312 
Journal oj Agricultural Research 
Vol. XXX, No. 4 
abruptly varying in width; occurring 
in nature within cortical cells of host, 
in nutrient solutions as extensive nebu¬ 
lous translucent mycelia. 
Sporangia in artificial culture arising 
by conversion of extensive portions of 
vegetative mycelium delimited by one 
or more septa; often including many 
ramifications; discharging through one 
or several (up to four) tapering 
branches, the distal portions of which 
measure usually approximately 4 /z- 
Zoospores cylindrical, in escaping 
from evacuation branches becoming 
attenuated to vermiform bodies, usu¬ 
ally 3.5 ju in diameter by 30 to 50 /z 
in length; forming spherical cysts at 
mouth of sporangium, measuring usu¬ 
ally 8 to 11 n in diameter, rarely up 
to 16 jjl; diplanetic, the empty spheri¬ 
cal wall being distinguished by a pro¬ 
truding evacuation tube 1 n long by 
2.5 to 3 m in diameter. 
Oogonium generally, if not always, 
terminal on a short lateral branch, 
from which it is delimited by a par¬ 
tition sometimes present as a simple 
septum, at other times as a columella¬ 
like structure protruding into the 
oogonial cavity; subspherical, meas¬ 
uring usually 25 to 35 a* in diameter; 
when mature exhibiting a heavy periph¬ 
eral wall with smooth outer contour 
and sinuous inner contour, hence of 
irregular thickness, this dimension 
varying between 1 to 5 /z, generally 
between 1 to 2.5 /z. 
Antheridia typically of diclinous 
origin, borne on a stalk frequently 
involved with the oogonial stalk, and 
often branching once or several times; 
measuring 8 to 10 a* in diameter by 
15 to 18 /z in length, or when con¬ 
siderably larger often more conspicu¬ 
ously arched, somewhat lobulate, and 
becoming compound by the insertion 
of transverse septa. 
Oospores subspherical or more rarely 
ellipsoidal owing to intruding columella¬ 
like seotum; 18 to 25 n (generally 20 to 
23 aO in diameter; provided with a 
wall of uniform thickness, this dimen¬ 
sion varying between 1.2 and 1.8 a * 
(generally 1.5 a*); slightly eccentric in 
internal structure (“subcentric”); ger¬ 
minating without protracted resting 
period either directly by 1 to 3 germ 
hyphae or by production of a single 
unbranched sporangial filament usually 
200 to 350 /z in length, in the latter event 
producing generally 13 to 18 zoospores, 
approximately half of which are de¬ 
limited within oospore wall. 
Collected repeatedly in diseased peas 
in Maryland, New York, Ohio, Indi¬ 
ana, Michigan, Illinois, Wisconsin, 
Montana, Idaho, Utah, and California. 
PHYSIOLOGY OF PARASITE 
GROWTH OF THE FUNGUS ON CULTURE 
MEDIA 
The fungus has been grown on sev¬ 
eral of the common culture media, on 
all of which it produces more or less of 
a sparse white surface growth with 
little aerial mycelium. A few imper¬ 
fectly formed oospores are usually 
found in older cultures on most media. 
However, on corn-meal agar, while the 
mycelial growth is rather less than on 
most substrata tried, there is a moder¬ 
ately abundant production of oospores 
which are capable of germination soon 
after they are formed. For this reason 
cultures on this substratum retain their 
vitality indefinitely, while those on 
other media soon perish. Semisolid 
media, such as cornmeal or oatmeal 
with varying amounts of water added 
also give good growth, but the oospores 
are less perfectly formed as the medium 
becomes more moist. The most use¬ 
ful liquid medium that has been found 
for the production of mycelium and 
sporangia is a pea decoction made by 
adding 10 to 20 peas, preferably of a 
wrinkled variety, to about 75 cc. of 
water in a flask. This liquid remains 
clear after sterilization in the auto¬ 
clave, and mycelium growing in it can 
readily be washed free from all nutrient 
material. The fungus grows as a sub¬ 
merged tuft in this medium until it 
reaches the surface, over which it 
spreads as a mat. In about 7 to 10 
days at room temperature oospores 
begin to form, and soon after most of 
the mycelium is found empty and dead. 
EFFECT OF TEMPERATURE UPON THE 
DEVELOPMENT OF THE FUNGUS 
Growth of mycelium. —The fact 
that this disease develops early in the 
spring indicates that the fungus must 
be able to thrive at comparatively low 
soil temperatures. In an experimental 
determination of the thermal limits 
within which the fungus will grow a 
semiliquid medium, composed of 1 part 
by weight of cornmeal with 12 parts of 
water, was used. The growth of the 
fungus in the surface of this gelatinous 
material is so inconspicuous that small 
increments of growth can not be 
measured accurately, and while the 
limits of growth were determined, the 
optimum could only be roughly esti¬ 
mated. In cultures held at a series of 
constant temperatures in incubators 
vigorous growth occurred at 34° C., 
but no growth was observed at 37°. 
At the lower end of the series no growth 
was observed after six days at 8° to 10°, 
