May 17,1924 
A Fusarium Bulb Rot of Onion 
687 
The fungus grows best in a slightly acid medium. There was very little 
change in the reaction of the medium during the experiment, even at the points 
where most profuse growth occurred. The acidity of the scale tissue of nearly 
mature onion bulbs of the red, yellow, and white types was found to vary between 
P H 5.3 and 5.4. The fungus grows well beyond the acidity limits ordinarily en¬ 
countered in the host tissue. Moreover one would expect the reaction of the soil 
to materially affect the fungus only in the case of very high acidity or alkalinity. 
PATHOGENICITY 
The repeated isolation of this species from decaying onion bulbs was evidence 
of its pathogenic properties. This fact was further substantiated by inoculation 
of growing onion plants and of mature bulbs. 
INOCULATIONS IN THE GREENHOUSE 
Apparently healthy onion sets were used; they were prepared by removing 
the dry outer scales, sterilizing in 1 to 1,000 mercuric chlorid solution for 15 min¬ 
utes, and rinsing several times in sterile water. Each bulb was wounded by 
piercing the base of the scales with a sterile needle. Those bulbs to be inocu¬ 
lated were placed in a water suspension of macroconidia of the fungus for an hour 
or longer; those to be used as controls were similarly exposed to sterile water. 
Subsequently the bulbs were removed and planted in pots of soil in a green¬ 
house in which the air temperature ranged from 18° to 24° C. 
Several experiments following this plan were conducted and the final results 
are given in Table II. Both inoculated and uninoculated bulbs produced leaves 
promptly, the top growth of the former, as a rule, being noticeably reduced from 
the outset. The first signs of disease usually appeared on the tenth to twelfth 
day. The green color at the tips of the leaves faded somewhat followed by a 
gradual loss of turgidity. Progressive wilting of the tops through several days 
resulted finally in complete death of the plant. (See PI. 2, A.) Simultaneously 
a decay of the leaf bases below ground occurred. The progress of the disease, 
as will be shown later, is materially influenced by temperature. 
Table II .—Results of greenhouse inoculation experiments with Fusarium cepae 
Experi¬ 
ment 
number 
Number 
plants in¬ 
oculated 
Plants dis¬ 
eased 
Number 
of con¬ 
trols 
used 
Condition 
of controls 
at end of ex¬ 
periment 
1 
18 
Per cent. 
100 
18 
Healthy. 
2 
24 
100 
24 
Do. 
3 
14 
100 
14 
Do. 
4 
32 
100 
24 
« Do. 
5 
10 
100 
8 
Do. 
a One bulb affected with bacterial decay; no evidence of Fusarium rot present. 
One hundred per cent infection resulted in each inoculation experiment while 
the controls remained free from Fusarium rot. The fungus was repeatedly reiso¬ 
lated from diseased plants and was proved to be identical with the original culture. 
When wounded bulbs were planted in sterilized soil inoculated with a spore sus¬ 
pension of the organism, the disease developed in a quite similar manner and 80 
per cent of the plants became infected. 
It is quite evident that the fungus is an aggressive wound parasite of onion 
plants. In order to determine whether or not the fungus is capable of infecting 
through the unbroken surface, 18 apparently sound sets were planted in inocu¬ 
lated soil. The same number of bulbs were planted in sterilized soil as controls. 
In four of the plants in inoculated soil the characteristic symptoms of the dis¬ 
ease developed and from them the fungus was reisolated. The other plants re¬ 
mained healthy, as did the controls. It thus appears that the fungus will invade 
apparently sound plants but the percentage of infection is much less than in the 
case of wounded plants. It is well to recall here the common observation that 
