Overview of the P. brassicae zoospore production procedure. From a P. brassicae culture grown on V8 agar (A) mycelial plugs (Ø 10 mm) were cut from the actively growing margin and gently pressed on the abaxial side of Brussels sprout leaves (B). From the foliar lesions (C) leaf discs were cut with a cork borer (Ø 25 mm) (D) and transferred to 6-wells plate (E). The infected leaf discs were submerged in water resulting in the formation of sporangia (F) that developed into zoosporangia (G) from which zoospores are released (H) after being exposed to the cold for several hours. Scale bar in (F) and (H) represents 40 μm and in (G) 100 μm. The white arrow in (G) points to a zoosporangium and the black arrow to a hyphal swelling.