Zoospores differentiate to become amoeba by losing flagella. Host cells are infected by fine hyphe from the amoeba. The resulting rhizomycelim occupies cells and is eventually converted to a sporangium. Stalk breakage may occur. These genera belong to the order Peronosporales in the Oomycetes .
Downy mildews are systemic diseases involving obligate parasites.
The pathogen has a wide host range, including 140 different grass species. Zoospores are produced and dispersed when soils are flooded. This occurs in soils flooded for as little as two days. Plants are susceptible from germination until they are approximately 20 cm tall. Seedborne transmission is not considered important.