Olpidium brassicae is a plant pathogen, it is a fungal obligate parasite. In 1983, the Alsike, Alberta area's clover (which is a major part of horses' diet) was struck by a fungus epidemic of Olpidium brassicae, previously not seen in Canada.
O. brassicae is the fungal vector for most, if not all, necroviruses.