June 23,1923 
Disease Resistance to Onion Smudge 
1027 
variety are as great as those between varieties. It is to be expected, 
therefore, that the volatile oil content of these varieties of onions is a 
factor not necessarily responsible for differences in susceptibility or 
resistance to smudge. 
From the foregoing experiments it may be concluded that the expressed 
sap from white onion scale inhibits germination and growth of Colletotrichum 
circinans and that this toxicity appears to be due, in large measure at least, 
to the volatile onion oil which is liberated upon crushing of the tissue. 
This toxic substance is thermolabile inasmuch as it is largely removed by 
heating the extract for 20 minutes in live steam. The fact that no 
appreciable difference in toxicity between sap from colored and that 
from white Globe scales was found indicates that the volatile oil has 
little to do with the difference in varietal susceptibility. It is believed, 
however, that this substance is not to be overlooked as a factor in 
limiting the parasitism of C. circinans. 
It is of interest in this connection to note that Bernard (2) found the 
bulb tissue of Loroglossum to have a fungicidal effect upon the mycorrhi- 
zal fungi isolated from a number of closely related species of orchid. 
This was demonstrated by placing a piece of the bulb on agar in proximity 
to the thallus of the fungus. Growth of the latter did not extend within 
a certain zone surrounding the bit of orchid tissue. 
RELATION OF SCALE PIGMENTS TO DISEASE RESISTANCE 
It will not be necessary to review the literature upon plant pigments 
since it has already been brought together by others, including Wheldale 
(25) and Wakeman (22). Although a few cases of correlation between 
resistance and the red or yellow pigments in plants have been noted 
(5, d, 77, 2J), in no previous case, so far as the writer is aware, has the 
relation of plant pigments to plant parasites been carefully studied. 
The fact that a water extract from pigmented onion scales has been 
proved to be highly toxic (25) to the smudge organism, has prompted a 
further inquiry into this subject. It is needless to point out that the 
widespread occurrence of closely related pigment compounds in the plant 
kingdom justifies further investigation of their relation to parasitism. 
NATURE OF THE RED AND YELLOW PIGMENTS IN ONION SCALES 
The red and yellow pigments of onion scales are solutes in the cell sap 
of the outer epidermal layer. They first appear when the plants are 
about half grown and continue to develop during the remainder of the 
growing season. At maturity the color is most intense in the outer 
scales which have dried down to a thin papery consistency, and it be¬ 
comes progressively less marked in the fleshy scales as the center of the 
bulb is approached. Intensity and color of pigment vary with varieties. 
Colored varieties may be roughly grouped into the large classes, the 
yellow and the red. There are numerous varieties, however, such as the 
Australian Brown, which lie on the border line between the two groups. 
Pigmented cells of the yellow varieties when treated with alkalies 
turn deep brownish yellow in color, a reaction typical of the flavones 
(25). The pigmented cells of the red varieties turn pink in acid and 
green in alkaline solutions, which reactions are characteristic of the 
anthocyans. Perkin and Hummel (14) isolated quercetin, a flavonol. 
