The fungus grows within the host and produces sporangia on the surface of diseased tissue, especially leaves. Sporangia are spread by splashing water from irrigation or rain. With moisture present, zoospores are formed and released. These zoospores swim for a few minutes to more than an hour before encysting. Environmental conditions such as water temperature, nutrition, pH, and other factors determine the length of time zoospores continue to swim.
Encysted zoospores germinate by producing a thin fungal hypha or thread.
The germ tube commonly penetrates the leaf through stomates which are natural opening in the leaf epidermis. Larger germ tubes produced by sporangia also penetrate the leaves. Sporangia are also moved within the field by contact with field equipment, clothing, gloves, tools, etc. Movement of soil from one field to another on equipment or boots will move this pathogen. The fungus survives in the soil in host debris.