Zoospores produced on leaf discs are infectious. (A) Foliar lesions (arrows) on Brussels sprouts 4 days post inoculation (dpi) with zoospores of P. brassicae isolate CBS686.95. (B) Colonization of Brussels sprout leaf tissue. Scale bar represents 100 μm. (C) Infection on Brussels sprout stem tissue 3 dpi with zoospores of P. brassicae. Developing lesions are indicated by arrows and incidental callus formation with a yellow star. (D) An Arabidopsis Col-0 leaf 6 dpi with zoospores of P. brassicae isolate CBS686.95. (E). Arabidopsis leaf colonization by intercellularly growing hyphae. Scale bar represents 100 μm. (F). Intercellular hyphal growth in Arabidopsis petiole tissue. A haustorium is indicated by an arrow. Scale bar represents 20 μm. (B, E, F) Intercellular hyphae and haustoria were visualized by trypan blue staining.