34 



Harold Thurston Cook 



TABLE 6. Effect of Temperature on Infection of Onions by Pebonospora 



DESTRUCTOR* 



• The minus and plus signs in the body of the table indicate the absence or the presence 

 of conidial formation following a test for infection. 



t The experiment was continued through twenty-nine days without further results. 



were discontinued after the twenty-ninth day, since by that time none 

 of the plants in the warm house had produced conidia. It is possible 

 that the fungus may have been killed by the high temperature in the 

 warm house. 



CONTROL 



The field application of fungicides which was formerly recommended 

 for the control of onion mildew was found during the investigation to 

 be unpractical, costly, and of doubtful value. It is unpractical to spray 

 or dust with the usual mixtures now available, because the waxy cover- 

 ing of the leaves prevents the materials from adhering; and the thick 

 top growth makes it very difficult, at least, to spray or dust without 

 causing serious injury to the plants. In order to protect the plant 

 properly, it would be necessary to begin spraying or dusting in the 

 seedling stage and to continue the treatment until shortly before harvest. 

 This would make the cost of growing onions prohibitive. Spraying 

 with 4-4-50 bordeaux mixture and dusting with 20-80 copper-lime 

 dust and Kolodust, in the course of this study, failed to give any indi- 

 cation of controlling the disease. 



A more practical and beneficial means of control is considered to be 

 the exclusion of the fungus from new land or from fields that have not 

 been used for onions for several years. Sanitation, rotation, and avoid- 

 ance of the environmental conditions favorable for the spread of mil- 

 dew, are additional measures to be recommended. 



It may be possible to accomplish the first of these measures by using 

 only disease-free seed and sets, and by avoiding the introduction of 



