186 
Journal oj Agricultural Research 
Vol. XXX, No. 2 
remains active apparently until the 
juice is destroyed by bacterial fermen¬ 
tation. The other type is volatile, and 
at ordinary room temperatures largely 
disappears from extracted juice within 
a few hours. It is destroyed by dis¬ 
tillation at ordinary pressure. A third 
type of substance toxic to fungi is found 
in the dry outer scales of colored onion 
bulbs, and studies on it are published 
in other papers ( 6, 9). While some of 
this third type may exist in the extract 
of fleshy colored scales it is probably 
too dilute there to be effective, and is 
considered distinct from the two types 
discussed here. These last two types 
appear to exist to about the same extent 
in white and colored onion bulbs. 
By testiifg the reaction of these fungi 
of the two types of toxic substances 
under as nearly uniform conditions as 
possible some interesting results have 
been noted. The response to the two 
kinds of toxin is in general very similar 
for each fungus. Comparing onion 
pathogens and nonpathogens no strict 
negative correlation was found be¬ 
tween pathogenicity upon onion and 
sensitiveness to the toxins. This 
simply indicates that other factors 
enter into the determination of the 
parasitic relation. 
But to compare the onion pathogens 
as among themselves, it must be re¬ 
membered that this method of exposure 
to the toxins is only an approximation 
to natural conditions. Whereas in our 
tests the fungus was exposed directly 
to the dissolved and volatile toxins, it is 
probable that in nature this exposure 
is not so severe nor direct. In the case 
of each of the bulb parasites considered 
here the host cells are affected some¬ 
what in advance by the fungus enzymes 
and in this interaction the inhibitory 
effects of these host toxins are probably 
weakened. But in any case, it is 
plausible to suggest that these toxins 
may retard the advance of the parasite 
according to the specific sensitiveness 
of the latter. In support of this is the 
fact that Colletotrichum cirdnans , al¬ 
though it is able to penetrate the onion 
scales readily, is the most sensitive to 
the host toxins and is among the least 
aggressive as a parasite. Aspergillus 
niger , though slightly less sensitive to 
the toxins, is also decidedly limited in 
its properties as a decay-producing 
organism. Botrytis allii , once estab¬ 
lished, is one of the most aggressive 
of the bulb parasites, and in addition 
to its advantage of possessing more 
active cytoltic properties, it is also 
much less sensitive to the host toxins. 
Botrytis sp. 108a is slightly less sensi¬ 
tive than Botrytis allii , although we 
should expect it to be somewhat more 
so should this factor be the only one 
which determines its weaker aggress¬ 
iveness. In this case, and indeed in 
all the forms considered, further study 
should be directed toward a considera¬ 
tion of the enzym activities and food 
requirements of the organisms. The 
other active bulb invaders, Fusarium 
cepae, Fusarium sp. 45, and Asper¬ 
gillus sp. 4660, fall into the general 
class with Botrytis allii as to their sen¬ 
sitiveness to the toxins and this is in 
correlation with their parasitic activity. 
It would appear from the evidence at 
hand that the toxic constituents of the 
host cell sap of the onion may exert 
an important influence on the aggress¬ 
iveness of the fungi pathogenic to that 
plant. Any broader generalization at 
this time would not be justified. 
SUMMARY 
1. This investigation is a continua¬ 
tion of one previously reported in 
which was demonstrated the toxic 
nature of juice extracted from fleshy 
onion bulb scales toward the onion 
smudge organism (Colletotrichum cir- 
cinans). 
2. These toxins are of two general 
types, one which is neithei removed 
nor broken down readily by heat and 
one which is volatile and passes off 
from the extracted juice at room tem¬ 
perature within a few hours. 
3. There is a gradual decline during 
storage of onion bulbs in the amount 
of volatile toxin which may be released 
from the extracted juice. This decline 
is also evident on sprouting. 
4. In general fungus spores become 
more sensitive with age to the volatile 
toxins. 
5. The temperature at which spores 
are tested may have some influence on 
their reaction to the volatile toxins and 
it thus becomes essential to provide as 
uniform conditions as possible for 
comparative studies. The depletion 
of volatile toxins in onion juice is has¬ 
tened by increase in temperature. 
6. The primary purpose of the 
investigation was to compare repre¬ 
sentative onion bulb parasites with one 
another and with other fungi nonpatho- 
genic to onion in their reaction to these 
toxins. The following bulb parasites 
were included: Colletotrichum circinans } 
Boytrytis allii, Botrytis sp. 108a ; Fusa¬ 
rium cepae, Fusarium sp. 45, Aspergillus 
niger. Aspergillus sp. 4660. Nonparasitic 
forms were: Colletotrichum lindemu- 
thianum, C. trifolii. C. pisi, Glomerella 
cingulate , Botrytis cinerea, and Fusar¬ 
ium graminearum (Gibberella sauhinetii ). 
