These are pictures of Peronospora parasitica infected Arabidopsis cotyledons. Peronospora structures are visualized by trypan staining which also stains plant cell walls and veins. The cotyledons are shown in decreasing order of resistance from no symptoms, where only a vein (V) can be seen within the tissue, to plants supporting both asexual (sporangiophores) and sexual (oospores) reproduction of the pathogen. The so-called “hypersensitive response” is the typical resistance response and consists of a rapid cell death at the site of pathogen ingress. The HR will typically prevent the pathogen from colonizing surrounding tissue. Trailing necrosis is an intermediate phenotype where the timing of plant cell death in response to the pathogen is not rapid enough to halt all hyphal growth.