A greater understanding of the mechanism behind phoma in oilseed rape plus developments in plant breeding are helping to pave a new way in dealing with this financially damaging disease. CPM finds out more.
“Understanding the different disease dynamics of Lm and Lb, and the different resistance mechanisms, are key to controlling phoma.” DR ALEX DOERING
By Rob Jones
While RLM7 resistance has worked well for UK growers in protecting crops against phoma in recent years, indications of its breakdown across Europe and the increasing grip of a new strain of the disease are causing concern.
While too early in the 2026 season to fully understand the extent of the problem, growers are urged to remain alert to the threat of phoma, particularly as the UK’s oilseed rape area looks set to increase again after a recent dip in output.
Professor Yongju Huang, professor of plant pathology at the University of Hertfordshire, believes that as OSR’s popularity rises again, so too does the impact of production loss caused by phoma.
“Phoma stem canker is a very damaging disease and when 100% of UK crops were affected with phoma leaf spots in the autumn of 2019, it was estimated this cost growers in the region of £60-90M,” she points out.
Although the disease is caused by closely related pathogens Leptosphaeri maculans (Lm) and Leptosphaeri. biglobosa (Lb) that co-exist on OSR, understanding the interactions between them in terms of disease development is important, she says.
“We know Lm and Lb attack OSR in different ways. Lm is generally considered more serious because it’s often associated with damaging stem base cankers, whereas Lb is generally linked to superficial upper stem lesions.
“In autumn and winter, Lm causes more damaging large phoma leaf spots with pycnidia (asexual fruiting bodies), while Lb only causes small dark phoma leaf spots with no or few pycnidia.”
Previous research showed that Lm can produce a phytotoxin sirodesmin PL to enhance its infection, whereas Lb doesn’t produce this phytotoxin, she adds. “Therefore, reducing the infection by Lm should be a priority to reduce the severity of phoma stem canker.”
In recent MAGIC (Multi-parent Advanced Generation Inter-Cross) field trials run by the John Innes Centre and Rothamsted Research, early indications were seen of the reduced effectiveness of Rlm7, she says.
“Incidence of phoma in one cultivar with Rlm7 increased from 20% to 50% during the three years of the trials,” highlights Yongju.
Relying less on Rlm7 and combining it with other resistance genes such as RlmS and LepR gene offers considerable potential for better phoma resistance in the future, she believes. “A combination of Rlm7 with quantitative resistance and RlmS or LepR genes will improve control of phoma in the UK, but there’s a need to monitor the change of pathogen populations for effective deployment of cultivar resistance.”
According to Dr Jon West, principal research scientist at Rothamsted Research, the phoma leaf spot phase is seen in the autumn and continues over winter, but very few new leaf infections occur after early spring.
Meanwhile, earlier leaf infections will have spread to the stem and remain invisible initially, he points out. “It’s too early right now (at the time of writing – April) to see the stem canker phase of the disease, which normally starts to appear on susceptible varieties in warmer locations in late May. So currently, it’s too early to draw conclusions for the current season.
“The Rlm7 resistance has worked well for a number of years in the UK and seems to have reduced the population of phoma, so it’s become less of an issue.
“In many parts of the country there was definitely less phoma leaf spot around in the past few years compared with say, 20 years ago. Some areas have escaped infection even when varieties being grown don’t have Rlm7 resistance,” says Jon.
This has happened, however, at the same time as greater incidence of L. biglobosa, he points out. “I’ve yet to see evidence of new variants of L. maculans that can overcome RLm7, although some have been found in other countries and infections were always possible on Rlm7 varieties when the leaves are a little old and about to senesce.
“Adding new resistance genes will certainly help, particularly because a new concern is that there’s been an erosion of the efficacy of azole (DMI) fungicides against both L maculans and L. biglobosa.
“It’s a subject we’re closely monitoring at Rothamsted, particularly as any further erosion of azole efficacy will put more pressure on the remaining modes of action.”
DSV’s Dr Alex Doering says combining a range of resistance mechanisms has been shown to be far more effective than relying on a single line of defence. “Phoma is a serious disease and there’s a genuine urgency for more robust control, not just in the UK but across Europe and worldwide.
“Understanding the different disease dynamics of Lm and Lb, and the different resistance mechanisms, are key to this. “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.”
Alex highlights that trials conducted by DSV in Dyngby, Denmark, have shown OSR varieties with Rlm7 are increasingly susceptible to phoma, with average susceptibility scores reaching 7.5 on a scale from 0-10.
“This is pretty much the same as varieties with no resistance at all, so it’s a real concern for the future as many of the most popular OSR varieties still rely solely on Rlm7 for their protection,” he says.
In response to this growing challenge, DSV has been working on ‘phoma blocker’ technology to assess how different genes can be combined to increase phoma resistance, he 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, with both approaches providing a robust multi-layered defence, delivering enhanced protection and longevity compared with other resistance mechanisms.”
In the Danish trials, such thinking has been shown to produce a susceptibility score of just 2.3 compared with the 7.5 for Rlm7 gene by itself, he points out.
“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.”
This article was taken from the latest issue of CPM. Read the article in full here.
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