Peronospora species are plant pathogens, one of the genera causing downey mildews. Peronospora is very common and is an obligate parasite on leaves, stems, flowers, and fruits of living higher plants. No information is available regarding health effects or toxicity. Allergenicity has not been studied. Peronospora may be identified in air on spore trap samples (spores have distinctive morphology), although because obligate parasites cannot grow on non living environmental surfaces, our laboratory does not include Peronospora on our spore trap report form. The spores may also be seen in dust as part of the normal influx of outdoor microbial particles.