Oct. 15, 1924 
Geranium Stemrot 
403 
Mar. 20, 1920.—Five fresh cuttings 
planted in pots previously containing 
soil and debris of infected plants of 
March 3; no direct inoculation made 
from cultures. Two cuttings planted 
in fresh pots as checks. Two of the 
former showed typical infection on 
sixth day; oospores present. Remain¬ 
der healthy. 
Sept. 15, 1921.—Ten cuttings inocu¬ 
lated at base. Nine infected. Prog¬ 
ress of discoloration shown in Figure 1. 
The three checks remained healthy. 
Aug. 17, 1922.—Ten cuttings, basal 
inoculation; soil sterilized three times. 
-Sunken base within two days; for dis¬ 
coloration progress see Figure 1. 
Sharp demarcation eighth day; pith 
and cortex rotted. Typical colonies 
obtained in reisolations made August 22. 
Five checks. These remained healthy. 
July 26, 1923.—Thirty inoculated. 
Ten plants (set A) were placed on 
moist sand for two days before inocu¬ 
lation. The remainder (set B) were 
inoculated immediately on removal 
from stock plants. All examined a 
month later, August 26. Seven of set 
A healthy. Four of set B completely 
Totted down; remainder showed sharp 
demarcation, still alive and turgid, 
over hollow blackened stem below; 
epidermal cylinder intact, fibrovas- 
cular bundles forming a shredded inner 
-cylinder; one plant had three adven¬ 
titious roots starting from above the de¬ 
marcation, which took place just above 
ground. Diseased areas 20 to 45 mm. 
long. Abundant typical oospores in 
tissues. The 10 checks remained sound. 
Sept. 24, 1923.—Three Coleus cut¬ 
tings inoculated. The inoculated 
plants rotted completely within five 
days, without demarcation. The one 
-check remained healthy. 
Oct. 18, .1923.—Seedlings of water 
cress, cucumber, and radish in dupli¬ 
cate 6-inch pots inoculated with and 
without scalpel injury. No infection 
in any after two weeks. 
Feb. 2, 1924.—Seedlings of cucum¬ 
ber and radish in glass moist cham¬ 
ber were inoculated, without injury. 
There was no infection on cucumbers. 
Black streaks appeared at points of 
inoculation on radish stems without 
progressing much or affecting turgidity. 
Feb. 16, 1924.—Seedlings of cucum¬ 
ber and radish (six each) grown under 
sterile conditions on Knopfs agar in 
test tubes, inoculated with and with¬ 
out wounding. The mycelium made 
some growth on the agar, but the cu¬ 
cumbers remained turgid and w^ere not 
discolored. Black streaks not present 
in the checks appeared on radish stems 
but without affecting turgidity. 
PATHOLOGICAL HISTOLOGY 
The hyphse are largely intracellular 
and show a constriction when passing 
through cell walls. They are found 
one or two cell layers ahead of cells 
with discolored walls, indicating that 
there is no lethal action in advance or 
that penetration is more rapid than 
visible degeneration and discoloration. 
Branched coils and nests of hyphae may 
be observed within the older infection 
area. Long strands run within and be¬ 
tween sieve tubes and companion.cells, 
with branches reaching out into cortex 
and pith. Oospores are present in 
great abundance within cells some dis¬ 
tance back of the advancing margin 
and may be observed in progressive 
stages of maturation in a single sec¬ 
tion. Xylem and the cuticularized 
epidermis are not invaded. Penetra¬ 
tion of the subepidermal cork layers 
from within rarely proceeds farther than 
the innermost, youngest, and least su- 
berized layer. 
Infected cells are killed immediately 
or very shortly after penetration, the 
walls soon taking up the brown dis¬ 
coloration. The thin-walled pith cells 
are the first to lose turgidity, resulting 
in the sunken appearance of the pith 
at the base of infected cuttings. Later 
the cortex also breaks down. The region 
of collapsed cells is often sharply de¬ 
limited from adjoining infected but still 
turgid tissues (PI. 2). Both pith and 
cortex are soon hollowed out by gela- 
tinization and solution of their walls. 
The host nucleus is discolored but not 
corroded, and may be recognized for a 
long time after the cell has collapsed. 
Starch appears to be as abundant in 
diseased as in healthy tissues, except 
as noted below; mature ^ospores in 
collapsed cells may often be found 
among masses of uncorroded starch 
grains still embedded in their plastids. 
Stained sections through the stem 
at the line of demarcation reveal an 
interesting condition. A cork cambium 
4 to 12 cells thick extends irregularly 
but completely across, marking off 
healthy from diseased tissues (PI. 3). 
It is usually indented upwards when 
crossing the fibrovascular bundles, 
along each side of which it may extend 
for some distance. The walls of the 
lowermost cambium layers (nearest the 
diseased tissues) are collapsed and take 
the gentian violet in the Flemming 
triple stain, indicating suberization. 
This conclusion has been confirmed by 
microchemical tests with Sudan III, 
Scarlet Red, alcoholic solution of 
chlorophyll, iodine and sulphuric acid, 
alkannin, and iodine. The walls of 
