The downy mildews are a large group of plant diseases caused by microscopic fungus-like (Oomycete) organisms related to the pathogen that causes tomato and potato blight. Despite a similar name and certain similarities in symptoms, they are unrelated to the powdery mildews. Pansies can also occasionally suffer from powdery mildew, although it is not usually as damaging to them as downy mildew.
Like other downy mildews, Peronospora megasperma is described as a biotroph; a pathogen that penetrates into host plant tissues over an extended period without killing them, while it extracts nutrients from the living host cells. During this period it releases airborne spores from the fuzzy fungus-like growth on the underside of leaves, which disperse to initiate new infections.
When the infected leaf tissues eventually die, it forms resting spores in the dead material which will then contaminate the soil. The pathogen can not grow in the absence of living plant cells.
There is little specific information on Peronospora megasperma, but it is likely that infections from resting spores are responsible for damage if pansies are replanted on sites previously affected. Infections are also caused by spores blowing in from other plants, possibly 1-2km (½-1 mile) away.
There is also a risk that commercially raised plants may have been treated with fungicides, which suppress the symptoms but do not kill the pathogen, resulting in the disease breaking out after purchase when the effect of these fungicides eventually wears off.