162 



FUNGOUS DISEASES OF PLANTS 



as well as many wild species. It frequently causes stem deformi- 

 ties, and in England it is often associated with Cystopus candidus 



on Capsella, while in this 

 country it is perhaps best 

 known as a pest in cauli- 

 flower culture under glass, 

 yet occasionally destructive 

 in cabbage cultures in the 

 open, and less frequently 

 occurring on radish, turnip, 

 etc. 



The conidiophores, shown 

 in Fig. 56, a, are character- 

 istic of the genus. The co- 

 nidia germinate (Fig. 56, b) 

 from one side by means of a germ tube. The development of the 

 oospores of this species has been carefully studied and would 

 correspond closely to that described for Cystopus candidus except 

 that in this Peronospora there is no true ccenocentrum. 



FIG. 56. PERONOSPORA ON CABBAGE: 

 CONIDIAL STAGE 



XVI. ONION MILDEW 

 Peronospora Schkideniana De Bary 



THAXTER, R. The Onion Mildew. Conn. Agl. Exp. Sta. Kept. (1889): 



155-158. 



TRELEASE, WM. The Onion Mold. Wis. Agl. Exp. Sta. Kept. (1884): 38-44. 

 WHETZEL, H. H. Onion Blight. Cornell Agl. Exp. Sta. Built. 218 : 138-161. 

 figs. 7-77. 1904. 



The onion mildew, blight, or mold is a disease which has 

 been recognized for more than half a century. At various times 

 since 1884 it has been reported as of consequence in parts of 

 the United States from New England to Wisconsin. It is proba- 

 bly far more common and destructive than has been supposed, as 

 shown by the observations of Whetzel in 1903. 



The disease, in the regions referred to, appears in late June or 

 July, and in the early morning while covered with dew it is in 

 young stages conspicuous by a " furry violet appearance " of the 

 affected leaves. Later the leaves become moldy in character, pale, 

 collapsed, and often broken. Fig. 57 shows a diseased plant in 



