COMPARATIVE STUDIES OF PYTHIUM DEBARYANUM 
AND TWO RELATED SPECIES FROM GERANIUM 1 
By Harry Braun 
Assistant Pathologist, Laboratory of Plant Pathology, Bureau of Plant Industry, 
United States Department of Agriculture 
INTRODUCTION 
Stem rots of geranium cuttings 
(Pelargonium spp.)have been frequently 
reported in the literature and ascribed 
to various microorganisms. Ward 
(20) 2 cultured Pythium debaryanum 
on live Pelargonium cuttings in the 
course of his classic studies with this 
fungus. Prillieux and Delacroix (18) 
described Bacillus caulivorus as the 
causal agent of a stem “gangrene ’ 1 
of Pelargonium, potato, clematis, and 
begonia, and reported B. pyocyanus on 
Pelargonium from Germany. Gallo¬ 
way (12) noted a stem discoloration, 
accompanied by shrivelling and soften¬ 
ing, which extended upward from the 
severed end until the whole stem was 
involved. Many bacteria but no fungi 
were observed filling the cells of the 
advancing area of discoloration. The 
disease was considered possibly identi¬ 
cal with that of Prillieux and Delacroix, 
which was also later described by 
Chifflot in a resume of geranium dis¬ 
eases (9). This writer also described 
a Pelargonium stem rot due to Botrytis 
sp. Peters (17) obtained P. debary¬ 
anum from diseased geranium stems, 
which are well illustrated in a colored 
plate showing typical progressive black¬ 
ening of the cuttings. He made suc¬ 
cessful pure culture inoculations and 
also reproduced the disease by placing 
healthy cuttings in soil infested with 
this organism. Inoculation usually re¬ 
sulted in early death; infected cuttings 
seldom remained partially diseased. 
Johnson (16) isolated this organism 
from blackened geranium cuttings. 
Buddin and Wakefield (5) obtained 
P. debaryanum , as well as Botrytis 
cinerea, on geranium stems. 
The writer has had four species of 
Pythium under observation since 1919, 
isolated from diseased geranium cut¬ 
tings. One of these, Pythium complec- 
tens , n. sp., was described in a previous 
paper (4). The second (labeled B 
throughout this study) corresponds 
morphologically to P. debaryanum as 
described by Hesse (15), De Bary (1 9 
2, 8), Ward (20), Butler (6, 7), and 
others who have worked with and 
defined the morphological characters 
of this fungus. The remaining two, 
which have also proved pathogenic in 
pure culture inoculation experiments, 
differ morphologically and physio¬ 
logically from P. debaryanum but are 
clearly related to it. The present 
paper is a report of a comparative 
study of organism B and the two re¬ 
lated fungi, which have not been found 
to be wholly identical with any previ¬ 
ously described Pythium species. 
THE DISEASE 
SIGNS 
The symptoms of disease caused by 
any of the Pythium species here de¬ 
scribed consist of a progressive black¬ 
ening, shrivelling, and necrosis, start¬ 
ing at the base of the cutting and rap¬ 
idly involving the entire stem and the 
petioles (pi. 1). The leaves wilt down 
when the petioles are reached and 
death of the entire plant soon ensues. 
The pith is hollowed out by a soft, 
wet rot. Epidermal cylinders (except 
in very young and succulent cuttings) 
and fibrovascular bundles are not at¬ 
tacked except in the late stages, when 
secondary organisms enter into consid¬ 
eration. Little difference in the char¬ 
acter of the lesions caused by any of 
these fungi can be observed except in 
the rate of progress, which is similar 
for B 3 (P. debaryanum) and A, but 
distinctly less rapid for D. The com¬ 
plete destruction of the cutting is in 
marked contrast with the stoppage of 
infection previously described for P. 
1 Received for publication July 29, 1924; issued August, 1925. 
2 Reference is made by number (italic) to “Literature cited,” 1082. 
3 For the sake of brevity the isolations were lettered A to D in their order of isolation, and will be so desig* 
nated in this paper as far as the section in which their taxonomy is considered; B referring to P. debaryanum , 
C to P. complectens , A to the large conidial Pythium, and D to the form producing the characteristically 
irregular, swollen, asexual fruiting bodies. 
Journal of Agricultural Research, 
Washington, D. C. 
( 1043 ) 
Vol. XXX, No. 11 
June 1, 1925 
Key No. G-486 
