New developments in phoma stem canker resistance could add to the growing confidence in UK oilseed rape and significantly increase the resilience of the crop, believes Sarah Hawthorne of DSV. Combining a range of resistance mechanisms has been shown to be far more effective than relying on single defences and a new generation of oilseed rape varieties with multi-layered protection could significantly reduce the impact of this yield sapping disease, she says.
"The AHDB has estimated phoma stem canker costs UK producers around £100 million annually through lost yields and control costs which highlights the urgency for more robust control of the condition. Phoma is caused by two fungal pathogens – Leptosphaeria maculans and Leptosphaeria biglobosa – with Leptosphaeria maculans now responsible for the majority of damage to OSR crops in Europe. The disease spreads from airborne spores, which infect the leaves of young plants and progress into the stems and we are already seeing evidence of the disease in this season's oilseed rape plants as a result of the wet weather early in the autumn. Growers are advised to apply plant growth regulators (PGRs) to very forward crops to manage excessive growth and reduce the risk of stem cankers caused by phoma developing further but clearly a longer term solution is required."
While RLM7 has been the go-to genetic resistance mechanism for many years, its effectiveness is now waning, with increasing evidence of resistance breakdown in popular varieties, she points out.
"Trials conducted by DSV in Dyngby, Denmark, have shown that OSR varieties with RLM7 are increasingly susceptible to phoma, with average susceptibility scores reaching 7.5 on a scale from 0 to 10, the same as varieties with no resistance at all.This is particularly concerning as many current RL varieties rely solely on RLM7 for their protection."
In response to this growing challenge, DSV has developed Phoma Blocker, a disease protection approach incorporating the RLM7 gene alongside RLMS or the LepR1 resistance mechanism, she explains.
“The LepR1 gene is a particularly strong defensive barrier offering promising resistance on its own but its true potential is realised when combined with other resistance traits. Combining RLMS with RLM7 also provides significantly enhanced protection. Both approaches provide a robust multi-layered defence, delivering enhanced protection and longevity compared to other resistance mechanisms. In the Dyngby trials Phoma Blocker has been shown to produce a susceptibility score of just 2.3 compared to the 7.5 for RLM7 gene by itself. The multi-gene approach not only strengthens protection against phoma but also extends the life of existing genetic traits, ensuring better resilience in the face of changing disease pressures."
As phoma pressure continues to rise, selecting the right seed variety will become increasingly critical in maintaining healthy crops and securing high yields, Sarah Hawthorne believes.
"By integrating Phoma Blocker into their crop rotation, growers can protect their investment, reduce the need for chemical treatments and ensure a more sustainable path forward for OSR production. We currently have two very promising Phoma Blocker varieties DSV Cognac and DSV Dompteur on the AHDB candidate list, so it is highly likely the first of these new generation varieties to be on the recommended list will be announced at the end of 2025. This is a significant step forward in safeguarding OSR crops and we therefore urge all growers to consider the long-term benefits of incorporating these breakthrough varieties into their future rotations."