FURTHER STUDIES ON THE RELATION OF ONION 
SCALE PIGMENTATION TO DISEASE RESISTANCE 1 
By J. C. Walker, Pathologist , Office of Cotton, Truck, and Forage Crop Disease 
Investigations, Bureau of Plant Industry, United States Department of Agri¬ 
culture, and Carl C. Lindegren, formerly Research Assistant in Plant Pa¬ 
thology, University of Wisconsin 
STATEMENT OF THE PROBLEM 
In earlier studies of the nature of 
parasitism and resistance associated 
with the smudge disease ( Colletotri - 
chum circinans [Berk.] Vogl.) of onion 
bulbs (3, 4) 2 it has been shown that 
the host tissue contains certain sub¬ 
stances which are toxic to the invading 
organism. These substances fall into 
two classes, (a) the volatile oils and 
certain associated nonvolatile com¬ 
pounds in the cell sap (6) and (b) 
the toxins associated with scale pig¬ 
mentation. The volatile substances 
which arise from the succulent tissue, 
especially upon wounding, are readily 
taken up by liquid suspensions of co- 
nidia and when present in sufficient 
concentration completely inhibit germi¬ 
nation and retard growth. No marked 
consistent difference in the amount of 
these volatile toxins given off by differ¬ 
ent varieties of onion has been noted, 
and the conspicuous resistance of 
colored-bulb varieties as compared 
with the white types is not to be ex¬ 
plained on this basis. The volatile 
substances, while generally toxic to 
fungi, apparently are not so effective 
upon rapid bulb-decaying organisms, 
such as Botrytis allii and Fusarium 
cepae, as upon milder parasites, such 
as Colletotrichum circinans. * It has 
been suggested therefore that these 
toxins in the juice of succulent onion 
scales may have a role in restricting 
the advance of certain invading organ¬ 
isms (6). 1 
The second group of toxic substances 
was found in the water ’extract from 
the dry outer colored-bulb scales from 
which the volatile substances had dis¬ 
appeared (4 ). Spores placed in ex¬ 
tracts of sufficient concentration were 
either entirely prevented from germ¬ 
inating or, if germination started, the 
process was so modified as to cause the 
exudation of naked cytoplasm from 
the tip of the young germ tube, thus 
inactivating the organism. As the 
concentration of the extract was re¬ 
duced, the toxic effects were propor¬ 
tionately decreased until eventually 
normal germination and growth oc¬ 
curred. In contrast to this, extracts 
from white scales showed little or no 
toxicity and supported good germina¬ 
tion and growth. It appeared there¬ 
fore that the natural resistance of 
colored onion bulbs to the smudge 
organism is based upon the toxic sub¬ 
stances associated with the pigmenta¬ 
tion of scales. It is presumed that in 
nature amounts of these toxins suffi¬ 
cient to inactivate the fungus (if 
present) dissolve into drops of meteoric 
or soil water which come in contact 
with colored scales. The outer colored 
scales therefore serve as a barrier to 
invasion by the parasite. 
In the course of this investigation it 
became of interest to extend the same 
line of experimentation to a number of 
other fungi, especially those which are 
parasitic upon onion bulbs. The re¬ 
lation of onion oils to a number of 
organisms has been reported (tf). The 
present paper deals with the relation 
of the pigment extracts to practically 
the same groups of fungi, namely, 
Fusarium cepae Hanzawa, Fusarium sp. 
No. 45 (an onion bulb-rotting organ¬ 
ism) , Fusarium lycopersici Sacc. (tomato 
wilt organism), and Fusarium gram- 
inearum Schwabe {Gibberella saubinetii 
(Mont.) Sacc.) (wheat scab organ¬ 
ism); onion neckrot fungi, Botrytis 
allii Munn, Botrytis sp. No. 110, and 
Botrytis sp. No. 108a; Aspergillus sp. 
No. 4660 3 and Aspergillus niger van 
Tiegh.; and an unidentified species of 
1 Received for publication May 2, 1924—issued February, 1925. This study has been supported jointly 
by the Office of Cotton, Truck, and Forage Crop Disease Investigations, Bureau of Plant Industry, U. S. 
Department 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. 514. 
3 As noted in another article ( 6 ), this species will readily decay onion bulbs. It is a yellow-spored 
form which is being studied by Dr. Charles Thom, Bureau of Chemistry, U. S. Department of Agriculture, 
and for the present is referred to by his number, 4660. 
Journal of Agricultural Research, 
Washington, D. C. 
(507) 
Vol. XXIX, No. 10 
Nov. 15, 1924 
Key No. G-438 
