Synergistic/antagonistic interactions between Neopseudocercosporella, Alternaria, Leptosphaeria, and Hyaloperonospora determine aggregate foliar disease severity in rapeseed

White leaf spot (Neopseudocercosporella capsellae), Alternaria leaf spot (Alternaria brassicae), blackleg (Leptosphaeria maculans), and downy mildew (Hyaloperonospora brassicae) commonly co‐occur on rapeseed (Brassica napus). In controlled environment studies, the synergistic/antagonistic interactions between these four pathogens were determined using two cultivars (Surpass 400, Thunder TT) with contrasting susceptibilities/resistances towards these pathogens. A spectrum of significant (p < 0.001) contrasting synergistic/antagonistic interactions between the four pathogens was observed and specific pathogen–pathogen interaction outcomes were determined by sequence of inoculation of different pathogens on each cultivar. On Thunder TT, Alternaria leaf spot was more severe (p < 0.001) when A. brassicae was applied prior to N. capsellae, but was reduced (p < 0.001) when A. brassicae was applied following N. capsellae. On Surpass 400, Alternaria leaf spot was enhanced (p < 0.001) when A. brassicae was applied following N. capsellae. For interactions involving H. brassicae on Thunder TT, downy mildew was reduced (p < 0.001) when co‐inoculated with N. capsellae, A. brassicae, or L. maculans, irrespective of co‐inoculating pathogen application sequence. This study highlights how disease severity of one or more co‐occurring pathogens is enhanced/decreased in the presence of other co‐occurring pathogens and emphasizes the importance of both pathogen inoculation sequence and relative host plant resistances/susceptibilities to different pathogens in together determining the total disease burden from these co‐occurring rapeseed diseases. Clearly, there is a need for rapeseed germplasm selection and breeding programmes to seriously consider the implications of co‐occurring pathogen complexes rather than solely relying upon single‐pathogen screening tests.