GERANIUM STEMROT CAUSED BY PYTHIUM COM- 
PLECTENS N. SP. 
HOST RESISTANCE REACTIONS; SIGNIFICANCE OF PYTHIUM 
TYPE OF SPORANGIAL GERMINATION 1 
By Harry Braun 
Assistant Pathologist, Laboratory of Plant Pathology, Bureau of Plant Industry 
United States Department of Agriculture 
INTRODUCTION 
Geranium cuttings (Pelargonium) 
are often affected with a blackening 
and decay of the roots and stems, which 
may result in complete rotting of the 
young plant. During the latter part 
of 1919, Pythium de baryanum and 
three other fungi belonging to the same 
genus were isolated from blackened 
geranium stems in the agricultural 
greenhouses at Washington, D. C., and 
were found capable of reproducing this 
condition in healthy cuttings. This 
paper is an account of one of these 
isolations, which differed very markedly 
in morphological and cultural char¬ 
acters from P. de baryanum, previously 
reported by Peters (14 ) 2 and recently 
by Buddin and Wakefield (5) as causing 
a geranium stemrot. It was further 
frequently characterized by the stimu¬ 
lation of a definite resistance reaction 
on the part of the host, evidenced by 
the formation of a cork cambium 
within and across the stem at some 
point in advance of infection, barring 
further progress of the hyphse after 
Infection and rotting had already pro¬ 
ceeded some distance from the point 
of inoculation. 
SIGNS OF THE DISEASE 
The early stage of the disease, as 
caused by any of the Pythium spp. 
isolated, consists essentially of a pro¬ 
gressive blackening and necrosis . ac¬ 
companied by wet rot of noncutinized 
and nonlignified tissues, usually com¬ 
mencing at the base of the cutting 
below the ground. Infected plants are 
not firmly embedded in the soil and 
offer little resistance to pulling, owing 
to their inability to form binding 
secondary roots or destruction of these 
when already present. General turgid- 
ity is not affected at first, so that 
diseased cuttings may appear normal to 
the eye until the discoloration has 
progressed above ground. 
After this stage the signs caused by 
the organism at present under con¬ 
sideration may be distinguished from 
those caused by the other Pythium spp. 
isolated, by the deeper dead-black dis¬ 
coloration, which progresses much more 
slowly up the stem and finally stops, 
usually 20 to 40 mm. above ground. 
This results in a sharp line of demarca¬ 
tion between the healthy, turgid green 
tissue and the shriveled black diseased 
portion below (PI. 1, . A). Under 
very warm, moist conditions, or when 
soil nematodes are abundantly present 
in the rotted tissues, infection may 
continue until it involves the entire 
plant, which wilts and rots on the 
ground. The characteristic limitation 
of infection is, however, more fre¬ 
quently observed in both naturally 
infected and artificially inoculated 
plants, often within six days after 
inoculation. Cuttings showing this 
type of infection may remain turgid 
above the dead portion of the stem so 
long as- four weeks. No further growth 
takes place, however, and the plants 
remain dwarfed but turgid until they 
finally topple over through some ex¬ 
ternal mechanical cause or through the 
continual weakening of the supporting 
base. If the stoppage of infection 
occurs sufficiently near the surface of 
the ground, secondary roots may be put 
forth just above the line of demarcation. 
A more detailed examination of a 
diseased cutting in the early stage of 
infection shows a narrow gray to brown 
advancing margin, involving all stem 
tissues. Below this the discoloration is 
a deeper brown to black, and is accom¬ 
panied by loss of turgidity in pith and 
cortex. These tissues are crushed in 
1 Received for publication April 16, 1924—issued January, 1925. 
2 Reference is made by number (italic) to “Literature cited,” p. 419. 
Journal of Agricultural Research, 
Washington, D. C. 
( 399 ) 
Vol. XXIX, No. 8 
Oct. 15, 1924 
Key No. G-465 
