Ninety-five percent of UK isolates of the oilseed rape pathogen Plenodomus biglobosus show resistance to the SDHI fungicides boscalid and fluxapyroxad, in what researchers at Rothamsted Research describe as the first confirmed report of SDHI resistance in this pathogen anywhere in the world.

Some UK isolates showed up to a 500-fold reduction in fungicide efficacy under laboratory conditions. Just 6% of Polish isolates screened in the same study carried resistance.

The discovery has direct consequences for phoma control in winter oilseed rape, one of the crop's most damaging diseases. Phoma is caused by two related fungal pathogens: Plenodomus lingam and P. biglobosus.

What did the researchers find?

Rothamsted scientists screened populations of P. biglobosus collected from the UK and Poland. While P. lingam has historically received greater attention, P. biglobosus has become increasingly important in UK and European crops in recent years.

The headline results:

• 95% of UK isolates were resistant to the SDHI (succinate dehydrogenase inhibitor) fungicides boscalid and fluxapyroxad
• Only 6% of Polish isolates showed resistance
• Some UK isolates displayed up to a 500-fold drop in fungicide performance in laboratory tests

The researchers also confirmed the genetic mechanism behind the resistance. Mutations in genes encoding the fungal succinate dehydrogenase (SDH) enzyme — the exact target of SDHI chemistry — are responsible. Crucially, the study found little evidence that these mutations reduce the pathogen's fitness, which means resistant strains can persist and multiply in field populations rather than dying out.

Rothamsted's Dr Kevin King, who led the research, said: "The scale of resistance we detected in UK populations was striking."

He added: "This is the first confirmed report of SDHI resistance in P. biglobosus anywhere in the world."

What does this mean for phoma control?

Phoma can cause substantial yield losses in oilseed rape, and the new findings raise concerns that control of the disease could become increasingly challenging for UK growers.

The picture is not limited to one chemistry. The findings follow recent Rothamsted reports of reduced sensitivity to azole fungicides in European populations of both phoma-causing pathogens. With two major fungicide groups now showing eroded performance against the disease, growers have fewer reliable actives in the tank.

Further research is now needed to determine what impact SDHI resistance has on phoma disease control under actual field conditions, where laboratory results do not always translate directly.

The study highlights the growing importance of resistance monitoring and integrated disease management strategies, rather than reliance on any single mode of action.

What should growers do now?

For most winter oilseed rape crops sown during August, emergence has been vigorous, according to Teagasc tillage specialist Shay Phelan. He identified grass weeds and volunteer cereals as the key management priority for growers at this stage of the season.

Phoma prevention is also on the immediate to-do list. Phelan said: "Spraying crops with a fungicide to prevent the onset of phoma is another important requirement at this stage."

But product choice matters more than spray timing alone, he warned. "However, choice of product is important in this regard. Some chemistries will also have a growth regulation impact," he explained.

"Growers may not want this to be the case as they will want individual plants to be large enough to fend off the impact of pigeon grazing later in the season."

What comes next?

Growers should watch for follow-up field trials that test whether the laboratory resistance translates into phoma control failures in commercial crops. Given the persistence of resistant strains, rotating modes of action and monitoring Rothamsted's resistance surveillance outputs will be essential when planning autumn fungicide programmes this season and beyond.