June 23, 1923 
Disease Resistance to Onion Smudge 
1023 
slight infection may occur on colored bulbs and even in rare occasions 
to a considerable degree if they are unduly exposed after matturity to 
the bleaching effects of direct sunlight as well as meteoric and soil water. 
RELATION OF ONION OIL TO THE PARASITISM OF COLLETOTRICHUM 
CIRCINANS 
The amount of research upon the chemical composition of the onion is 
limited. According to Remington and Wood (i5, p, 1 52 ^-1^26 ): 
Fourcroy and Vanquihn obtained from the ordinary onion a white acrid volatile sub¬ 
stance containing sulphur, albumen, much uncrystallizable sugar and mucilage, 
phosphoric acid both free and combined with lime, acetic acid, calcium citrate, and 
lignin. The expressed juice is susceptible of vinous fermentation. The oil is essen¬ 
tially the same in chemical composition as the oil of Allium Sativum and consists 
largely of allyl sulphide (C3H5)2S. 
According to Gildemeister and Hoffman (7, p, 546), Semmler has found 
that onion oil, contrary to the reports given in most textbooks, consists 
not of allyl sulphid but of a series of sulphids, chiefly CqHjjSj. Kooper 
(ji) found thiocyanic acid in the onion as well and secured positive tests 
for thiocyanic acid allyl ether, but no trace of acetic, formic, or allyl 
aldehyde. The germicidal effect of onion juice was noted by Pasteur 
RELATION OF EXPRESSED ONION JUICE TO COLLETOTRICHUM CIRCINANS 
The relation of the constituents of the onion tissue to the fungus was 
first investigated experimentally by studying the effect of the expressed 
sap from onion scale upon spore germination. White onion scale tissue 
was macerated and the sap expressed by forcing through cheesecloth. 
Portions of the sap were diluted 10, 100, and 1,000 times with distilled 
water. Spores from a pure culture of the fungus were placed in drops 
of the expressed sap and in its various dilutions on glass slides in moist 
Petri dishes. No germination occurred in the sap as expressed from the 
tissue, nor in the i to 10 dilution. In the i to 100 dilution fair germina¬ 
tion took place, and in the i to 1,000 dilution normal germination occurred. 
This experiment was repeated many times with practically the same 
results. When bits of fleshy onion scale tissue were added to drops of 
water containing spores, germination was also inhibited. 
In order to determine the effect of heating on this inhibitive substance, 
a small amount of onion extract was secured in the usual way and divided 
into two equal parts, one of which was placed in live steam for 20 minutes. 
No germination occurred in the fresh extract as usual, while in the heated 
extract a high percentage of the spores germinated normally. From the 
results of these experiments it seemed quite certain that there was present 
in the extract a toxic substance which was readily removed by heat. 
Since germination of the spore normally takes place in the soil or in 
water outside the plant, and not in contact with the host cell sap, and since 
the fungus would thus not come in contact with the inhibitive substance 
until its mycelium had penetrated the host, the question arose whether 
this toxic substance within the cell is capable of inhibiting the growth of 
the germ tube. Spores were germinated in distilled water until the germ 
tubes were several times the length of the spore, then a number of them 
were transferred to onion sap extract. The growth of the latter group 
was checked at once, while those left in water continued to grow at the 
normal rate. This showed that the mycelium could be checked by the 
