DISEASE RESISTANCE TO ONION SMUDGE^ 
By J. C. WAI.KER ^ 
Pathologistf Office of Cotton, Truck, and Forage Crop Disease Investigations, Bureau of 
Plant Industry, United States Department of Agriculture 
INTRODUCTION 
In an earlier paper on onion smudge {24) ^ a description of the symp¬ 
toms and seasonal development of the disease, and of the morphology 
and life history of the causal organism is given. In connection with the 
above studies it was found in confirmation of the earlier observations of 
Berkeley (r), Thaxter (18, p. 163-165), Halsted {8), and Selby (16, />. 364- 
366, 414), that all white varieties of onion observed are highly susceptible 
to smudge while varieties with colored bulbs—yellow and red—show a high 
degree of resistance. Further study as to the nature of this resistant quality 
has shown that there are at least two substances or groups of substances 
within onion tissue which have a marked inhibitive effect upon the growth 
of the causal fungus, Colletotrichum circinans (Berk.) Voglino. One of 
these is the volatile pil,^ and the other is a group of one or more com¬ 
pounds closely associated or identical with the flavone and anthocyan 
pigments of the scales. The present paper contains the results of inves¬ 
tigations upon this subject. 
OCCURRENCE OF VARIETAU RESISTANCE AND SUSCEPTIBILITY TO 
ONION SMUDGE 
Observations on the relative susceptibility or resistance of onion 
varieties to smudge in field and storage have been continued since 1914^ 
chiefly in Wisconsin and Illinois but incidentally in a number of other 
onion-growing sections in the United States. In 1922, the opportunity 
was given to secure field data from certain onion centers in Europe and 
the Canary Islands. In all cases there, as in this country, where colored 
and white varieties were grown simultaneously under conditions favorable 
for the development of smudge, white bulbs were uniformly infected 
while colored bulbs were completely free or infected to only a slight 
degree. 
It should be pointed out in this connection that the pigment in colored 
varieties of onion appears in the outer bulb leaves or scales when the 
plants are about half grown, or earlier. Several of the first leaves of the 
plant acquire no basal thickness and the first of these slough off, without 
the production of much pigment, during the plant's early growth. With 
. age the coloring becomes more intense until a short time before maturity^ 
1 Accepted for publication Feb. 20, 1923. This study has been supported jomtly by the D^artment of 
Plant Pathology, University of Wisconsin, through a special grant from the general research fund of the 
university, and by the Office of Cotton, Truck, and Forage Crop Disease Investigations, Bureau of Plant 
Industry, U. S. Department of Agriculture. _ . , , ^ 
* The writer is indebted to Prof. U. R. Jones of the University of Wisconsin and to others m both the 
university and the Department of Agriculture for valuable suggestions and criticisms durmg the progress 
of the investigation. 
» Reference is made by number (italic) to “ Literature cited,” p. 1038-1039. 
< Throughout this paper the term " volatile oil ” is used to designate the volatile substances which or¬ 
dinarily arise from onion tissue, some of which, at least, are responsible for the characteristic onion odor. 
Journal of Agricultural Research, 
Washington, D. C. 
afh 
(1019) 
Vol. XXIV, No. i» 
June 23, 1923 
Key No. G-316 
