DOWNY MIDDEW ON LETTUCE IN CALIFORNIA 
By D. G. Milbrath 1 
Assistant Pathologist, Cotton, Truck, and Forage Crop Disease Investigations, Bureau 
of Plant Industry, United States Department of Agriculture 
INTRODUCTION 
Downy mildew caused by Bremia lactucae Regel was reported several 
decades ago by Arthur (i), 3 Burrill (4), Halsted ( 6 , p . 175-176 ), and 
Farlow (5, p. 513 ), as occurring in this country on lettuce grown under glass. 
During a long period following these early reports, investigators appear, 
not to have given any intensive attention to this disease outside of 
Europe, where Marehal (7) obtained partial control of it by the applica¬ 
tion of chemicals to the soil. Recently, downy mildew was reported 
as causing slight losses, usually in greenhouses, in most of the North¬ 
eastern States and in Ohio, Indiana, Iowa, and Texas. With the 
exception of these notes on the occurrence of the disease, referring 
to it entirely as a greenhouse problem, discussions of B . lactucae in litera¬ 
ture have been meager, and no attention appears to have been given 
to it as a pathological problem of lettuce grown in the field on a com¬ 
mercial scale. In California, where extensive acreages are annually 
planted to lettuce, downy mildew has become an important factor both 
in production and transportation. 
EARLY CLASSIFICATION OF THE FUNGUS 
Bremia lactucae was first recorded as a parasite on lettuce and other 
compositae in Europe in 1843. Although the fungus retained its 
position among the Peronosporaceae, according to Saccardo (8, p. 244 ) , it 
was discussed later by Berkeley (3), de Baiy (2, p. 108), and others under 
the names of Peronospora gangliformis (Berk.) de Bary, Botrytis gang - 
lioniformis Berk., Peronospora ganglioniformis Berk., P. nivea , Botrytis 
lactucae , Botrytis geminata, Botrytis sonchicola Schlecht., and Actinobotrys 
tulasnei Hoffm. 
DISTRIBUTION OF THE DISEASE IN CALIFORNIA 
The most important commercial lettuce-growing areas of California 
are located in the Imperial Valley and the Eos Angeles, Sacramento, 
and Watsonville districts. During the season of 1919-20, there were 
approximately 19,000 acres of lettuce in these districts, and the major 
portion of the product of this acreage was destined for carlot shipments 
to markets in all parts of the country. The lettuce planted consists 
largely of the variety known as New York. 8 During the past few 
years about 85 per cent of the total annual acreage has been planted to 
1 Accepted for publication July a, 1921. 
1 Reference is made by number (italic) to “literature cited." 993. 
* The variety New York is also known locally as Los Angeles and Los Angeles Market. In Eastern 
markets it is frequently erroneously called Iceberg. Other synonyms of this variety are Henderson’s New 
York. Bonanza. Schwill’s Bonanza. Queen. Faust’s Queen, Sterling, Hastings’s Drum Head, Wonderful, 
and Neapolitan. 
Journal of Agricultural Research, 
Washington, D. C. 
abt 
(989) 
Vol. XXIII, No. 1, 
Mar. 24, 1923 
Key No. G-293 
