STUDIES ON THE DOWNY MILDEW OF ONIONS, 



AND THE CAUSAL ORGANISM, 

 PERONOSPORA DESTRUCTOR (BERK.) CASPARY 1 



Harold Thurston Cook 



Downy mildew of the onion has been known for many years and 

 frequently has been a subject of study. In spite of this, our knowledge 

 of the life history of the pathogene and its environmental relations is 

 still incomplete. The disease continues to be one of the most destructive 

 and widespread diseases of the onion, since, for the most part, the 

 measures recommended for its control have been impracticable and have 

 yielded uncertain results. With heavy losses occurring in the vicinity 

 of Elba, in western New York, during the summers of 1926 to 1929 while 

 the writer was engaged in investigating diseases of muck crops, 2 a study 

 of the disease and the pathogene was undertaken. The results of the 

 investigation, together with a review of results reported by other work- 

 ers, are here set forth. 



NOMENCLATURE OF THE CAUSAL ORGANISM 



Although the fungus causing downy mildew of onions was placed in 

 its proper genus soon after its discovery, various specific names have 

 been given to it. A chronological review of the taxonomy of the onion- 

 mildew organism, and the writer's conclusions regarding the valid name, 

 follow. 



So far as is known, Botryiis destructor is the first name applied to 

 the pathogene. It was published in 1841 by Berkeley, with a Latin de- 

 scription and an illustration of the conidial stage of the fungus. The 

 perfect stage was not included in this description. 



The name Botrytis {parasitica?) is used by Schleiden in connection 

 with a description and a figure of the fungus in Grundziige der Wis- 



1 Also presented to the Faculty of the Graduate School of Cornell University. June. 

 1931, as a major thesis in partial fulfillment of the requirements for the degree of 

 doctor of philosophy. 



Author's acknowledgment. The author wishes to express his appreciation for the 

 helpful suggestions and criticisms given by Dr. W. H. Burkholder, at whose suggestion 

 these studies were undertaken. He is indebted also to Dr. II. M. Flfzpa trick for 

 criticism and assistance in the preparation of the section on the nomenclature of the 

 causal organism : and to Dr. C. Chupp and other members of the staff of the Depart- 

 ment of Plant Pathology of Cornell University, for assistance given in the course of 

 the work. 



'Acknowledgment is made of the cooperation of members of the Genesee-Orleans 

 Vegetable Growers' Association, whose support of a special temporary fellowship made 



these studies possible. 



