16 Harold Thurston Cook 



few hours, and almost certainly do not function in carrying the fungus 

 over winter. 



Mycelium in the soil is a possible, but not probable, means by which 

 the fungus overwinters. No evidence in support of this theory is known. 



INFESTED SEED 



Oospore-infested onion seed may be a source of inoculum. Chapman 

 (1910) reported the presence of spores of Peronospora destructor with 

 onion seed, but did not specify whether he found oospores or conidia. 

 Chupp (1925) states: "An examination of the seed has revealed an 

 admixture of oospores." If this statement is based on Chapman's 

 paper it is erroneous, since the kind of spores found was not specified. 

 Murphy and M'Kay (1926) also assumed that Chapman meant oospores. 

 Katterfeld (1926) found that oospores occur in the pedicels of the 

 flowers, and Hiura (1930 a) reported that oospores are occasionally 

 found in the flower stalks and the capsules. This being the case, it 

 is easily understood how the oospores might become mixed with seed 

 during the threshing process. L. D. Leach 4 stated that oospores are 

 present in large numbers in California. This fact is especially sig- 

 nificant, since California is the largest onion-seed-produeing center in 

 the world. 



In order to determine definitely whether or not oospores do occur 

 with the onion seed, a number of samples of commercial seed were ex- 

 amined by the writer. This was done by shaking a quantity of seed 

 with about twice its volume of water, which was then poured off, cen- 

 trifuged, and the sediment examined for the presence of spores. A few 

 oospores were found in several of the samples. 



The behavior of the disease at Elba suggests dissemination of the 

 mildew fungus with the seed, since, as was shown by field observations, 

 the disease occurred on scattered individual plants in new and isolated 

 onion fields, which is characteristic of seed transmission. Such dissem- 

 ination may occur partly by oospores mixed with the seed. 



Conidia mixed with the seed are a possible, but not probable, source 

 of primary inoculum. Since they are very sensitive to drying, they 

 would not be able to survive storage with seed. 



INFECTED SEED 



A considerable amount of evidence was found in support of the 

 theory that infected seed may be one source of primary inoculum. The 

 distribution of the disease early in the season, which is characteristic 



* In a letter dated August 5, 1930. 



