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

White leaf spot (Neopseudocercosporella capsellae), Alternaria (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 pathogens. A spectrum of significant (p < 0.001) was observed specific pathogen–pathogen interaction outcomes by sequence inoculation different each cultivar. On TT, more severe when A. brassicae applied prior to N. capsellae, but reduced following capsellae. Surpass enhanced For involving H. co-inoculated brassicae, or L. maculans, irrespective co-inoculating pathogen application sequence. This study highlights how disease severity one co-occurring is enhanced/decreased in presence other emphasizes importance both relative host plant resistances/susceptibilities together determining total burden from diseases. Clearly, there a need for germplasm selection breeding programmes seriously consider implications complexes rather than solely relying upon single-pathogen screening tests.