jan. 6,1923 A Phytophthora Footrot of Rhubarb 9 
Inoculations No. 21 to 28 were made similarly, except that the 
inoculum was applied in each case to the crown at the points from which 
stalks were pulled. From three to five points were inoculated in each 
plant. Immediately after the plants were inoculated every other plant 
was sprayed at its base with 4-4-50 Bordeaux mixture, the intent being 
to determine whether plants could be protected from infection by this 
means. Inoculation and spraying were done on July 12. Five days later 
observations were made and final notes were taken in the spring of 
1921. Eventually three of the four plants inoculated and not sprayed 
succumbed to the disease. Spraying served to prevent infection, for 
none of the sprayed plants died. 
In connection with the motion-picture film illustrating zoospore emer¬ 
gence, mentioned by Godfrey and Harvey ( 11 ) another inoculation, No. 
29, was made on July 20. Three or four stalks were pulled from a large 
plant and inoculum was applied from a Petri dish culture at the broken 
stubs in the crown. This inoculation resulted in the spread of the organ¬ 
ism to other parts of the plant and the ultimate death of the plant. 
Plate 6, B, shows this plant photographed on July 26. Plate 4, B, is 
from a photograph of a plant that became diseased as the result of 
inoculation 11 (Table I) photographed July 17. 
As indicated in the tabulations after inoculations No. 10 and 14, 
reisolations were made from affected parts. In the case of the leaf 
inoculated by application of zoospores (No. 10) reisolation was accomp¬ 
lished by cutting out a portion of the base of a large leaf vein that had 
become infected, washing it for about a minute in mercuric chlorid 1 to 
1,000 and then in three changes of sterile water, and applying in pieces 
broken apart with sterile forceps to an agar plate. This resulted in a 
pure culture of the same organism as was used in making the inoculation. 
An isolation was made from the base of the affected stalk in No. 14 in 
practically the same way. One of the bits gave a pure culture. These 
cultures were used for successful inoculations in the greenhouse during 
the following winter. Thus Koch’s rules of proof of pathogenicity were 
completed and this organism was established definitely as the cause of 
the disease. 
Still further inoculations and reisolations were made in the spring of 
1921 on some potted rhubarb plants in the greenhouse. At this time 
an easy method of isolation to free quickly from contaminating organ¬ 
isms was worked out. On the chance that it may be useful to other workers 
on Phytophthora, this method is here described. It consists simply of 
inoculating with the mixed culture a good sound apple, according to the 
methods described completely in connection with another phase of the 
problem on page 15 of this paper. The Phytophthora quickly outgrew 
the contaminating bacteria and in about two days was obtained abso¬ 
lutely pure by transferring from the advancing edge of decay in the 
interior of the apple. Plate 7, B, shows the fungus in pure culture, grow¬ 
ing from such transfers of apple tissue, compared with the contaminated 
culture A, made by transferring from near the point of inoculation. 
DESCRIPTION OF THE FUNGUS 
So much has been written from time to time of the characters peculiar 
to members of the genus Phytophthora and of their behavior in culture 
that an effort will be made here to avoid useless repetition by reference 
to previous papers wherever possible. An exception will be made of 
