Jan. 6, 1923 
A Phytophthora Footrot 0} Rhubarb 
21 
that these two species (P. terrestris Sherb. and P. melongenae K. Sawada) 
are synonymous but that no definite conclusion could be reached without 
more complete descriptions or a comparison of cultures. Likewise Butler 
(4, p. 82) has stated that P. terrestris is identical with P. parasitica . Be 
that as it may, the sporangia of P. melongenae , as noted from Tanaka’s 
translation (22), average 42.4 by 33.9 p, or somewhat larger, and the 
oospores are from 17 to 21 p in diameter, or distinctly smaller than the 
writer’s. Other differences determined from a study of the original paper 
(22) with the aid of Dr. Tanaka are the apparent lack of haustoria in 
P. melongenae , and differences in the effect on other hosts, particularly 
tomato and potato. The oospores of P. terrestris are likewise smaller, 
being 18 to 21 p in diameter. The conidia and chlamydospores average 
practically the same. A comparison of cultures affords further evidence 
that the two differ. P. terrestris invariably produces on corn meal agar 
plates the peculiar “tufted” growth pictured and described by Sherba- 
koff (23, p. 123-126). The rhubarb form on this medium starts almost 
at once to produce an aerial growth. The contrast between the two is 
shown in Plate 6, C. Another difference is in their effect on tomato 
fruits. P. terrestris produces a profuse aerial growth on inoculated green 
fruits held in the moist chamber, as pictured by Sherb^koff (23, p. 120). 
This was verified with the same organism by the writer. Those inocu¬ 
lated with the rhubarb fungus produced no aerial growth except very 
slightly at the wound itself, its spread being entirely beneath the surface. 
These fruits were held as parallel inoculations in the same moist chamber 
with fruits inoculated with P. terrestris. 
The conclusions to be derived from this lengthy comparison with other 
described species of Phytophthora are that the strain from rhubarb is 
distinct from all except what might be called the Phytophthora parasitica 
group. It seems sufficiently like P. parasitica Dastur, morphologically, 
to be included for the time being at least, as a variety of that species. 
If this is done, then, as Pethybridge and Lafferty (19) have suggested, 
P. melongenae Saw. and P. terrestris Sherb. should be included in the 
same category. 
A detailed description of the rhubarb organism follows: 
Phytophthora parasitica var. rhei, n. var. 
Mycelium at first continuous, later sparingly septate; 5 to 15/x in diameter, mostly 
intercellular, producing haustoria which may be small and subspherical, or finger¬ 
like; sporangiophores not distinguishable from the mycelium; sporangia terminal 
sometimes intercalary, normally ovate and papillate, sometimes spherical and lacking 
papillae, 21 to 42 fx by 27 to 54^, mostly 24 to 35/* by 36 to 48/z, germinating by zoo¬ 
spores, rarely by germ tube; zoospores biciliate, 8 to gp by n to 13^, becoming globoid, 
8 to iop in diameter; chlamydospores globose, 27 to 42 p; walls thick; oogonia in abun¬ 
dance in old cultures, 24 to 33/z in diameter, pale to brownish in color, penetrating 
through antheridium which remains attached at base; antheridia variable; oospores, 
globose, thick-walled, 21 to 30/*, mostly 24 p in diameter; growing profusely on most 
vegetable culture media. 
Parasitic in base of leaf petioles and roots of rhubarb ( Rheum rhaponticum L.) in 
Maryland, District of Columbia, and Virginia, United States of America; type local¬ 
ity, District of Columbia; type culture in Bureau of Plant Industry, Washington, D. C. 
CONTROL MEASURES 
Spraying experiments with Bordeaux mixture 4-4-50 and with Bor¬ 
deaux dust were started on a small scale at Arlington Farm, Va., in the 
summer of 1920 during the height of the season favorable for infection. 
Conditions in no wise favored an extensive spraying experiment at this 
time, however. In the first place, there was no assurance early enough 
