Jan. 6,1923 
A Phytophthora Footrot of Rhubarb 
i 5 
Various sterilized vegetable plugs were used. Dastur has noted 
(jo, p . 217) that Phytophthora parasitica failed to develop on sterilized 
carrot and sterilized sweet potato. The writer’s strain grew very readily 
on these media, producing a tough superficial mat of hyphae at first, and 
after a few days a dense white cottony growth. The growth was similar 
on string bean plugs and on sterilized potato. On none of these vegetable 
plugs were sporangia produced to any great extent. 
The organism is very tolerant to acid, at least to that found in the rhu¬ 
barb stem, as might be expected from its habits in nature. It grew in 
pure culture, though somewhat slowly, in rhubarb decoction that titrated 
roughly to phenolphthalein 0.07 N (4- 70 Fuller’s scale). Haas ( 12 ) gives 
the following figures for the acidity of different parts of the rhubarb leaf 
stalk; basal part, actual acidity or concentration of free hydrogen ions, 
determined electrometrically, 0.0007 N; intermediate part, actual acidity 
0.0005 N, total acidity 0.1578 N; green part below leaf blade, actual 
acidity 0.00022 N, total acidity 0.1681 N. As Haas states, there is here 
shown a great difference between the total and actual acidities, which 
difference is probably advantageous to the plant’s metabolism in that it 
helps to maintain fairly constantly that actual acidity which is most 
favorable. Even the actual acidity is unusually high, however, as is 
shown by the same author in another paper (j j), in which he compares it 
with soybean, the former having a Ph of 3.36 (acidity 0.00044 N) and 
the latter of 5.85 (acidity 0.0000014 N). The acidity, however, does 
not prevent the active invasion of the Phytophthora into all parts of the 
stalk, as has been shown repeatedly by observations in the field and inoc¬ 
ulations into rhubarb stalks in the laboratory. Neither is high acidity 
requisite to the rapid growth of the organism, as is shown by its ability 
to grow on a wide variety of artificial culture media and on many common 
fruits and vegetable roots. 
In the winter of 1917 inoculations were made into the starchy food¬ 
storing roots and tubers of various plants. This was repeated in 1920-21 
with a wider range of hosts, including certain fruits. The roots and fruits 
were first thoroughly surface sterilized by immersing for 10 to 15 minutes 
in 1 to 240 formaldehyde, then wrapped in a dry towel and allowed to 
stand overnight. The inoculations were made by cutting a deep slit with 
a sterile scalpel and inserting the inoculum from a vigorous culture about 8 
days old. The specimens were then placed in moist chambers and allowed 
to stand 2 to 4 days. 
Stayman Winesap and Grimes Golden apples were both rapidly 
attacked, the fungus growing in toward the center as rapidly as it spread 
beneath the skin. The decay of the fruit was light brown or tan in color 
and of a soft and mealy consistency. Typical hypae could be found at 
any point in the advancing edge of the decayed area. Reisolations were 
easily secured free from contaminations, from the advancing edge 
beneath the skin, or from the interior of the fruit. Seeds that had been 
reached by the fungus in one of the fruits when planted in a plate of 
corn meal agar gave a pure culture of it. Parsnips (Pastinaca saliva L.) 
were readily attacked with a soft, wet, but not watery decay and a 
characteristic pinkish or light purplish discoloration of the surface. 
Carrots (Daucus caroia L.) also were rapidly decayed, with a clear watery 
condition developing darker in color than the normal. The decay spread 
to the growing leaves and quickly involved them. Turnips (Brassica 
rapa E.) decayed more slowly and did not lose their hard texture, but a 
distinct blackening of the interior occurred. Sweet potatoes (Ipomoea 
