University of California

Onion downy mildew


C. E. Yarwood

Author Affiliations

C. E. Yarwood was Assistant Professor of Plant Pathology and Assistant Plant Pathologist in the Experiment Station.

Publication Information

Hilgardia 14(11):595-691. DOI:10.3733/hilg.v14n11p595. February 1943.

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Downy mildew of onion, caused by Peronospora destructor Berk., is the most important disease of the onion seed crop in California. It is serious on onions grown for bulbs and greens not only here but also in other onion-growing regions throughout the world. Though the disease probably occurs in most regions every year, severe losses are rather sporadic, as is the case with many diseases caused by downy mildews. Most previous attempts to devise control methods for the disease have been unsuccessful. The present study was started in 1935 and is devoted to various biological aspects of the disease and to its control. The work was done in the greenhouse in Berkeley unless otherwise mentioned. The principal literature concerning onion mildew is briefly reviewed, more attention being given to the controversial aspects. Most field observations were made on onions grown for seed, and generalizations in this paper refer to and are based on the California seed crop.

California grows from about 1,000 to 7,000 acres of onions for seed annually (72)3 with a production of perhaps 300,000 to 1,500,000 pounds and a value of perhaps $300,000 to $1,000,000 (no official estimates available). From 1918 to 1929 (72) California produced about 95 per cent of the total onion seed for the United States. Since then, partly because of destructive onion-mildew epidemics in California, some of the seed industry has been moved to Oregon and Idaho. The California bulb crop, varying from about 5,000 to 10,000 acres, produces about 1,000,000 to 1,5001,000 sacks at a value of about $1,000,000 to $1,800,000 (7). California produced about 9 per cent of the total United States crop of onion bulbs during 1935 to 1940.

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Yarwood C. 1943. Onion downy mildew. Hilgardia 14(11):595-691. DOI:10.3733/hilg.v14n11p595
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