm is often associated with damaging stem base cankers, whereas Lb is generally associated with superficial upper stem lesions. It is known that Lm can produce a secondary metabolite Sirodesmin PL which plays a role to enhance its infection, whereas Lb does not produce Sirodesmin PL. Results of our recent experiments showed that Lb can inhibit production of Sirodesmin by Lm, resulting in reduced infection by Lm. This opens a new opportunity to investigate new methods for managing phoma stem canker This PhD project aims to understand mechanisms of interactions between Lm and Lb for improving control of phoma stem canker on winter oilseed rape. To achieve the aim, there are five related objectives. (1). To investigate interactions between Lm and Lb during phoma leaf spot and phoma stem canker development in controlled conditions. Plants will be inoculated with Lm or Lb or both Lm & Lb. The effects of Lb on severity of phoma leaf spot and phoma stem canker caused by Lm will be assessed. The amounts of Lm and Lb DNA in each of the inoculation treatments will also be assessed by species-specific qPCR. (2). To investigate effects of Lb on control of Lm during phoma leaf spot and phoma stem canker development in natural conditions. Plants will be grown in outdoor pots and field experiments, inoculated with Lb at different time points depending on the timing of airborne ascospore release. The severity of phoma leaf spot and phoma stem canker, and yield of each treatment will be assessed. (3). To investigate effects of Lb on the production of the phytotoxin Sirodesmin PL by Lm. Plants will be inoculated with Lm or Lb or both; inoculated leaves will be sampled at different time points for extraction of secondary metabolites and composition analysis. Leaves will also be sampled for RNA extraction and analysis of gene expression related to Sirodesmin production. (4). To investigate effects of Lb on function of Lm effectors. Successful infection of oilseed rape by Lm relies on its effectors (proteins that are involved in virulence) being able to avoid recognition by host resistance receptors. Effects of Lb on the function of Lm effectors will be investigated using the Rlm7 (host resistance gene) and AvrLm7 (pathogen effector gene) interactions. (5). To transfer new knowledge into new disease control strategies. New knowledge obtained from this work about interactions between the two phoma stem canker causal pathogens in controlled and natural conditions will be used to improve control strategies and delivered to growers through ADAS and Hutchinsons’ agronomists. With limited available effective fungicides and observed breakdown of host resistance against Lm, investigation of the possibility of using Lb for biocontrol of Lm will not only reduce the use fungicides to protect the environment but also improve the profit margin to benefit farmers.