22 Harold Thurston Cook 



Since obtaining the above evidence that Peronospora destructor may 

 be transmitted in the seed, a review of the literature and further work 

 with other downy mildews has shown that seed transmission of mem- 

 bers of the Peronosporales is probably of frequent occurrence and is 

 one of the principal means by which they overwinter and are dis- 

 seminated. Although seed transmission of Phytophthora phaseoli has 

 not been actually demonstrated, the work of Clinton (1906) has shown 

 that it probably does occur. Angell (1929) gives relatively conclu- 

 sive experimental evidence that the organism causing blue mold of 

 tobacco is carried in or with the seed. The manner in which this 

 disease made its first appearance in this country would indicate also 

 that the inoculum was widely distributed and needed only the proper 

 environmental conditions for its development. Wolf and Lehman 

 (1924), in their studies on downy mildew of soybean, found circum- 

 stantial evidence that Peronospora manshurica is transmitted with the 

 seed. Leach (1931) found mycelium and oospores of Peronospora 

 schachtii in the sepals, pericarp, filaments, and ovules of beet flowers, 

 but was unable to find them in the nucellus and the embryo. By con- 

 trolled germination tests, however, he was able to obtain evidence of 

 seed transmission of the beet-downy-mildew fungus. According to the 

 Plant Disease Reporter, beet mildew is as severe on beet seed-plants 

 in California as is onion mildew on onion seed-plants. Eriksson (1925) 

 also cites evidence of seed transmission of beet mildew. The writer's 

 own observations on the downy mildews of spinach, crucifers, and 

 cucurbits point to seed transmission of the causal organisms of these 

 diseases. It has been observed frequently that cucumbers are attacked 

 by mildew as early and as severely when grown on new and isolated 

 fields as when grown on old land or near other fields of cucumbers. 

 Peronospora parasitica frequently was found fruiting on the seed pods 

 of kale and cabbage, and it is probable that the fungus enters some of 

 the seed. Observations have shown that Peronospora effusa is present 

 on practically all spinach seed-plants, and that it fruits on some of 

 the seed. Sections of these seed have demonstrated the presence of 

 the fungus in the ovary and in the seed coat. The seed of commerce 

 consists of the ovule surrounded by the ovary. 



ENVIRONMENTAL FACTORS 

 FIELD OBSERVATIONS 



In studying onion mildew in the field, an attempt was made to 

 determine the climatic conditions favoring the development and spread 

 of the disease. However, with so many variable factors only general 

 conclusions could be reached. It was observed that, as a rule, mildew 

 was not found until late July or early August, when the nights were 





