01 October 2026, London: Scientists at Rothamsted Research in the UK have confirmed the first case anywhere in the world of a major oilseed rape disease pathogen developing resistance to a widely used class of fungicides, complicating disease control for one of Europe’s most important oilseed crops just as growers were already managing resistance to a separate fungicide group in the same pathogen.
The research, led by plant pathologist Dr Kevin King with Professor Jon West, found that 95 percent of UK isolates of the fungus Plenodomus biglobosus showed reduced sensitivity to succinate dehydrogenase inhibitor (SDHI) fungicides, a widely used group of crop protection products that work by blocking an enzyme the fungus needs to produce energy. Some UK isolates showed up to a 500-fold drop in sensitivity to the active ingredients boscalid and fluxapyroxad under laboratory testing, compared with just 6 percent of isolates collected from Poland. The findings, published in the journal Pest Management Science and announced by Rothamsted in late September, mark what the team describes as the first confirmed report of SDHI resistance in this pathogen species anywhere in the world.
Plenodomus biglobosus is one of two closely related fungi, alongside Plenodomus lingam, that cause Phoma stem canker, also known as blackleg, in oilseed rape (canola). The disease infects plants through leaf spots in autumn and then grows slowly down the stem over winter, eventually girdling and weakening it so severely that plants can lodge or die before harvest. Left unmanaged, blackleg can cut oilseed rape yields by a third or more in bad years, which is why fungicide sprays timed around autumn leaf infection remain a core part of crop protection programmes across the UK and Europe.
Mechanism and methodology
SDHI fungicides, which include boscalid and fluxapyroxad, are grouped by the Fungicide Resistance Action Committee as Group 7 and work by blocking succinate dehydrogenase, an enzyme complex the fungus relies on in its mitochondria to generate energy. Resistance typically arises from mutations in the genes that encode this enzyme, altering its shape enough that the fungicide molecule can no longer bind effectively while the enzyme still functions for the fungus. Rothamsted’s team screened field-collected P. biglobosus isolates from the UK and Poland, exposing them to the fungicides in laboratory bioassays and linking the resistance levels they observed back to mutations in the succinate dehydrogenase genes, the same general approach used to track resistance evolution in other crop pathogens.
The discovery follows an earlier Rothamsted study, also led by King and West, which found the first case of reduced sensitivity to azole fungicides (also called DMI fungicides, for demethylation inhibitors) in the same pathogen, linked to a mutation in a gene called CYP51 that caused a roughly seven-fold drop in sensitivity. Azoles and SDHIs are the two fungicide groups UK growers lean on most heavily to manage blackleg, often alternating or mixing them specifically to delay resistance development in the pathogen population. Finding meaningful resistance to both groups in the same species within the space of a couple of growing seasons removes much of the safety margin that rotation strategies were designed to provide.
“The scale of resistance we detected in UK populations was striking,” King said. “This is the first confirmed report of SDHI resistance in P. biglobosus anywhere in the world.” West added that SDHI-resistant P. lingam, the related blackleg pathogen, has already been detected in Australia but not yet in Europe, underlining how resistance can emerge independently in related fungal populations on different continents before spreading further.
Implications for growers and industry
For now, Rothamsted’s team is calling for closer surveillance of fungicide resistance in UK and European blackleg populations rather than an immediate change in spray recommendations, since field performance does not always decline in lockstep with laboratory sensitivity results. But the practical message for agronomists and input suppliers is that the margin for relying on SDHI and azole chemistry alone to manage blackleg in oilseed rape is narrowing. That raises the priority of integrated approaches: using resistant or partially resistant varieties, rotating away from oilseed rape for longer periods to reduce the pathogen’s survival on crop debris, timing sprays more precisely around infection risk, and bringing in fungicide groups with different modes of action, including newer multisite or biological products, as part of a mixture rather than a straight rotation. The research was funded through BBSRC’s Growing Health and Resilient Farming Futures Institute Strategic Programmes at Rothamsted.
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