Understanding resistance to decrease risk of severe phoma stem canker on oilseed rape

This project has identified methods to decrease the risk of severe phoma stem canker epidemics on UK winter oilseed rape crops, both in the short-term and the long-term. These methods have been communicated to the UK oilseed rape industry, including farmers, AHDB, crop advisors and breeders.

The overall aim of this project was to decrease the risk of severe phoma stem canker on oilseed rape at increased temperature by improving understanding of operation of Brassica napus resistance against Leptosphaeria maculans. There were three related objectives/tasks:

1. To identify L. maculans races in UK regions with different climates to optimise the deployment of oilseed rape resistance genes

2. To investigate phenotypes of R gene-mediated and quantitative resistance (QR) against L. maculans in leaf and stem tissues in different environments

3. To develop an experimental system to investigate mechanisms of operation of R gene-mediated and QR against L. maculans

This work has identified a number of ways to decrease the risk of severe phoma stem canker epidemics on UK winter oilseed rape crops, both in the short-term and the long-term. These are discussed in the context of a series of recommendations, which could be implemented by different sections of the UK oilseed rape industry, including farmers, the AHDB, crop advisors and the breeding industry.

Short-term benefits to farmers
To guide regional deployment of oilseed rape cultivars with different resistance genes, there is a need to monitor regional populations of L. maculans, especially in relation to virulence at the AvrLm1, AvrLm4 (populations currently differ between regions) and Avrlm7 (to decrease risk of breakdown of Rlm7 cultivar resistance) loci. It is recommended that AHDB deploys a set of spore samplers at some Recommended List (RL) winter oilseed rape trials sites to monitor races present in L. maculans populations in different regions. This should be done each growing season to detect frequencies of virulent isolates by using PCR of samples of airborne spores. Such information could then be made available to farmers to guide their choice of cultivars for the next growing season.

There is a need to develop a web-based scheme for guiding regional deployment of winter oilseed rape cultivars with different types of resistance. There is a need to continue monitoring ascospore release to provide better guidance for timing of fungicide applications. Results of this project suggest that deployment of a small number of spore samplers would be sufficient to provide such guidance. It is likely that a new generation of spore samplers will be developed that can provide ‘real time’ information about the timing of release of specific spores but, even if it is necessary to send weekly spore sampling tapes to a laboratory for quantitative PCR analysis, results could improve the accuracy of guidance for spray timing.

Long-term benefits to farmers
While some recommendations could be implemented quite quickly to decrease the risk of phoma stem canker, others are more long-term, since they need to be implemented by oilseed rape breeders.

R gene-mediated resistance should always be combined with QR to improve its effectiveness in different environments. To provide farmers with cultivars that have effective resistance against the stem canker pathogen at different locations and in different growing seasons, it is essential that R gene-mediated resistance is incorporated into backgrounds with good QR that is environmentally stable.

Relationships between temperature-resilience and effectiveness of resistance should be investigated. If factors that influence the effectiveness of resistance (differences between R genes; background QR) are associated with the temperature-resilience of that resistance, then breeders could screen potential cultivars/lines for temperature-resilient resistance as a means of selecting for effective resistance. This would save time and expense by comparison with field experiments at multiple locations over several seasons that are currently used to select for effective, environmentally stable resistance. Such screening could be used in initial selection stages, although the best material would then need to be tested in field experiments.

There is a need to understand interactions between the two pathogens (L. maculans and L. biglobosa) and cultivar R gene-mediated resistance to identify cultivars with resistance against both pathogens. Furthermore, although L. maculans is generally regarded as a more damaging pathogen than L. biglobosa, there is evidence that L. biglobosa was the dominant pathogen, responsible for severe stem base and upper stem lesions, in the 2011/2012 growing season.

There is a need to exploit new genomic information and genetic resources to improve our understanding of the operation of resistance against Leptosphaeriaspecies and other extracellular pathogens. There are now unprecedented opportunities to exploit this novel genomic information through bioinformatics and biocomputational methods, together with the new host materials that have become available to improve our understanding of resistance against the phoma stem canker pathogens.

This new understanding can be exploited by breeders to develop new cultivars with more effective, durable, temperature-resilient resistance for the benefit of farmers and the whole agricultural industry in the UK. 

 ahdb.org.uk/phoma