62 
Journal of Agricultural Research 
Vo 1 . XXIX No 2 
CAUSAL ORGANISM 
ISOLATION 
In preparations made by macerating 
lesions in a drop of water, the invaded 
tissues were found, upon microscopic 
examination, to be teeming with 
bacteria. These bacteria swarm out 
in such abundance as to cloud the water 
and form a turbid suspension, a 
phenomenon characteristic of diseases 
of bacterial origin. If an inoculum is 
prepared by direct maceration of 
young lesions in sterile water and a 
loopful of this inoculum is transferred 
to an agar plate and spread over its 
surface with a zigzag stroke, discrete 
colonies of the pathogen form near the 
end of the stroke. These can then be 
selected for transfer to tube cultures. 
A considerable number of strains were 
isolated by this procedure during the 
course of the present investigation. 
Subsequent comparative study of these 
strains to determine their cultural 
characters and their ability to infect 
soybean showed them all to be identi¬ 
cal. Since it was apparent from the 
preliminary studies that the soybean 
pustule organism is closely related to 
Bad. phaseoli , several strains were 
compared with Bad. phaseoli isolated 
from pod lesions of Lima bean, Phase - 
olus lunatus L., and garden bean, 
Phaseolus vulgaris L.; and with a 
strain of bean blight isolated by Miss 
Hedges. The following parallel studies 
of the morphology and physiology of 
all strains both from soybean and from 
Phaseolus show that all are practically 
identical. 
MORPHOLOGY 
Vegetative cells. —The primary 
cause of soybean bacterial pustule is 
a yellow rod-shaped organism with 
rounded ends. When taken directly 
from lesions it is found to occur singly 
or in pairs, but tends in bouillon to 
form short chains. The organism 
stains readily with all of the more 
common bacteriological stains. When 
stained from 24-hour potato-agar cul¬ 
tures with carbol fuchsin, the elements 
are 1.3 to 2X0.6 to 0.75 n. Such 
preparations, too, show the presence 
of investing material, as is manifest 
also with Welch's capsule stain; but 
this envelope can not be interpreted 
as indicating a well-defined capsule. 
When stained by Gram's method, the 
organism is decolorized. Endospores 
and marked involution forms have not 
been noted. 
When the organism from young 
lesions, from bouillon cultures, or from 
24-hour agar cultures is examined for 
motility, it is seen to possess the power 
of rather active locomotion. That 
this movement is due to the presence 
of a polar flagellum about twice the 
length of the cell has been demon¬ 
strated by several modifications of the 
method of Loeffler. 
Cultural characters.— The vari¬ 
ous media employed in the cultural 
studies were prepared by the methods 
employed by the writer and his asso¬ 
ciates in their studies on the physiology 
of certain plant pathogenic bacteria 
(17). The nutrient broths contained 
1 per cent Difco peptone and 0.3 per 
cent Liebig's beef extract; the nutrient 
agars the same, with the addition of 
1.8 per cent of bacto-agar. The hy- 
drion concentration of cooled media 
was measured colorimetrically and the 
media were not heated after adjust¬ 
ment of reaction. The carbon com¬ 
pounds were sterilized in distilled water 
and added with aseptic precautions to 
cooled media. The cultures were in¬ 
cubated at room temperatures which 
approximated 20° to 25° C. 
Nutrient agar. —The colonies may 
appear in agar plates within 24 hours, 
but are not prominent, nor do they 
show the characteristic yellow color 
until they are 48 hours old. They 
are circular in outline with entire 
margins and a glistening surface but 
with no surface markings. Their con¬ 
sistency varies from nonviscid to 
slightly viscid. In potato agar or 
other nutrient agar containing 3 per 
cent or more of agar, the colonies are 
convex with internal markings (PL 3). 
With media of a low degree of viscosity, 
the colonies are flat and spreading with 
no striking internal convolutions. In 
agar slants the growth is filiform, 
spreading at the base of the slant, with 
an entire or somewhat contoured mar¬ 
gin, glistening and translucent. No 
odor is developed and the agar does 
not become pigmented. 
Potato cylinders. —On steamed po¬ 
tato cylinders, growth is first manifest 
by a faint yellowish streak. This 
becomes abundant within 24 to 36 
hours, is spreading, yellow, and has a 
striking whitish zone along the border 
of the growth. Within six to eight 
