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Peronosporales (class Oomycetes) An order of fungi in which a well developed, coenocytic, branching mycelium is formed. Asexual reproduction occurs usually by the formation of zoospores, sometimes by the formation of conidia. Species are chiefly parasites of vascular plants, but some can live saprotrophically in soil or water. The order includes a number of important plant pathogens, including the downy mildews.