Decreased DMI sensitivity of Plenodomus biglobosus (phoma of oilseed rape) associated with CYP51 substitution G476S

BACKGROUND
Phoma leaf spot / stem canker is an international oilseed rape (Brassica napus) disease caused by Plenodomus lingam (Pl) and P. biglobosus (Pb). Phoma management can include fungicide applications, often sterol 14α-demethylase (CYP51) inhibitors (DMIs). Pl and Pb isolates (collected throughout Poland in 2024) were screened for in vitro sensitivity to the DMI prothioconazole-desthio, and molecular mechanisms associated with altered sensitivity investigated.

RESULTS
Pl isolates (2024) were less sensitive (4-fold) than older (<2002) isolates. All Pl (2024) isolates carried CYP51 promoter inserts, likely a Sahana transposable element (TE) fragment, previously associated with CYP51 overexpression and decreased DMI sensitivity. Interestingly, Pb isolates with decreased DMI sensitivity were also identified, but without such TE inserts. CYP51 gene sequencing instead revealed substitution G476S that was absent in more sensitive G476 isolates. G476S homologues have been linked to decreased DMI sensitivity in multiple other fungi. Pb G476S isolates were less sensitive (7.3-fold) than Pb G476, widespread throughout Poland, and comprised 14/24 (58%) Pb isolates tested. Although CYP51 promoter inserts were detected in 2/24 (8%) Pb isolates, these were promoter sequence duplications (not TEs) and not obviously associated with decreased sensitivity. Pl isolates carrying promoter inserts (predominant in recently tested European populations) were more sensitive (2.6-fold) than Pb G476S, but less sensitive (2.8-fold) than Pb G476.

CONCLUSION
We provide first evidence for a CYP51 substitution associated with decreased DMI sensitivity in Pb. Result implications for phoma management, how DMI usage might influence pathogen population structure, resistance management strategies, and future research required, are discussed.