Studies on the Downy Mildew of Onions 7 



ernment to investigate the cause of the heavy losses to the onion crop 

 in Bermuda. He began his study by visiting the Canary Islands, 

 where the onion seed used in Bermuda was grown, and found onion 

 mildew to be one of the most serious onion diseases there. He made 

 careful observations of the effect of the disease on the plants, and of 

 the environmental conditions favoring it. In the following year he 

 investigated the disease in Bermuda, where his earlier observations were 

 confirmed. 



Onion mildew was first reported in America by Trelease (1884). 

 In the first annual report of the Wisconsin Agricultural Experiment 

 Station, he devoted several pages to a discussion of the disease, which 

 apparently was important in the Middle West at that time. Shortly 

 afterward, Dudley (1889) found that onion mildew was destructive in 

 New York, and Thaxter (1890) reported it on seed onions in Connecti- 

 cut. L. R. Jones (1896) found it very destructive in Vermont. 

 Whetzel (1904) gave the most complete account of the disease that 

 had been published up to that time. No other work of consequence 

 relating to onion mildew appeared for the next seventeen years. It is 

 strange that such an important disease should have been so com- 

 pletely neglected during this period, when plant-pathological activi- 

 ties were expanding rapidly. 



Beginning with the publication of the Plant Disease Reporter in 1917, 

 there have been numerous reports of the seriousness of this disease in 

 various parts of the country. It has been present to some extent every 

 year, and in some years serious losses were caused by it. 



More recently Murphy (1921) and Murphy and M'Kay (1926) in 

 Ireland, Katterfeld (1926) in Russia, and Hiura (1930, a and b) in 

 Japan, have added materially to our knowledge of onion mildew. 



SUSCEPTS 



Peronospora destructor appears to be confined to the genus Allium. 

 It has been reported in literature on the following species: 3 

 Allium ascalonicum Linn. (Shallot). Ritzema Bos (1898). 

 Allium cepa Linn. (Common onion). De Bary (1863). 

 Allium cepa var. bulbellifera Bailey (Egyptian or tree onion). 



Murphy and M'Kay (1926). 

 Allium cepa var. multiplicans Bailey (Potato or multiplier onion). 



Murphy and M'Kay (1926). 

 Allium fistulosum Linn. (Welsh onion). Schleiden (1842). 

 Allium porrum Linn. (Leek). Schoyen (1901). 

 Allium sativum Linn. (Garlic). Caballero (1922). 



3 The scientific names of the suscepts are taken from The Standard Cyclopedia of 

 Horticulture, 1922, by L. H. Bailey. 



