SDHI fungicide-resistant phoma discovered in OSR

SDHI resistance has been discovered for the first time in the world in one of the key pathogens that cause phoma in oilseed rape.

The discovery raises concerns that control of the disease, one of the most important diseases of oilseed rape, could become increasingly challenging.

Especially as it follows the discovery two years ago of phoma resistance to azoles in western Europe, another key group of fungicides used to manage the disease.

See also: Phoma resistance to azole fungicides found in western Europe

Phoma stem canker can cause substantial yield losses in oilseed rape and is caused by two related fungal pathogens, Plenodomus lingam and P. biglobosus.

While P lingam has historically received greater attention, P biglobosus has become increasingly important in UK and European crops.

In research published in Pest Management Science, scientists from Rothamsted Research screened populations of P biglobosus collected from the UK and Poland.

They found that 95% of UK isolates were resistant to the SDHI fungicides boscalid and fluxapyroxad, compared with just 6% of Polish isolates.

Some UK isolates showed up to a 500‑fold drop in how well fungicides work against them under laboratory conditions.

The researchers showed that resistance was linked to mutations in genes encoding the fungal succinate dehydrogenase (SDH) enzyme, the target of SDHI fungicides.

The study also found little evidence that these mutations reduce the pathogen’s fitness, suggesting resistant strains may persist in field populations.

“The scale of resistance we detected in UK populations was striking,” said Rothamsted’s Dr Kevin King, who led the research.

“This is the first confirmed report of SDHI resistance in P biglobosus anywhere in the world.

He said further research is now needed to determine what impact this may have on phoma disease control under field conditions.

The findings follow recent Rothamsted reports of reduced sensitivity to azole fungicides in European populations of both Phoma stem canker pathogens, highlighting the growing importance of resistance monitoring and integrated disease management strategies.