In 2018 a study by the University of Hertfordshire and Innovate UK found that around 25% of phoma isolates were virulent on the Rlm7 phoma gene, which once had 100% efficacy against the yield-robbing leptosphaeria maculans strain of the disease.
There is heavy reliance on the Rlm7 gene in the UK and with some 70% of varieties on the AHDB Recommended List (RL) list containing it, pressure is mounting on the genetics, with some farmers reporting breakdowns in their crops.
In France, breakdowns of inbuilt phoma resistance are ongoing and widespread, with both Rlm6 and Rlm1 rapidly losing efficacy against the disease in just three to four years.
For Rlm7, the breakdown has been much more gradual, but it is estimated around 90% of the pathogen population in France is now virulent on Rlm7, says Chris Guest, managing director of LS Plant Breeding (LSPB).
This decline in resistance is also evident in UK stem canker resistance ratings in varieties with Rlm7, slipping from mean scores of 8.4 to 7.6 in the past decade.
Now, in a bid to bring more genetic diversity to fighting the disease, the company that brought the Mendel resistance gene for clubroot to market is using this new source of phoma resistance in its varieties.
Resistance gene The RlmS gene, which was first discovered by LSPB’s breeders and used in its European germplasm, is a major resistance gene in oilseed rape which confers adult plant resistance to phoma.
Mr Guest says: “We don’t think Rlm7 is a problem and that it will break down tomorrow, but the introduction of RlmS is trying to give longevity to that resistance gene through diversity.
“OSR has faced multiple challenges over the last few years, which is perhaps why phoma has been pushed slightly under the radar.
There is good chemistry to control the disease and a suite of good genetics that have good resistance.
However, there are signs of change and Rlm7 is in danger of being overcome.”
The resistance gene works by preventing colonisation of the leaf by the phoma pathogen, which then stops it moving into the stem of the plant.
Canker “Everyone knows the telltale sign is on the leaf but very few have actually seen true levels of canker in the field in the stem,” adds Mr Guest.
LSPB has two varieties on the RL with the RlmS resistance – Respect, the first RL hybrid to feature the new gene, and Flemming, which was added to the 2022/23 RL and combines RlmS with resistance to turnip yellows virus (see table).
And two new varieties are currently on the Candidate List and are said to combine impressive agronomic merit scores with gross output.
Murray and Vegas both have high gross output for the East/West region, good oil content and high phoma resistance conferred by the RlmS gene.
Mr Guest adds: “These are also both vigorous varieties, particularly in spring.”
Agronomic merit Vegas has the highest agronomic merit score on the Candidate List, with Murray not far behind.
There will be some availability of both varieties this autumn.
Mr Guest adds: “One of the exciting things for us is the future potential to get a variety on the list with both Rlm7 and RlmS stacked together.
RlmS is very much an alternative and it’s not about using one or the other.
We’re still breeding with Rlm7 because it is still effective against a percentage of the phoma pathogen.”