Nov. 15, 1924 
Onion Scale Pigmentation 
511 
noted. A high percentage of the white 
bulbs was infected with this form in 
both years, while in no case was the 
fungus found on normally pigmented 
bulbs of the colored varieties. In 1920 
the Yellow Globe seed used was not 
of a pure strain. A small number of 
the bulbs formed very little yellow 
pigment. B . sp. 108a developed on 
these, illustrating further the close cor¬ 
relation between pigment and resist¬ 
ance. 
In spite of this marked expression of 
resistance, it is not uncommon to find 
cases where a fairly large percentage of 
colored bulbs are decayed with neckrot. 
That the extent of resistance to neckrot 
is limited is further substantiated by 
experiments in which spores and my¬ 
celium of Botrytis allii and Botrytis sp. 
110 were injected through needle 
wounds into the succulent neck tissue of 
white, red, and yellow bulbs. Under 
such conditions the organisms were only 
-slightly, if at all, exposed to the water- 
extractable toxins in the dry outer 
scales. In all of these experiments both 
organisms caused colored and white 
bulbs to decay with apparently equal 
rapidity. It is evident, therefore, that 
the resistance of colored bulbs to B. 
allii and B. sp. 110 is a resistance to 
initial infection, and where this is not 
prevented and the fungus becomes 
established in the succulent tissue all 
forms are equally susceptible. 
Data secured from a study of the 
effect of outer-scale extracts upon germ¬ 
ination and growth are of interest at 
this point. A strain of Botrytis drierea 
isolated from cyclamen and nonpatho- 
genic to onion was included in these 
tests. Table III shows that exposure to 
white scale extract had no detrimental 
effect on any of the forms tested; and 
in some instances the result was bene¬ 
ficial. In the case of yellow and red 
scale extracts the toxic effects are quite 
as marked as already noted for Fusa- 
rium, Aspergillus, Helminthosporium, 
and Colletotrichum. In two experi¬ 
ments with B. sp. 110 a high percentage 
of germination occurred in yellow scale 
extract, but it is to be noted that very 
meager growth resulted in these cases. 
From the data just cited, resistance 
of colored bulbs is to be expected where 
the organisms are exposed to the 
toxins present in the outer colored 
scales. To what extent does this cor¬ 
relation exist in nature? The three 
forms of Botrytis parasitic on onion do 
not ordinarily attack the growing 
plants, but subsist as saprophytes until 
harvest time. As the tops of the 
onion mature, entrance is gained 
through the “neck” of the bulb. 
Comparable lots of White Globe bulbs, 
with the tops* removed from one lot 
and left intact in the other, showed 
that in an appropriate environment 
the infection occurs as readily through 
an intact neck as through a wounded 
one. Therefore as the top dies down 
these organisms may be expected to 
invade the dying tissue at the neck and 
gradually enter the succulent tissue of 
the bulb. A considerable amount of 
moisture is necessary for growth at this 
point, as the senior writer has demon¬ 
strated ( 5 ) that Botrytis sp. 110, at least, 
can be almost completely checked by 
artificial drying of the neck tissues after 
incipient infection has taken place. In 
the necks of colored bulbs at this stage 
of maturity there is ordinarily a con¬ 
siderable quantity of intensely colored 
dead leaf or scale tissue. The extreme 
outer scales of colored bulbs usually 
develop little or no pigment at the 
Table III.— The effect of water extracts from dry outer scales of White, Yellow r 
and Red Globe varieties of onion on the germination and growth of several species 
of Botrytis as compared with Colletotrichum circinans 
Organism 
i 
Experi¬ 
ment 
No. 
Germination in— 
Length of germ tubes in— 
Con¬ 
trol 
- 
i 
! 
! White 
scale 
extract 
Yellow 
scale 
extract 
Red 
scale 
extract 
Con¬ 
trol 
White 
scale 
extract 
Yellow 
scale 
extract 
Red 
scale 
extract 
Per cent 
Per cent 
Per cent 
Per cent 
Microns 
Microns 
Microns 
Microns 
Colletotrichum circinans _ 
1 
99 ! 
80 
0 
2 
414 
224 
0 
a 
f 1 
64 ! 
55 
0 
0 
280 
294 
0 
0 
Botrytis allii _ 
\ 2 
97 
94 
4 
1 4 
491 
554 
5 
28 
l 3 
49 
55 
0 ! 
0 
f 1 
53 
99 ! 
1 
0 
24 
209 
5 
6 
B. sp. 110 _ 
l 
53 
90 
100 ! 
97 ! 
M ol 
0 
0 
24 
82 
10 
0 
1 4 
90 
100 ' 
93 
10 
f 1 
100 i 
100 , 
4 1 
14 
341 
839 
27 
39 
IS . Sp* jUou _ - _ — 
l 2 
72 
90 1 
8 
2 
125 
394 
15 
8 
B. cinerea _ 
1 1 
93 
96 
0 
3 
■ 143 
166 
0 
7 
