How to Manage Pests

UC Pest Management Guidelines


Downy Mildew

Pathogen: Peronospora trifoliorum

(Reviewed 3/17, updated 3/17)

In this Guideline:

Symptoms and signs

The upper surface of leaves infected with downy mildew becomes lighter in color, in some cases almost a mottled yellow. Bluish-gray areas of mycelial mats and spores can be seen with a hand lens on the underside of the affected area. Spores are more often found in the morning when humidity is high. Sometimes entire buds and leaves become infected, resulting in distortion of leaves and general yellowing. Infected leaves drop off the plant, reducing yield and quality.

Comments on the Disease

Downy mildew is a cool-season foliar disease. It occurs when temperatures are cool and humidity is high enough for the pathogen to infect alfalfa and produce spores. For this reason it is rarely seen in some years, and even in "wet" years it is usually a problem only for a few to several weeks in spring.

Spring-planted fields are harmed the most because plants are in the seedling stage when weather tends to be most favorable for the disease. Stand survival is usually not affected. In established fields, harvest early to reduce losses.


[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Alfalfa
UC ANR Publication 3430


R. M. Davis, Plant Pathology, UC Davis (Emeritus)
C. A. Frate, UC Cooperative Extension, Tulare County (Emeritus)
D. H. Putnam, Plant Sciences, UC Davis

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