Evolution of decreased sensitivity to azole fungicides in western European populations of Plenodomus lingam (cause of Phoma leaf spot / stem canker on oilseed rape)

Plenodomus lingam (Leptosphaeria maculans) and P. biglobosus (L. biglobosa) are related fungal pathogens causing Phoma leaf spot and stem canker, an internationally damaging disease of oilseed rape (Brassica napus) and other brassicas. In Europe, fungicides used for disease management are mainly sterol 14α-demethylase (CYP51) inhibitors (DMIs/azoles); quinone outside inhibitors (QoIs), and succinate dehydrogenase inhibitors (SDHIs) are also used. Decreased DMI sensitivity has emerged in Australian and eastern European P. lingam populations. Decreased sensitivity is mediated by promoter inserts in CYP51 resulting in target site overexpression. In the present study, based on in vitro sensitivity testing, we report decreased DMI (prothioconazole-desthio and mefentrifluconazole) sensitivity in modern western European isolates of P. lingam (collected 2022-23) compared to baseline historical (1992-2005) isolates. Around 85% of the modern western European P. lingam isolates collected, for which the CYP51 promoter region was sequenced, carried a promoter insert but target site alterations were not detected. Six different CYP51 promoter inserts were identified, with a 237 bp fragment of the Sahana transposable element most frequently detected. Inserts were typically associated with a 3 to 10 fold decrease in sensitivity to the DMIs tested. In contrast to P. lingam, PCR screening revealed that CYP51 promoter inserts were absent in modern western European P. biglobosus isolates (collected 2021-23). The combined data indicate P. lingam isolates lacking an insert were similarly (or slightly more) sensitive to the DMIs tested for P. biglobosus, whereas those carrying an insert were slightly less sensitive than P. biglobosus. No clear evidence for substantive sensitivity shifts to the QoI (pyraclostrobin) or SDHI (boscalid) fungicides tested was obtained for either Plenodomus species.