Kevin M King, Jonathan S West
This study provides the first confirmation of SDHI resistance in P. biglobosus. These findings have important implications for PSC and resistance management, highlighting the need to target both species within this pathogen complex for sustainable disease control. © 2026 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.
BACKGROUND: Phoma stem canker (PSC) of oilseed rape (Brassica napus) is a disease caused by a complex of two related fungal pathogens: Plenodomus lingam (Leptosphaeria maculans) and P. biglobosus (L. biglobosa). While P. biglobosus is increasingly recognised as an important PSC pathogen, its sensitivity to succinate dehydrogenase inhibitor (SDHI; FRAC group 7) fungicides remains poorly understood.
RESULTS: In vitro sensitivity testing of P. biglobosus isolates from the UK (2025) and Poland (2024) revealed that resistance to boscalid and fluxapyroxad was predominant in the UK, with 95% (19/20) of isolates classified as resistant, compared with 6% (1/17) in Poland. Resistance factors (RFs) were up to ~500 for boscalid and 265 for fluxapyroxad, with strong positive cross-resistance observed (R2 = 0.96). Sequencing of the SDHB, SDHC and SDHD subunits in a subset of isolates spanning the observed SDHI sensitivity range revealed SDHC and/or SDHD mutations, while SDHB remained wild type. Low-resistant and moderate-resistant isolates carried single SDH mutations, whereas high-resistant isolates carried double SDH mutations; all sequenced resistant isolates exhibited distinct SDH mutation profiles. The SDH mutations identified in P. biglobosus were all homologous to mutations associated with SDHI resistance in other fungi. In contrast, fluopyram showed partial cross-resistance, with modest sensitivity shifts (RF ≤ 4.5). In vitro fitness assays revealed no consistent effects of SDH mutations on mycelial growth or asexual sporulation.
CONCLUSION: This study provides the first confirmation of SDHI resistance in P. biglobosus. These findings have important implications for PSC and resistance management, highlighting the need to target both species within this pathogen complex for sustainable disease control. © 2026 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.