Mar. 34,1923 
Downey Mildew on Lettuce in California 
991 
K. Link, in an unpublished report, stated that the fungus was abundant 
on lettuce shipped from California to Houston, Tex., New Orleans, La., 
and Chicago, Ill. On arrival at these points, the heads were badly 
decayed. That such decay occurs readily was established in the labora¬ 
tory, where lettuce heads, some severely affected with lesions caused by 
Bremia lactucae and some normal, were placed under similar conditions 
and where the affected heads always decayed more rapidly than normal 
heads. A number of secondary organisms in the form of species of 
Macrosporium, Cladosporium, Botrytis, Fusarium, and Aspergillus were 
isolated from lesions caused by Bremia lactucae , and of these several are 
capable of causing decay. 
CAUSAL FUNGUS 
Bremia lactucae has long been known as one of the Peronosporaceae. 
En masse, the fungus when first appearing on the outer portion of the 
leaf is of a snowy white color. Microscopically, both mycelium and 
conidia are hyaline. In the host the mycelium grows between the 
cell walls, projecting modified club-shaped haustoria into the cells. 
Aerial portions of the fungus proceed generally from stomatal openings, 
at which point a single issuing hypha forms a bulbous swelling. From 
this swelling two or three conidiophores arise, with an average length 
of 190 microns from the base to the first branches. The conidiophores 
are nonseptate and profusely branched, both dichotomously and tricho- 
tomously. At the distal end of each branch a cuplike swelling projects 
four or five sterigmatic and radiating branchlets, each bearing a single 
conidium. The conidia are ovate, hyaline, papillate, and average 18.5 
by 17.5 microns in size (PI. 2). 
Reproduction takes place through direct germination of conidia by 
means of a germ tube, by the production of zoospores, and by formation 
of oospores. Heretofore, the first and third methods of reproduction 
have been mentioned in literature, while the second, the production 
of zoospores, has not been recorded up to this time. Direct germination 
was readily obtained when freshly collected conidia were placed in a 
hanging drop culture for a period of 24 hours. While the formation 
of germ tubes by conidia under aquatic conditions was being studied 
free-swimming zoospores were frequently observed. Numerous and 
careful preparations of cultures through selection of individual conidia 
led the writer to conclude that under certain conditions conidia de¬ 
veloped zoospores, while under others they germinated directly. It 
was found that conidia produced during the cool months, December 
to March, inclusive, formed zoospores more readily than those conidia 
collected during the warm season of the year. The most favorable 
conditions for the development of zoospores were found to be darkness 
and a temperature in the vicinity of io° 0 . After the preparation 
had been subjected to these conditions for a period of 24 to 48 hours, 
an abundance of motile ciliated spores could be observed. These spores 
were hyaline, globular, ciliated, motile for several hours, and about 
4.2 microns in diameter. The exact period of motility was not deter¬ 
mined. After emergence of the zoospores, the epispore collapsed quickly. 
The number of zoospores formed in each conidium seemed to vary; 
however, it was greater than eight. The media employed in this study 
were physiological salt solution, tap water, 0.1 per cent dextrose solu¬ 
tion, and 0.1 per cent bacto-peptone solution. Common tap water 
