36 Harold Thurston Cook 



Field observations were made on the occurrence of the disease on 

 market onions, set onions, seed onions, and Egyptian onions. The 

 disease was found to be favored by abundant moisture and relatively 

 low temperatures. Market onions were found to become diseased as 

 early and as frequently on new as on old land. The first diseased 

 plants were found to be scattered throughout the fields. 



There are four sources of primary inoculum: systemically infected 

 plants, infested soil, infested seed, and infected seed. Evidence pre- 

 sented points to the fact that mycelium in the seed is an important 

 source of primary inoculum. 



The fungus was found to fruit over a wide range of temperatures. 

 Water on the leaves was necessary for the formation of conidiophores 

 and conidia. 



Lake water was found to be the most satisfactory medium tested for 

 spore germination. The optimum temperature for spore germination 

 was found to be 11° C, but the full range over which the spores 

 germinate was 3° to 27° C. 



An abundance of moisture, and low temperatures, were found neces- 

 sary for infection. The incubation period was about eleven to fifteen 

 days. 



Fungicidal treatment is not considered a satisfactory method of 

 control. Exclusion of the fungus from new areas, sanitation, and reg- 

 ulation of the environmental conditions, are considered the most promis- 

 ing means of combating the disease. 



REFERENCES 



[Anonymous] Investigations of plant diseases. Mauritius Agr. Dept. 



Ann. rept. 1923:10-11. 1924. 

 Angell, H. R. Blue mould of tobacco: investigations concerning 



seed transmission. Australia Council Sci. and Indus. Research. 



Journ. 2:156-160. 1929. 



Bary, A. de. P. schleideniana linger. In Recherches sur le developpe- 



ment de quelques champignons parasites. Ann. sci. nat. Bot, 4: 



20:122. 1863. 

 Berkeley, M. J. Botrytis destructor, n. s. In Notices of British 



fungi. Ann. and mag. nat. hist. 6:436. 1841. 

 Peronospora, Cd., Casp. In Outlines of British fun- 



gology, p. 348-349. 1860. 

 [Boyd, 0. C] Downy mildew caused by Peronospora schleideni Ung. 



U. S. Plant Indus. Bur. Plant disease reporter, Suppl. 41: 



249-250. 1925. 



