Studies on the Downy Mildew of Onions 35 



soil from infested fields on tools and on transplants. It is possible 

 that disease-free seed may be procured from sections of the country 

 in which onion mildew does not occur, or perhaps it can be grown 

 from disease-free seed bulbs. Such bulbs may be obtained by selection 

 or by treating with hot air at 40° C. for eight hours. Murphy and 

 M'Kay (1926) found that this treatment would kill the fungus in the 

 bulb. Disease-free sets also may be obtained in this manner. 



Sanitation and rotation will aid in reducing the amount of inoculum 

 in the soil. The onion refuse consists largely of tops which may con- 

 tain oospores and of culls which may be systemically infected. Prac- 

 tically 100 per cent of the onion refuse was burned at Elba in the 

 fall of 1927, 1928, and 1929 for the control of onion maggots. This 

 practice was accompanied by a decrease in the severity of onion mildew 

 during those years. At present the length of time that the fungus 

 will remain viable in the soil is not known, and so the length of rota- 

 tion necessary for the elimination of the fungus from the soil is 

 uncertain. 



The severity of the disease can be reduced to some extent by regulat- 

 ing the environmental conditions. The location of the onion field has 

 much to do with prevalence of the disease. As a rule, onions grown in 

 fields that are well drained and are exposed to the sun and wind suffer 

 less from mildew than do those in fields on low ground or surrounded 

 by high windbreaks. Furthermore, no work should be done in the 

 onion fields while the leaves are wet with dew or rain, because the 

 spores are viable under such conditions and may be carried on the 

 clothing of the workmen to other plants in the field. 



SUMMARY 



Downy mildew, caused by Peronospora destructor (Berk.) Caspary, 

 is one of the most serious diseases of the onion crop. 



Evidence has been presented to show that Peronospora destructor 

 (Berk.) Caspary is the valid name of the causal fungus. 



The disease is widely distributed, occurring in nearly all parts of the 

 world and in most of the principal onion-growing sections of the United 

 States. 



A brief history and review of the literature of onion mildew is given. 



Peronospora destructor has previously been recorded on seven species 

 and varieties of Allium. Allium schoenoprasuni* is here recorded 

 as a new suscept. Fifty-three varieties of the common onion were 

 tested and found to be equally susceptible to onion mildew. 



Losses may be sustained at any stage in the life of the plant. Data 

 are presented to show that the bulbs are considerably stunted by the 

 disease. 



