June 73, 1923 
Disease Resistance to Onion Smudge 
were added. This is best shown in figme 2, which contains camera- 
lucida sketches of representative spores and thalli, made at the timp 
when the scale tissue was added to the drops of spore suspension and 
at the conclusion of the experiment. It is quite evident that the toxic 
substance (or substances) here concerned inhibits the growth of hyphae 
as well as the normal germination of spores of Colletotrichum circinans. 
EFFECT OF COLORED AND OF UNCOLORED PORTIONS OF THE SAME SCALE 
It has already been pointed out that ordinarily little or no pigment is 
fomed in the outer scales at the neck of the bulb. Moreover, under 
suitable conditions this particular portion of an otherwise resistant plant 
is r^dily attacked. It was important, therefore, to know whether the 
inhibitive substance aheady demonstrated in the pigmented tissue could 
be found in this closely adjacent uncolored area. The outer scale of a red 
onion set (Red Wethersfield variety) which had become naturally 
infected in the uncolored portion at the neck was carefully removed. 
Fig. 2.— Effect of colored outer scale extract upon growth of hyphae of the smudge fungus. Spores were 
germuiated m drops of distilled water to the point illustrated by camera lucida sketches in A. To some 
of the hits of red outer scale were then added. Growth was recorded by further sketches 24 hours 
later. In B are shown typical spores from drops to which red scale tissue has been added, while in C 
are shown typical spores from control drops. Growth was almost, if not entirely, checked when the 
scale tissue was added. 
Bits of tissue were then cut from the colored part of the scale just below 
the area of infection, and from the uncolored part about i inch above 
this point. The scale after the removal of these bits of tissue is shown 
in Plate 4. The bits of tissue from the two points were placed in drops 
of spore suspension according to the previously described method. In 
the drops containing uncolored tissue normal germination occurred, 
while in those containing colored tissue practically all of the spores 
developed ruptured germ tubes. Representative spores from each lot 
are illustrated in Plate 4. The experiment has been repeated several 
times with similar results, showing conclusively the close correlation 
existing between the pigment and the toxic entity. 
EFFECT OF DILUTING THE PIGMENT EXTRACT 
In order to secure a more nearly imiform basis of comparison between 
white, yellow, and red bulbs, an equivalent amount of dry scale tissue 
from each variety was weighed. The three lots were then allowed to 
steep for 24 hours at room temperature in distilled water added at the 
