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

Plenodomus lingam (Leptosphaeria maculans) and P. biglobosus (L. biglobosa) are fungi causing Phoma leaf spot/stem canker, an international damaging disease of oilseed rape (Brassica napus) and other brassicas. In Europe, fungicides used for disease management are mainly sterol 14 alpha-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 and is mediated by CYP51 promoter inserts resulting in target site overexpression. In this study using in vitro sensitivity testing, we report decreased DMI (prothioconazole-desthio, mefentrifluconazole) sensitivity in modern western European P. lingam isolates (collected 2022-2023) compared to older baseline (1992-2005) isolates. Around 85% of 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, most commonly a 237 bp fragment of the Sahana transposable element. Inserts were associated with an approximately 3- to 10-fold decrease in sensitivity to the DMIs tested. In contrast to P. lingam, PCR screening revealed CYP51 promoter inserts were absent in modern western European P. biglobosus isolates (2021-2023). 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 evidence for substantive sensitivity shifts to the QoI (pyraclostrobin) or SDHI (boscalid) fungicides tested was obtained for either Plenodomus species.