PR‐1 (pathogenesis‐related gene 1) expression in leaf material of Arabidopsis wild‐type and mutant lines following Australian (Col‐0, cpr1, dnd1, NahG, npr1) (A) and ‘e3’ (Col‐0, sid2) (B) Plasmodiophora brassicae infection and infection together with salicylic acid (SA) treatment. Expression levels were determined at 2 and 3 weeks post‐inoculation. Plant actin was used as reference for the Australian isolate, whereas plant YLS was used as reference for ‘e3’.