|Erysiphe species are plant pathogens, one of the genera causing powdery mildews. Erysiphe 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. The asexual phase Oidium 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 Erysiphe on our spore trap report form. The asexual spores are also seen in dust as part of the normal influx of outdoor microbial particles.