Studies on the Downy Mildew op Onions 



table l. 



Varieties of the Common Onion Tested for Susceptibility to 

 Peronospora destructor* 



1. Ailsa Craig 



2. Australian Brown 



3. Autumn Queen 



4. Bedfordshire Champion 



5. Blood Red 



6. Bronze Globe 



7. Ciboule Blanch Native 



8. Crystal White Wax 



9. Dark Red Brunswick 



10. Earliest White Queen 



11. Early Dan vers 



12. Early Large Red 



13. Ebenezer 



14. Extra Early Pearl 



15. Extra Early Red 



16. Froxfield 



17. Giant Gibraltar 



18. Giant Rocca 



19. Golden Globe Tripoli 



20. Holborn 



21. Iwatsuki 



22. James Long Keeping 



23. Large Red Italian 



24. Michigan Yellow Globe 



25. Mountain Danvers 



26. Mountain Red Globe 



27. Natsu-negi Nebuka 



28. New Mammoth Silver 



King 



29. Nuneham Park 



30. Ohio Yellow Globe 



31. Pear-Shaped 



32. Pearl Pickler 



33. Prizetaker 



34. Record 



35. Red Wethersfield 



36. Round Yellow Danvers 



37. Rousham Park Hero 



38. Senju-negi Nebuka 



39. Southport Red Globe 



40. Southport White Globe 



41. Southport Yellow Globe 



42. Straw-colored Flat Keep- 



ing 



43 



The Perfect " 

 Keeper 



44. Tokyo Nebuka 



45. Trebons 



46. Up-to-Date 



47. White Portugal 



48. Winterheck 



49. White Welsh 



50. White Emperor 



51. Yellow Bermuda 



52. Yellow Danvers 



53. Yellow Globe Danvers 



Long 



* Some of these observations were made on varieties grown by E. L. Felix in testing 

 for resistance to other diseases. 



IMPORTANCE AND NATURE OF THE LOSSES 



The frequent and heavy losses caused by the onion mildew have been 

 reported by numerous writers from the time when the disease was first 

 recorded by Berkeley (1841). Many investigators have stated that the 

 disease causes serious damage to all stages of the common onion and 

 other species of Allium. The actual reduction in yield is sometimes 

 as high as 60 to 75 per cent of the normal crop. 



The onion plant may be attacked and destroyed in the seedling stage, 

 as has occurred several times in Georgia (Boyd, 1925, and Higgins, 

 1925). In greenhouse experiments it was found that infected onion 

 seedlings usually died soon after the fungus fruited. Possibly some of 

 the damping-off of the seedlings is attributable to mildew instead of to 

 Botrytis, to which it usually is attributed. 



When older plants are attacked by mildew, the bulbs are reduced in 

 size. This fact was noted in the first written account (Berkeley, 1841), 

 in which it was stated that the disease prevents the onions from coming 

 to perfection. Whetzel (1904) discussed this point in considerable 

 detail, emphasizing the fact that the fungus destroys the leaves at the 

 time when they should be manufacturing food for the bulb, which then 

 ceases to grow while new leaves are being developed. In turn these 

 new leaves may be destroyed by the fungus soon after they are formed, 

 thus further retarding the development of the bulbs. Whetzel stated 

 that there is little or no increase in size of the bulbs following a severe 

 attack of the fungus. 



Observations by the writer also indicated that the bulbs of mildewed 

 plants are considerably reduced in size, and an experiment was con- 

 ducted in 1927 to obtain more definite information on this point. On 

 August 4, approximately ten days after the mildew was found in the 



