ONION SMUDGE 



By J. C. Walker 

 Assistant Professor of Plant Pathology, University of Wisconsin, and Pathologist, Office 

 of Cotton, Truck, and Forage Crop Disease Investigations, Bureau of Plant In- 

 dustry, United States Department of Agriculture 1 



INTRODUCTION 



Smudge is a common disease of onions occurring both in the field and 

 in storage or transit. It is confined for the most part to the bulbs and 

 is characterized by dark green to black spots of variable size and shape 

 on the outer scales. The spots may be homogeneous in appearance or 

 may consist of numerous individual stromata scattered miscellaneously 

 or arranged in concentric rings. The disease is most common on the 

 white varieties of onions and damages materially the appearance and 

 market value of the crop. The causal fungus has heretofore generally 

 been known as Vermicularia circinans Berkeley, but as explained later 

 in this paper it should more properly be termed Colletotrichum circinans 

 (Berk.) Voglino. 



The present investigations have been carried on with special reference 

 to the disease as it occurs in the districts of southeastern Wisconsin and 

 northeastern Illinois where onion sets are grown. The growing of white 

 onion "bottom sets" is an industry of considerable importance in these 

 sections, and the methods used in growing and handling the set crop are 

 often conducive to the excessive development of smudge during and 

 immediately following harvest. In this study attention has been given 

 primarily to the mycological and physiological aspects of the causal 

 organism, the relation of the parasite to the host tissue, the life history 

 of the fungus with relation to the production of disease, and the develop- 

 ment of remedial measures. 



THE DISEASE 

 COMMON NAMES 



A number of common names have been used in American and Eu- 

 ropean literature for this disease — namely, "onion Vermicularia" (j) 2 , 

 " Vermiculariose " (29), "black spot" (7, 50), "scab" (17, 21), "an- 

 thracnose" (7,36, 37, j^and "smudge" (26). The name " anthracnose " 



1 This study was begun in the Department of Plant Pathology at the University of Wisconsin in 1914, 

 and the major portion was completed in 191 7. Since the writer entered the Office of Cotton, Truck, and 

 Forage Crop Disease Investigations in the latter year, observations have been extended to sections outside 

 of Wisconsin. Grateful acknowledgments are expressed to Dr. L. R. Jones, under whose immediate direc- 

 tion the work has been done, and to Drs. J. J. Davis and E. M. Gilbert, who have given valuable aid and 

 suggestions on the mycological phases of the problem. 



2 Reference is made by number (italic) to " Literature cited, " p. 719-721. 



Journal of Agricultural Research, Vol. XX, No. 9 



Washington, D. C. Feb. 1, 1921 



^r Key No. G-218 



(685) 



