512 
Journal o f Agricultural Research 
Yol. XXIX, No. 10’ 
neck and, as already noted (4), these 
are commonly affected with smudge 
during the latter part of the growing 
season. Nevertheless, some pigment 
develops in the underlying neck scales, 
and it becomes more intense with the 
desiccation of these tissues at maturity. 
The neckrot fungi entering at this 
latter period are thus exposed to the 
soluble toxins of dry colored scales, 
from which it is certain that the 
soluble toxins readily diffuse into any 
drops of moisture present. 
It is believed that at this point the 
toxins associated with the scale pig¬ 
ments become the principal factor of 
resistance to Botrytis and serve as a 
barrier to infection. It is obvious that 
conditions are not always favorable to 
the expression of this resistant charac¬ 
ter. Frequently bulbs are harvested 
and the tops removed before complete 
maturity. This condition is unfavor¬ 
able to the exclusion of the neckrot 
fungi, since it leaves fewer dry colored 
scales on the necks of colored bulbs 
and exposes a greater amount of succu¬ 
lent wound tissue, which is most con¬ 
ducive to the establishment of the 
fungus. 
From this analysis of the question it 
may be assumed that the colored varie¬ 
ties of onion possess a quality of resis¬ 
tance to the three species of Botrytis 
causing the various types of neckrot. 
This resistance functions as a barrier 
to initial establishment of the fungi in 
the succulent tissues. Certain con¬ 
ditions of environment and handling of 
the crop may be expected to overthrow 
this means of defense at times. Resis¬ 
tance, therefore, in this case, as in most 
others, is not to be considered as 
absolute. There is, however, a definite 
substance (or group of substances) 
associated with pigmentation of the 
bulbs of colored varieties of onion 
which is toxic to the neckrot fungi, and 
this accounts in the main for the 
reputed resistance of these types to 
neckrot as compared with the well- 
known susceptibility of white varieties. 
DISCUSSION OF RESULTS 
Certain outstanding facts in the in¬ 
vestigation to date of disease resistance 
in the onion may well be reviewed at 
this point. There exist in the onion 
tissue substances which, when ex¬ 
tracted, exhibit a surprising amount of 
toxicity not only to fungi nonpatho- 
genic to onion and to its mild para¬ 
sites, but to its more aggressive para¬ 
sites as well. . Yet a fungus parasite 
is frequently found invading the tissue 
of the onion bulb, often in close proxim¬ 
ity to these toxic substances or to sub¬ 
stances from which the toxins are rap¬ 
idly formed upon injury to the tissues. 
Therefore the mere presence of toxic 
substances in the host tissue does not 
necessarily imply resistance. The ave¬ 
nue of invasion and the device by 
which the organism parasitizes the 
host cell influence the ultimate effect 
which toxic substances within this cell 
may have upon the invader. 
The studies on volatile onion oil (6) 
show that the toxicity in this case is 
general rather than specific to a given 
organism. Although the more rapidly 
invading bulb parasites are as a rule 
less sensitive to the volatile toxins 
than are the milder parasites, they do 
not ordinarily germinate or grow in 
the extracted onion juice, and are re¬ 
tarded materially by the volatile toxins 
from undiluted juice. Moreover, there 
is no correlation between the conspicu¬ 
ous resistance of colored varieties to 
smudge and neckrot and the amount of 
volatile toxins in the tissue. While it 
is postulated that the different degrees 
of retarding effect of the volatile toxins 
upon the different organisms may be 
responsible in part for the variation in 
parasitism of these organisms, this 
variation may be influenced by other 
factors as well, such as (1) mode of 
entrance, (2) method of attacking the 
host cell, (3) advance lethal effects of 
fungus enzymes, (4) food elements in 
the host. 
The study of the range of toxicity of 
outer colored scale extract shows that 
this substance, as in the case of the 
volatile oil, is quite generally toxic to 
the fungi. With one notable excep 
tion (Aspergillus niger), the germina¬ 
tion and growth of all of the organisms 
tested (those pathogenic and those non- 
pathogenic to onion) are decidedly in¬ 
hibited in colored scale extracts, while 
they thrive in the extract of white 
scales. But here again the correlation 
between color and resistance is not 
general for all the bulb diseases of onion. 
It would appear from the present 
studies that the toxic substance in col¬ 
ored scales becomes functional as a 
repellent principle in the dry outer 
bulb scales. There it diffuses readily 
into soil or meteoric water and inac¬ 
tivates the fungus, preventing its en¬ 
trance into the living tissue of the 
underlying succulent scales. Conse¬ 
quently, those parasites which by na¬ 
ture of their respective channels of 
invasion come into direct contact with 
this toxic substance upon attacking 
the onion bulb would be expected to 
appear less frequently on colored than 
on white varieties. 
