FURTHER STUDIES ON THE TOXICITY OF JUICE EX¬ 
TRACTED FROM SUCCULENT ONION SCALES 1 
By J. C. Walker, Pathologist, Office of Cotton , Truck , and Forage Crop Disease 
Investigations , Bureau of Plant Industry , United States Department of Agricul¬ 
ture, and Carl C. Lindegren and Freda M. Bachmann, formerly Research 
Assistants, Department of Plant Pathology , University of Wisconsin 
SCOPE OF THE INVESTIGATION 
In a previous paper reporting studies 
on the nature of resistance to onion 
smudge, the senior author (7) 2 has 
discussed the toxicity of volatile sub¬ 
stances of the host to the causal fun¬ 
gus, Colletotrichum circinans (Berk.) 
Vogl. It was pointed out at the time 
that, although these substances did not 
apparently contribute to the con¬ 
spicuous varietal resistance of highly 
pigmented, as compared with unpig- 
mented, varieties of onions studied, they 
probably did play a role in restricting 
the parasitism of the organism. The 
obvious question arose during the 
course of these studies as to the effect 
of these volatile substances upon other 
parasites of the onion bulb. As one 
approaches these fundamental ques¬ 
tions of parasitism the complexities 
become increasingly apparent. The 
interrelations with the host are proba¬ 
bly specific for each parasite. Each 
invader attacks by its own method 
and the defense of the host may in 
each case be different. The present 
investigation was undertaken primarily 
with the idea of throwing further light, 
if possible, on the general question of 
parasitism. 
In this work the crude mixture of 
volatile substances arising from onion 
tissues, usually after crushing, was 
used. It is often erroneously stated 
in textbooks that the chief volatile 
constituent of the onion is allyl sul¬ 
phide (C 3 H 5 ) 2 S). Semmler (4), how¬ 
ever, found that the volatile oil from 
onion contains no allyl sulphide, but is 
made up principally of a disulphide, 
C6 Hi 2 S 2 . It also contains a higher 
sulphide of the same radicle and another 
sulphur-containing compound which is 
probably identical with one of the 
higher-boiling asafetida oils. Since any 
attempt at isolation of the individual 
substances is likely to alter consider¬ 
ably the complex as it exists in the 
plant, it was considered best for the 
purpose of this investigation to study 
first their combined effect upon the 
fungi. In considering the results, there¬ 
fore, it is evident that one or more 
compounds may be responsible for the 
toxic effects observed. 
It is of interest to note that since the 
completion of the senior author’s last 
paper (7), Brown (2) has published his 
studies on the effect of volatile sub¬ 
stances from various plant parts upon 
spore germination. He notes a stimu¬ 
lating effect in the case of most tissues 
tried, but when onion tissue was used 
marked inhibition was found to occur. 
Brown also points out that in an ordi¬ 
nary moist chamber, formed by placing 
moist filter paper in the bottom of a 
closed dish, there is a biotic reaction 
which results in the fermentation of 
the filter paper and this process yields 
a volatile substance toxic to the fungi 
which he studied. He did not find 
this to occur, however, until several 
days after the filter paper was 
moistened. Since all of the writers’ 
observations were made within 24 
hours after initiation of the experi¬ 
ment, and since all experiments were 
controlled, they do not consider that 
this factor has entered into the re¬ 
sults. With the method of experi¬ 
mentation used it was impossible to 
duplicate actual conditions which the 
parasite encounters upon entering the 
host tissue. It is quite probable that 
the volatile substances released upon 
crushing the tissue do not occur thus 
exactly as they existed in the host cell. 
The manner in which the fungus is 
exposed to the onion oil may differ 
widely as between the experiment and 
nature. Many of the bulb parasites 
1 Received for publication May 3, 1924; issued April, 1925. This study has been supported jointly by 
the Office of Cotton, Truck, and Forage Crop Disease Investigations, Bureau of Plant Industry, U. S. Dept, 
of Agriculture, and by the Department of Plant Pathology, University of Wisconsin, through a special 
grant from the general research fund of the University. 
2 Reference is made by number (italic) to literature cited, p. 87. 
Journal of Agricultural Research, 
Washington, D. C. 
( 175 ) 
Vol. XXX, No. 2 
Jan. 15, 1925 
Key No. G-443. 
