Cooler nights and days of fall can create conditions that encourage the disease known as downy mildew.

This problem is caused by an organism that is more closely related to brown algae than to fungi: Hyaloperonospora parasitica. It infects plants from plant debris left in the soil, or it may be carried on the surface of seeds. All stages of the plant are susceptible, including the very first leaves that emerge from the seed (the cotyledons), but symptoms may be overlooked. As the infection progresses, the organism produces spores that initiate new infections. The structures on which the spores are produced can be seen when the temperatures are mild and relative humidity is high. Under these conditions, the sporulation structures (conidiophores) and spores will emerge from the plant tissue and appear as a tiny whitish fuzz on the undersides of the leaves or on stems.

Infection of the roots causes dark, irregularly shaped spots on the roots that often crack, and scattered or more concentrated dark areas in the flesh of the root. Similar spots occur on the seed pods.

This organism cannot live without plant tissue but will persist in plant debris until it completely degrades. For home gardens, removal and destruction of infected plants will help to prevent spread. For commercial crops, see the PNW Plant Disease Management Handbook for more information.