Downy Mildew

Downy mildews are water molds in the Oomycota class that affect foliage. Downy mildew appears on the upper leaf as a yellow, discolored section between veins, and on the lower leaf as white or purple-brown mold. (Downy mildew is not the same as Powdery mildew, which is a fungus that appears on the upper leaf as a white powder). Downy mildews flourish in moist conditions, between 40 – 60 degrees F, so are prevalent in some areas of the San Francisco Bay Area. The Downy mildew microbes are obligate parasites, meaning that they need a living host, so their strategy is to feed, but not kill, the host plant. Frequent hosts include grape, cucurbits (pumpkin, cucumber, etc.), and impatiens.

Powdery mildew sporulation on alfalfa leaves.
Photo by Gerald Holmes. This work is licensed under a Creative Commons
Attribution-Noncommercial 3.0 United States License.

The Downy mildews require water for infection, and at last 90% humidity for sporulation. The pathogen exists as an oospore in leaf debris or soil between growing seasons. When conditions are right, the spores germinate and produce sporangia (tree-like structures), which in turn produce zoospores (swimmers with flagellum, capable of motility in water). The sporangia and zoospores are airborne, or splashed onto lower leaves. The spores enter stomata or damaged tissue on the underside of the leaf, and infect the leaf (generally within 8 – 12 hours). Additional waves of sporangia and zoospores follow during the growing season as a secondary inoculation. The mold that you see on the underside of the leaf is the sporulation, or collection of spores. At the end of the season, sexual reproduction produces oospores that can survive without a host until the next growing season.

Downy mildews are typically host specific, meaning that a particular pathogen attacks a specific host (if your pumpkin exhibits Downy mildew, you do not need to worry that it will jump to your grapes; however, the same environmental conditions may exist, making your grapes susceptible to their own Downy mildew pathogen). For management tips, see the Integrated Pest Management (IPM) site, and search for a specific host. In general, recommendations include planting disease-resistant seeds and plants; rotating crops with plants that are not susceptible to the pathogen; and using practices to minimize leaf wetness. Chemical control methods are also available; consult the IPM for recommendations about your particular host.