How to Manage Pests

Pests in Gardens and Landscapes

Downy mildew on spinach—Peronospora farinosa f. sp. spinaciae

Downy mildew causes light green to yellow angular spots on the upper surfaces of leaves. White fluffy growth of the fungus develops on the underside when moisture is present, but disappears soon after leaves dry. With time these lesions turn brown and dry up. Older leaves are attacked first. Severely infected leaves may die. On rare occasions the pathogen can become systemic, causing dark discoloration of stem tissue.

Life cycle

Damp, moist conditions and cool temperatures are ideal for the development and spread of downy mildew. Spores, produced only under conditions of subdued light or darkness and a near-saturated atmosphere, are dispersed by wind and, in cool, moist conditions, they can survive several days. Free moisture on the leaf is necessary for spore germination and infection, but moisture is not required for vegetative growth once the fungus is within the leaf. Areas with year-round vegetable culture and favorable climate often have the most serious outbreaks. Dry, desiccating winds and clear, warm days inhibit the growth and spread of downy mildew.

Solutions

The best way to prevent downy mildew is to avoid the environmental conditions that favor it. Freely circulating air, allowing plants to dry between irrigations, and keeping leaves as dry as possible are good ways to limit the disease. Use raised beds and avoid sprinkler irrigation. This fungus attacks only spinach; keeping spinach out of the garden for a year will lower disease incidence. Cool, moist weather in early spring and late fall favor the disease, so adjusting planting times may help in some areas. Destroy infected crop debris immediately. Resistant cultivars are available.

Spores of downy mildew on the underside of a spinach leaf
Spores of downy mildew on the underside of a spinach leaf

Yellow spots on upper surface of leaf
Yellow spots on upper surface of leaf


Statewide IPM Program, Agriculture and Natural Resources, University of California
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