Understanding changes in pathogen populations to improve control of phoma stem canker on oilseed rape in the UK (The Perry Foundation)

Phoma stem canker is a damaging disease of oilseed rape (OSR) in the UK, causing annual yield losses of winter oilseed rape worth £60-80M. Improving disease control to reduce yield losses is crucial for making OSR a more profitable crop for UK farmers.

The disease is caused by two fungal pathogens, Leptosphaeria maculans (Lm) and L. biglobosa (Lb). However, previous research mainly focused on Lm with little work on Lb. Recent research has showed that management practices targeted only towards Lm have led towards selection for Lb, which increased the risk of phoma stem canker caused by Lb. Furthermore, a new Lb subclade was recently detected in the UK. Recently, shifts in Lm populations from avirulence to virulence were also observed. There is a need to investigate the changes in Lm and Lb populations to develop improved disease management strategies.

This PhD project aims to understand changes in Lm races and Lb subclades in the pathogen populations and investigate the mechanisms that underpin these changes for improving control of phoma stem canker. To achieve the aim, there are three related objectives.

Objective 1. To understand changes in virulent races of Lm for effective use of cultivar resistance. Major gene mediated resistance against Lm is effective only when the corresponding avirulence gene in the pathogen population is predominant. To detect virulence races of Lm in current pathogen populations, Lm isolates will be collected from field experiments. To understand mechanisms of evolution of Lm to overcome host resistance, newly identified virulent isolates will be sequenced and compared with historic isolates to identify molecular changes leading to increased virulence.

Objective 2. To investigate the emerging threats posed by the newly identified Lb subclade.
Previously, only the subclade Lbb (Lb ‘brassicae’) was present in the UK. However, the subclade Lbc (Lb ‘canadensis’), which mainly occurred in Canada and Australia, was recently detected for the first time in the UK. Studies suggest that Lbc is more damaging than Lbb. New diagnostic methods will be developed to rapidly detect Lbc and explore the distribution of Lbc and Lbb in the UK. The pathogenicity of different Lb subclades will be tested to assess the threat of the newly identified Lbc to OSR cultivars. The sexual compatibility between Lbc and Lbb will be investigated.

Objective 3. To investigate novel strategies for effective control of Lm and Lb subclades.
Considering climate change, effects of temperature on resistance against Lb, especially the new Lbc will be investigated. With the emerging of virulent Lm races and the new Lbc subclade, interactions between Lbc and Lm on disease severity will be investigated. Recently, emergence of azole resistance only in Lm was detected in western Europe. Work is now required to explore changing sensitivity to azole fungicides in UK Lm populations and in Lbb or Lbc subclades.

New knowledge obtained about Lm and Lb subclades (Lbb, Lbc) will be used to improve control strategies by targeting both causal pathogens (Lm and Lb subclades) delivered to growers for improving the profit margin.