990 
Journal of Agricultural Research 
Vol. XXIII, No. xa 
this variety, while the remaining 15 per cent consisted largely of the 
varieties known as Iceberg and Big Boston. Both New York and Ice¬ 
berg are cabbage-heading varieties, the former wholly green and the latter 
greenish with a brownish tinge. Synonyms for Iceberg are Burpee’s 
Iceberg, Curled India, and Weaver’s Market Gardener’s. Synonyms 
were taken from the work of W. W. Tracy, jr. (9). 
With the concentration of the industry upon one variety to the extent 
of 85 per cent, an increasing amount of downy mildew has developed, 
particularly since the variety New York is highly susceptible to that 
disease. In the Imperial Valley the disease was found in a milder form 
than in other districts. In the Los Angeles district the fungus was found 
on lettuce during all months of the year but appeared to be most active 
from October to May. During this period, in many fields where the 
plants were approaching condition for harvest, 90 per cent of the crop 
was affected severely, that is to say the fungus was found on most of the 
exposed leaves regardless of age. In a survey of the whole district 
during the season of 1919-20 an estimate of 40 per cent severe infection 
was made. In the Watsonville and Sacramento districts the severity of 
the disease was similar to that in the Los Angeles district. 
SYMPTOMS OF THE DISEASE 
The first indications of infection appear as scattered light green to 
yellow areas on the upper surface of the leaf. Any exposed leaf may be 
attacked, although the oldest leaves frequently show the first signs of 
infection. Shortly after the appearance of these discolored areas, the 
fungus develops on the lower surface of the leaves as patches of downy 
white directly beneath the discolored areas. Conidial development is 
almost simultaneous with the appearance of the fungus on the lower 
surface. The size of the spots varies from f/i to $4 inch in diameter, 
the surface development of the organism being limited by the extent of 
spread within the tissues of the host. Frequently spots coalesce and 
when of long standing take on a brown color, giving rise to the term 
“brown margin” recently employed by inspectors of the Bureau of 
Markets (PI. 1). 
DESTRUCTIVENESS OF THE DISEASE 
The frequency and severity of infection of lettuce heads makes selec¬ 
tion in the field and close trimming in packing for shipment impracti¬ 
cable. In packing, the outer leaves, usually three to four in number, 
are removed. This amount of trimming does not reduce appreciably 
the amount of infection, since not only do the outer leaves contain brown 
lesions or spots but many of the younger leaves are covered with later 
infections. 
For immediate local consumption lettuce so affected as to have the 
margins of the outer leaves browned and white patches of fungus on 
other exposed parts is not greatly reduced in value, but when such 
lettuce is stored or shipped, rapid deterioration through decay is likely 
to occur. That a large portion of the crop is shipped out of the State 
is shown by the record of over 5,789 cars of lettuce shipped during the 
year from July, 1919, to July, 1920. 
Bremia lactucae was found to remain vigorous and even to flourish on 
lettuce in cars under refrigeration on arrival at destination. Dr. G. K. 
