BACKGROUNDPhoma stem canker is caused by two coexisting pathogens, Leptosphaeria maculans and L. biglobosa. They coexist because of their temporal and spatial separations, which are associated with the differences in timing of their ascospore release. L. maculans produces sirodesmin PL, while L. biglobosa does not. However, their interaction/coexistence in terms of secondary metabolite production is not understood.RESULTSSecondary metabolites were extracted from liquid cultures, L. maculans only (Lm only), L. biglobosa only (Lb only), L. maculans and L. biglobosa simultaneously (Lm&Lb) or sequentially 7 days later (Lm+Lb). Sirodesmin PL or its precursors were identified in extracts from ‘Lm only’ and ‘Lm+Lb’, but not from ‘Lm&Lb’. Metabolites from ‘Lb only’, ‘Lm&Lb’ or ‘Lm+Lb’ caused significant reductions in L. maculans colony area. However, only the metabolites containing sirodesmin PL caused a significant reduction to L. biglobosa colony area. When oilseed rape cotyledons were inoculated with conidia of ‘Lm only’, ‘Lb only’ or ‘Lm&Lb’, ‘Lm only’ produced large gray lesions, while ‘Lm&Lb’ produced small dark lesions similar to lesions caused by ‘Lb only’. Sirodesmin PL was found only in the plant extracts from ‘Lm only’. These results suggest that L. biglobosa prevents the production of sirodesmin PL and its precursors by L. maculans when they grow simultaneously in vitro or in planta.CONCLUSIONFor the first time, L. biglobosa has been shown to inhibit the production of sirodesmin PL by L. maculans when interacting simultaneously with L. maculans either in vitro or in planta. This antagonistic effect of interspecific interaction may affect their coexistence and subsequent disease progression and management. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Phoma stem canker (also known as blackleg) is caused by two closely related coexisting ascomycete fungal pathogens, Leptosphaeria maculans and L. biglobosa (Fitt et al., 2006a;Shoemaker & Brun, 2001). This disease causes damage to oilseed rape worldwide (Fitt et al., 2006b) and accounts for annual yield losses to UK oilseed rape growers worth approximately £100 million, despite the use of fungicides and resistant cultivars (www.cropm onitor.co.uk) (Zhang et al., 2014). In the UK, phoma stem canker is a monocyclic disease that is initiated by ascospores as primary inoculum in autumn or winter. These ascospores are released from pseudothecia (sexual fruiting bodies) that mature after harvest on infected stem debris, such as stubble left in fields from the previous cropping season. Once mature, ascospores