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UC Pest Management Guidelines

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Floriculture and Ornamental Nurseries

Downy Mildew

Pathogens: Peronospora spp. and Plasmopara spp.

(Reviewed 1/02, updated 1/02)

In this Guideline:

Symptoms
Comments on the disease

SYMPTOMS
The name downy mildew is somewhat descriptive of the appearance of the white, tan, lavender, and purple sporulation that occurs usually on the undersides of leaves. Downy mildew infections are sometimes angular in shape and delimited by veins. Pale yellow or necrotic areas sometimes are visible from the upper side of the leaf.

In some plants when young shoots are infected the fungus may become systemic and the resultant growth is stunted, malformed, and discolored.

COMMENTS ON THE DISEASE
The downy mildews are more specific as to hosts than the powdery mildews, but the big difference between the two is that downy mildews require very wet conditions to flourish. Water is required for infection and humidity above 90% is required for sporulation. Growth of the fungi is also favored by cool temperatures.

Spores of the fungus are usually short-lived although they may survive several days under cool, moist conditions. They are airborne and when they land on a susceptible plant and there is free water, germination and infection occurs, generally in 8 to 12 hours. The fungus also produces a sexual spore that can survive dry conditions, which enables the fungus to survive in absence of the host. Downy mildews are favored by moist and cool conditions (40° to 60°F).

Common name

(trade name) Amount to Use

A. MANCOZEB

(Dithane) 75 1-1.5 lb/100 gal water

COMMENTS: A protectant fungicide.

B. COPPER 50% 1-2 lb/100 gal water

COMMENTS: A protectant fungicide.

C. FOSETYL-AL

(Aliette) 2.5 lb/100 gal water

COMMENTS: For control of downy mildew on roses. Spray to wet using no more than 400 gal/acre.

PUBLICATION

UC IPM Pest Management Guidelines: Floriculture and Ornamental Nurseries
UC ANR Publication 3392
Diseases
R. D. Raabe (emeritus), Environmental Science, Policy, and Management (ESPM), UC Berkeley
M. E. Grebus, Plant Pathology, UC Riverside
C. A. Wilen, UC IPM Program, UC Cooperative Extension, San Diego Co.
A. H. McCain (emeritus), Environmental Science, Policy, and Management (ESPM), UC Berkeley

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