508 
Journal of Agricultural Research 
Vol. XXIX, No. 10> 
Helm i nth oxpori um which occurs on the 
dry outer scales of white onions. Vari¬ 
etal resistance to these groups is not 
uniform. No conspicuous resistance 
to the Fusaria has been observed ( 7 ). 
In the case of neckrot, however, the 
disease is more prevalent on the white 
than on colored varieties. Aspergillus 
niger is common on both colored and 
white varieties. Helminthosporium sp. 
has been found only on white varie¬ 
ties. The data secured for each group 
and their possible bearing upon para- 
istism and resistance will be consid¬ 
ered separately. 
METHODS OF EXPERIMENTATION 
The method previously described was 
employed (4). Spores of the organism 
were suspended in sterile water, and 
equivalent drops of the suspension were 
transferred to glass slides in moist 
chambers. Uniformly white or colored, 
dry, outer scales were secured from 
White, Red, and Yellow Globe onions. 
These were cut into 2 mm. squares. 
Rapid diffusion of toxin into the liquid 
followed the addition of these frag¬ 
ments to the drops. To gain a grada¬ 
tion of concentration, one, two, and 
four fragments, respectively, were 
added to a series of three drops for 
each type of onion scale. This method, 
while obviously subject to some ex¬ 
perimental error, gave the general com¬ 
parison which was desired. Control 
drops were included in each series. 
As will be shown presently, the tox¬ 
icity of the pigmented extracts was 
quite general and the toxic effects were 
of the same general type as those noted 
with the smudge organism (4). 
The effect on germination was ex¬ 
hibited either as complete inhibition or 
by the exudation of cytoplasm from 
the tip of the germ tube, usually shortly 
after the germ tube had formed. The 
latter reaction was not constant, how¬ 
ever. It would seem that a very deli¬ 
cate balance in the concentration of the 
toxin is necessary to bring about the 
phenomenon. Too high a concentra¬ 
tion of toxin evidently completely in¬ 
hibits germination; too low T a concen¬ 
tration allows the more normal hyphal 
development. At a certain point in¬ 
itiation of growth is permitted, but 
this is soon checked in such a way as 
to bring about the exudation of cyto¬ 
plasm. The nature of this phenome¬ 
non is not entirely explained. It is 
suggested that the toxin retards or 
stops the wall-forming process at the 
tip of the germ tube, w hile imbibition 
of liquid by the spore protoplast con¬ 
tinues. This leads to “rupturing” of 
the cell membrane at the weakest 
point followed by exudation of the 
cytoplasm. In certain cases the 
presence of a membrane around the 
exuding cytoplasm is suggested, but 
more often no such confining membrane 
can be distinguished. 
When hyphal development does oc¬ 
cur, the rate of growth is influenced by 
the concentration of the toxin. The 
most reliable criterion for comparison 
w r as found to be the percentage of nor¬ 
mally germinating spores and the rate 
of germ-tube growth of the spores 
which germinated. These were esti¬ 
mated after a given interval by count¬ 
ing 100 or more spores and measuring 
the length of 40 or more germ tubes in 
each drop. As previously noted with 
Colletotrichum circinans, decrease in 
concentration of the toxin caused all 
the organisms studied to return gradu¬ 
ally to normal germination and growtji. 
Consequently the effects were not the 
result of lack of food but were due to 
the presence of inhibitory substances- 
In this report it has been considered 
sufficient to give the data secured from 
the exposure of the various organisms 
to the highest concentration of the 
toxins, that is, where four fragments 
of scale tissue were added to the drop. 
RELATION OF PIGMENT TO DEVEL¬ 
OPMENT OF CERTAIN SPECIES 
OF FUSARIUM 
Two strains of Fusarium which had 
been proved to be pathogenic to onion 
bulbs were used. One of these was 
isolated from bulbs collected in Illinois 
and agrees with the description of 
Fusarium cepae Hanzawa. Further 
studies of this disease are reported in 
another paper (7). The second strain 
was isolated from a decaying onion at 
Valencia, Spain. Since it has not as 
yet been identified with any previously 
described species, it is referred to in 
this paper as Fusarium sp. No. 45. 
Two species of the genus, nonpatho- 
genic to onion, were included—one a 
vascular parasite of tomato, Fusarium 
lycopersici Sacc., and the other a 
parenchymatous invader of wheat and 
corn, Fusarium graminearum Schwabe 
(■Gibberella saubinetii (Mont.) Sacc.). 
Tests with Colletotrichum circinans were 
included for comparison. 
From the data given in Table I it 
wdll be seen that the addition of white 
scale tissue to the spore suspension did 
not materially affect the percentage of 
spore germination of any of the 
organisms studied, with the exception 
of the tomato Fusarium, in which case 
there was some reduction. The addi- 
