Downy mildew fungi are microscopic members of the class Oomycetes, oospore fungi.
They produce thread-like tubular mycelia that form branching sporangia: small sacs containing asexual or sexual spores, depending on weather conditions.
These obligate parasites overwinter inside infected leaves, stems and fruit in the form of spores and mycelia.
In spring, the mycelia resume growth and the spores are dispersed by wind and rain, causing an initial infection.
During the growing season, when weather conditions are favourable, the mycelia proliferate and produce zoospores, asexual spores with flagella that make them free-swimming. A film of water on plant tissues promotes their dispersal and germination.
This is the most active phase, when the downy mildew multiplies quickly and consolidates its infection of the plant.
In late summer, the fungi produce oospores, sexual spores that are able to survive during unfavourable conditions and serve to maintain the genetic diversity of the species.