992 
Journal of Agricultural Research voi. xxin, No. %* 
appeared to be as efficient as any other medium for the inducement 
of zoospore formation, while in dextrose solution there was a greater 
tendency to direct germination of the conidia. After coming to rest, 
the zoospores germinated with regularity (PI. 2). 
The complete pathological significance of zoospore production must 
be determined through further investigation. It is very evident that 
environmental conditions play an important part in the determination 
between direct germination and zoospore formation. Although direct 
germination occurs in the absence of light and at io° C., zoospore for¬ 
mation predominates under those conditions, and one is led to conclude 
that zoospores provide for the reproduction ot the fungus at low tem¬ 
peratures. 
SUSCEPTIBILITY OF LETTUCE VARIETIES TO THE ORGANISM 
The disease was found on lettuce during all parts of the year, but 
the severest infections occurred during the winter months. It was 
thought that the control of soil moisture during irrigation might inhibit 
the development of the fungus. In order to accomplish such control, 
seed beds were constructed about 8 inches above the irrigation furrow, 
on the experiment field of the Office of Cotton, Truck, and Forage Crop 
Disease Investigations, located at Alhambra, Calif. Irrigation water 
was then allowed to rise in the furrow to a line 4 inches below the surface 
of the seed bed. This attempt to reduce soil surface moisture and 
subsequent condensation of moisture on the plant's leaf surfaces was 
futile in the winter months, because of rains and frequent fogs. During 
the rainless months a considerable amount of the fungus developed, 
but not to as great an extent as during the rainy season. 
In carrying out varietal tests of lettuce, it was soon found that the 
variety New York was highly susceptible to mildew, while several other 
varieties showed evidence of resistance. During two consecutive years 
six separate plantings were made each year. In each test 200 plants 
of each variety were kept under observation. In each test the contrast 
in susceptibility between the varieties New York and Iceberg was mark¬ 
edly evident. Iceberg appeared to be highly resistant. The relative 
susceptibility of four varieties of lettuce in two tests during the season 
of 1919-20 is expressed in the following table. 
Variety. 
Number of 
plants. 
Number of plants 
infected. 
Test x. 
Test 2. 
New York . 
200 
196 
I 
170 
0 
Iceberg ... •.. 
200 
Big Boston ... 
200 
10 
6 
Hanson .*. 
200 
12 
8 
The contrast in susceptibility between New York and Iceberg (PI. 3) 
is obviously in favor of the inferior variety. New York has supplanted 
Iceberg in recent years on account of the production of a firmer and 
larger head during the winter months, the main growing season. Ice¬ 
berg excels during the summer months, when high temperatures retard 
