Nov. 15, 1924 
513 
Onion Scale Pigmentation 
This is strikingly true in the case of 
smudge (C olletotrichum circinans ), 
where the parasite normally attacks at 
any point on the scale surface. In the 
case of black mold ( Aspergillus niger ) 
there is slight if any correlation be¬ 
tween color and resistance. The or¬ 
ganism attacks by almost precisely the 
same avenue as smudge, but the wri¬ 
ters’ tests have shown that the colored 
extracts have no consistent toxic effect 
upon this fungus. In the case of neck- 
rot the entrance is usually through the 
neck of the bulb at about harvest time. 
Contacts with the toxins may be ex¬ 
pected in a majority of cases, although 
as pointed out, this status may be 
changed under some conditions. Field 
observations show that there is a strong 
correlation between color and resistance 
to neckrot, but probably it is not so gen¬ 
eral as in the case of smudge. 
Helminthosporium sp., which is es¬ 
sentially a saprophyte capable of at¬ 
tacking only the dead outer scales of 
onion bulb, has been found only on 
white varieties, but little is known as to 
its general occurrence. From its sensi¬ 
tiveness to the colored scale extract it 
is likely to be limited largely to the 
white varieties. There are probably 
other saprophytic soil organisms which 
occasionally develop on the outer scales 
of white varieties and which under cer¬ 
tain conditions may act as mild para¬ 
sites of the bulb in storage, but from 
which colored bulbs are protected by 
the repellent toxins in the outer scale. 
The Fusarium bulb-rot organisms 
which invade most commonly through 
the stem plate, or through insect 
wounds, probably come under little or 
no influence of the toxins from the 
outer colored scales at the point of infec¬ 
tion. Though they are very sensitive to 
these toxins, a strong expression of dis¬ 
ease resistance would not logically be 
expected in this case. The limited evi¬ 
dence at hand points in this direction. 
SUMMARY 
(1) The marked resistance of colored 
types of onions to smudge (Colletotri¬ 
chum circinans ) is due to the presence 
in the outer scales of certain toxic sub¬ 
stances closely associated with the pig¬ 
ment compounds or identical with them. 
The functioning of this resistant prin¬ 
ciple depends upon the ready diffusibil- 
ity of these toxins from the dead outer 
scale into drops of meteoric or soil 
water, where they inactivate the fungus 
before it can attack the onion tissue. 
The purpose of the present investiga¬ 
tion has been to determine the effect of 
these soluble toxins on certain other 
fungi, especially those which attack 
onion bulbs. 
(2) The fungi considered, besides the 
smudge organism, were two onion-bulb- 
rotting species of Fusarium, namely, 
F. cepae Hanzawa and an unidentified 
form, referred to as Fusarium sp. 45; 
the tomato-wilt organism, Fusarium 
lycopersici; the wheat-scab organism, 
Gibberella saubinetii (Fusarium grami - 
nearum); the onion black-mold organ¬ 
ism, Aspergillus niger; a garlic and onion 
bulb-decaying organism, Aspergillus sp. 
4660; Helminthosporium sp. which occurs 
on the outer scales of white varieties; 
and three forms of Botrytis allii , Botrytis 
associated with onion neckrot, Botrytis 
sp. 110, and Botrytis sp. 108a. 
(3) With one exception, Aspergillus 
niger (see below), the organisms germi¬ 
nated and grew quite as well in the ex¬ 
tracts from dry outer white scales as in 
the control drops; but when extracts 
from dry outer colored scales were used, 
germination and growth were greatly 
retarded or entirely inhibited. 
(4) The function of these toxins in 
rendering the bulb resistant to a given 
organism depends upon the organism’s 
mode of attack. 
(5) In the case of the onion Fusaria, 
which enter through insect wounds or 
through openings in the stem plate, 
there is little chance of contact with 
the soluble toxins in the outer scales. 
No evidence of varietal resistance cor¬ 
related with* pigmentation has been 
noted in this instance. 
(6) Aspergillus niger , which nor¬ 
mally attacks the outer scales, and 
which seems to be little affected by the 
outer scale toxins, develops quite as 
well on colored as on white varieties. 
Helminthosporium sp., which is sen¬ 
sitive to the toxins, is evidently limited 
or restricted largely to the outer scales 
of white varieties. 
(7) The neckrot fungi by their mode 
of entrance through the neck tissues 
ordinarily encounter in colored var¬ 
ieties a certain amount of the outer 
scale toxins. The generally observed 
escape of colored varieties from neck¬ 
rot is probably due to this factor. 
More or less neckrot on colored vari¬ 
eties does occur in nature, however. 
Experimentally, it was shown that 
upon inoculation with Botrytis allii 
and Botrytis sp. 110 directly into the 
succulent tissue (thus eliminating the 
influence of the soluble toxins in the 
dry outer scales) infection occurred 
with approximately equal rapidity in 
colored and in white bulbs. It would 
appear therefore that the explanation 
for the occasional occurrence of neck¬ 
rot epiphytotics in colored varieties will 
