June 1, 1925 
Comparative Studies of Pythium debaryanum 
1061 
retraction of protoplasm with laving 
down of successive septa, germinating 
by germ tube; minimum temperature 
6° C. at 24 hours, lower than Pythium 
debaryanum, from which it is further 
distinguishable culturally, particularly 
in a more compact habit of growth. 
Parasitic on Pelargonium sp., causing 
a stem rot. 
In the case of A, the smooth oospore 
lying free in the smooth oogonium 
(specific according to Butler) separates 
it immediately from other large- 
conidial and multi-ant he ridial species. 
Conidia are unknown in Pythium 
proliferum, whose zoosporangia are 
large, spherical, and proliferating, 
with oospores not filling the oogonium. 
P. megalacanthum is multi-antheridial, 
but the oogonial walls are sculptured. 
Type A differs from Pijthium de¬ 
baryanum and the allied species both 
morphologically and physiologically, 
in the following respects: (1) The 
constant larger size of the fruiting 
bodies compared with those of ^the 
other fungi studied on the same 
media; (2) constant and recognizable 
cultural differences; (3) preponderance 
of conidial production; (4) specific 
temperature relations, expressed in a 
higher minimum, and in more luxuri¬ 
ant growth than the others at 20° to 
35.5° C.; (5) difference in the size ratio 
of oospore to oogonium; (6) constant 
accompaniment of the oogonium by 
more than one antheridium, ranging 
from three to eight under conditions 
identical with those at which P. 
debaryanum produced one or rarely 
more than two; it differs from P. 
ultimum in parasitism, measurements 
and absence of intercalary conidia, 
and from P. intermedium in the 
absence of catenulate conidia. It is 
considered a distinct species, for which 
the name Pythium splendens, n. sp., is 
proposed. The technical description 
is appended. 
Pythium splendens, n. sp.: 
Hyphae 3.5 to 9.2 p wide, average 
6.4 p; oospores spherical to ellipsoid, 
smooth walled, lying free in the smooth 
oogonium; ratio oospore to oogonium 
1:1.2; oospores thick walled, produced 
sporadically in cultures; three to eight 
antheridia, clavate, arising at the 
oogonial stalk and from neighboring 
branches; oospores germinating by 
germ tube within a month of forma¬ 
tion; conidia spherical, always terminal, 
smooth walled, large, 21.7 p to 48.9 p 
in diameter, average 36.2 p; hyaline, 
darker on rich media, germinating by 
two to six germ tubes; sickle-shaped, 
clavate bodies formed singly and in 
chains on plate media two days after 
inoculation; temperature ranges 10° to 
37.5° C., optimum 30°; minimum 
higher than that of P. debaryanum 
and more luxuriant growth between 
20° and 35.5; readily distinguishable 
by specific cultural characters. Para¬ 
sitic on Pelargonium, in the tissues of 
which conidia are abundantly formed. 
SUMMARY 
Stem rots of Pelargonium cuttings 
caused by Pythium debaryanum, P. 
debaryanum var. Pelargonii, nov. var., 
and P. splendens, n. sp., are described 
and the comparative morphology and 
physiology of these organisms is 
detailed. 
The signs of the disease consist of a 
blackening and shriveling, starting at 
the cut base and ultimately involving 
the entire plant in a soft rot. P. 
debaryanum var. Pelargonii differs, so 
far as infection is concerned, only in 
the slower progress of the discoloration. 
Inoculation experiments with single¬ 
spore cultures and subsequent reisola¬ 
tions have demonstrated the pathogen¬ 
icity of these organisms on Pelargonium, 
coleus, begonia, cucumber seedlings, 
and radish seedlings, P. debaryanum 
proving the most pathogenic on the 
last two. 
Cultural characters on 16 media are 
described. The three fungi are readily 
distinguishable macroscopically. 
Pythium debaryanum var. Pelargonii 
is characterized by the lowest minimum 
temperature, a character sufficiently 
constant and specific to identify it 
experimentally within 24 hours. P. 
splendens shows the most luxuriant 
growth of the three at 20° to 35.5° C. 
Pythium debaryanum shows the high¬ 
est viability on media and was readily 
recovered after months at room 
temperature. The other two were 
dead after 5)^ months, except P. splen¬ 
dens, which was recovered in one case 
after 8 months. The correlation be¬ 
tween oospore production and viability 
is pointed out. 
Pythium debaryanum var. Pelargonii 
is characterized morphologically by 
greater size variation of conidia and 
their preponderance over oospores, 
which are produced only rarely and are 
often aborted; its oospores are thin- 
walled; one to four antheridia are 
present; irregular swollen intercalary 
bodies are formed through retraction 
of the protoplasm, which lays down 
successive septa; these resting bodies 
are not related to aging of the media 
but are produced within seven days of 
plate inoculation. 
