COLONY DIAMETER IN MILLIMETERS 
1060 
Journal of Agricultural Research 
Vol. XXX, No. 11 
ing 7 mm. at 144 hours; no growth was 
observed at 3°. 
Isolation A showed the highest 
minimum, showing no growth at 6° 
even after 144 hours and at 11° only 
after 72 hours. 
The optimum temperatures of the 
three forms do not differ greatly, best 
growth taking place between 27° and 
30°. Between 20° and 35.5°, however, 
the growth of A constantly outstripped 
that of the other two fungi. It was 
also characterized by a higher 
maximum. 
The temperature experiments indi¬ 
cate distinct physiological differences 
between these fungi, which are parti¬ 
cularly evident and constant in the 
ability to grow at low temperatures, and 
in the greater luxuriance of growth of 
A within the range of temperatures at 
which cultures are usually incubated. 
0 3 6 1011 W 16 18 20 23 2 7 30 33 35.5 373 
TEMPERATURE (° C) 
Fig. 3.—Temperature relations plotted in colony 
diameter growth for 25 hours 
TAXONOMY 
The morphological character of these 
fungi places them in Butler’s group of 
Pythium species which includes forms 
with a smooth-walled oospore lying free 
in a smooth-walled oogonium, and with 
spherical conidia or sporangia. These 
include (1) P. debaryanum, (2) P. vex- 
ans , (3) P. ultimum, (4) P. anguillulae 
aceti. The first is considered identical 
with the writer’s isolation B in meas¬ 
urements, relative abundance of 
oospores and conidia, in cultural char¬ 
acters so far as detailed by Hawkins 
(14) and by Edson (10), and in appear¬ 
ance as figured by De Bary, Ward, and 
others. Zoospores were not obtained, 
but neither were they found by Ward, 
Edson, or Johnson (16). It also corre¬ 
sponds with P. debaryanum as figured 
from geranium by Peters (17), although 
the originality of his drawings is not 
clear in view of the fact that he repeated 
Hesse’s mistake of placing a single 
cilium on the zoospores, the biciliate 
nature of which was pointed out by 
De Bary (3). 
Isolation D, while corresponding 
closely to P. debaryanum in measure¬ 
ments, differs in the following respects: 
(1) Constant and readily distinguish¬ 
able cultural differences; (2) specific 
temperature relations, evident by 
growth at the lowest temperature of the 
four species of Pythium studied; (3) a 
distinctly slower infection rate as com¬ 
pared with B and A; (4) a preponder¬ 
ant tendency toward a sexual reproduc¬ 
tion, oospores appearing only rarely and 
then often aborted; (5) a thinner 
oospore wall; (6) greater variation in 
the size of conidia as compared with B; 
(7) the appearance, within a short time 
after inoculating plates, of irregular 
swollen bodies formed by the retraction 
of protoplasm with the laying down of 
successive septa, germinating by a germ 
tube and also differing in size and ab¬ 
sence of lobing from the “presporangia’' 
of P. butleri . 
In the above respects D does not 
correspond with any of the other species 
grouped with but distinguished from 
Pythium debaryanum. In its parasitism 
and scarcity of oospores it differs 
from P. ultimum; P. intermedium, in 
which oospores are unknown, is charac¬ 
terized by catenulate conidia, which are 
not present in this fungus. The writer 
considers it a variety of P. debaryanum , 
to which it is clearly closely related, and 
proposes the name Pythium debaryanum 
var. pelargonii. The technical descrip¬ 
tion follows: 
Pythium debaryanum var. pelargonii 
nov. var. 
Hyphae coenocytic, hyaline, richly 
branching from thick main hyphae, 
3.8 fx to 9.8 n wide, average 6.2 m; 
oospores spherical or subspherical, 
smooth, diameter 15.9 m to 19.9 m, 
average 17.8 m; free in the smooth- 
walled oogonium, thin-walled, scarce in 
culture media, often aborted; oogonial 
diameter 17.4 m to 21.9 m, average 20.1 m; 
one to four antheridia, clavate, often 
adhering to the oogonium along their 
entire length, usually arising from a 
neighboring branch; conidia spherical 
or subspherical, terminal and inter¬ 
calary, smooth, diameter 12.8 m to 27.7 m, 
average 20.1 m; hyaline, germinating by 
one to three tubes, formed abundantly 
on media; sickle-shaped bodies formed 
singly or in chains two days after inocu¬ 
lations of plates; irregular swollen 
intercalary bodies formed in plates 
within seven days after inoculation, by 
