6 
Journal of Agricultural Research 
Vol. XXIII, No. i 
the fungus grew into contact with these blocks of agar, which were soon 
after turned again over a clean part of the plate and after several hours' 
growth had been permitted, were found to be free from all contamina¬ 
tions. These sources were used to inoculate several clean plates, and 
from them a large number of cultures were made on corn meal, oatmeal, 
and lima bean agars. Two more strains indentical in character with the 
first were obtained in this manner, one from a garden and the other 
from a commercial field, both in the District of Columbia. 
PROOF OF PATHOGENICITY 
A large number of rhubarb seeds were planted in flower pots in the 
Washington pathological greenhouses, and about the middle of October, 
1917, they were large enough for some preliminary inoculation experi¬ 
ments. 
Inoculation No. i. —A seedling planted August 31 was inoculated 
on October 19 by puncturing with a sterile needle a leaf petiole near its 
base and applying a small bit of a culture from a corn meal agar tube. 
Five days later, not only the inoculated leaf but all the others on the plant 
had fallen to the ground. Plate 6, A, shows this plant, together with 
the control similarly treated except for the application of the fungus, 
photographed October 26. 
Inoculation No. 2. —On October 19 a similar plant was inoculated 
by the application of the culture without injury at the base of a leaf 
stalk. The plant was covered with a bell jar which was removed in 
24 hours. This treatment was applied to two plants. On October 24 
one of the plants was dead, and the other showed the outer leaves wilted 
down. The control was perfectly normal. 
Inoculation No. 3 .—A plant similar to the others was inoculated as 
in No. 2, but the bell jar was not applied, moist earth being piled up 
around the inoculum. At the expiration of five days this plant was dead. 
Inoculation No. 4 was identical in method and results with No 2., 
the plants being killed and the controls remaining healthy. In inocula¬ 
tion No. 5 the root was injured, and the inoculum applied at this point 
resulted in its decay and the death of the plant as before. In No. 6 the 
fungus was applied to the root without injury, and at the end of five 
days the plant was still healthy. In No. 7 the root was injured as in 
No. 5, with similar results. In No. 8 the petiole was injured, no bell jar 
was used, and infection was positive. Controls of No. 5, 6, 7, and 8 
remained healthy. 
On November 24 an artificially infected seedling inoculated about 
three days previously and showing typical signs of the disease was 
removed from the greenhouse and an attempt was made to reisolate the 
organism. One leaf stalk showed a lesion extending up the stem about 
inch and a rot extending into the root from inch on one side. 
In a microscopic mount from the advancing edge of the decay, hyphae of 
Phytophthora could be seen apparently free from other organisms. 
Saprophytic fungi, bacteria, and nematodes were abundant at the points 
of earliest decay. Isolation attempts on rhubarb juice agar and potato 
agar plus rhubarb juice, made by the tissue fragment method, failed to 
get the organism into pure culture, though the Phytophthora developed 
and fruited. 
These results were so strongly indicative that the right organism had been 
found as to justify confining further operations practically to this fungus. 
