Jan. 6, 1923 
A Phytophthora Footrot of Rhubarb 
19 
Phytophthora infestans (Mont.) De Bary is clearly distinct from the 
writer’s species. Besides the sharp morphological differences, including 
the enlargements in the conidiophores so typical of that species, the 
smaller conidia 27 to 30^ by 15 to 20ju, and the larger oospores, 34 to 50^ 
by 24 to 35JU, there are physiological differences that clearly distinguish 
them. P, infestans produces oospores to only a slight extent, while in 
rhubarb Phytophthora they are abundant. The optimum temperature 
for zoospore production with P. infestans is from 12 0 to 13 0 C., according 
to Melhus (16), while with the writer's they form very quickly at room 
temperature, the lower temperatures inhibiting their development. 
Phytophthora phaseoli Thaxt. also is clearly different, judging by 
Clinton's account of that organism (6). With it, too, there are nodal 
swellings in the conidiophores, these being entirely absent in the writer's 
form. The sporangia appear to average somewhat smaller (17 to 35^ 
by 28 to 42ju) and the oospores considerably larger (26 to 28/4). The 
antheridia are only temporary with phaseoli, whereas they are persistent 
with the writer's species. 
No very complete account is given anywhere of Phytophthora thalictri 
Wilson and Davis; but from the description given by Wilson {29, p. 392) 
and Clinton’s (7, p . 894) subsequent reference to it, it seems safe to 
consider it distinct. In the first place its conidiophores are distinct, 
developing in fascicles from the stomata; the sporangia are very much 
smaller than on the writer's species (20 to 27 \x by 13 to 17/z), and the 
oospores, unknown at first and then later described by Clinton, are 
somewhat smaller (18.5 to 25 p). The fact that it appears to be a leaf- 
attacking form primarily rather than a soil-inhabiting and decay- 
producing fungus, indicates in itself that they are different. 
Phytophthora arecae (Colem.), Pethyb., found on the Areca palm in 
India (8), has oospores that are much larger (23 to 38/1), the sporangia, 
too, running considerably larger in both dimensions than those of the 
writer’s species (20 to 45^ by 30 to 71 /*). 
The rhubarb Phytophthora is similar to Phytophthora erythroseptica, 
Pethyb., in some respects. Its effect when inoculated through a wound 
into potato tubers is almost identical with that organism. But the 
conidia are distinctly different, the rhubarb species having distinct 
papillae, the other not ( 18 ). The oospores of this form are distinctly 
smaller than those of the other. 
The morphological differences are not marked between the rhubarb 
fungus and Phytophthora parasitica Dastur (10). P. parasitica, however, 
produces distinct spots on affected leaves of the castor bean, the under 
side of which bears conidiophores issuing from the stomata. Inoculations 
made with the rhubarb Phytophthora resulted in the appearance of 
spots different from those produced by P. parasitica and did not result 
in the production of sporangia. The growth of the two organisms on the 
various media was similar, as indicated in general by Dastur’s account 
{10 p. 211—2 if) except that his organism did not give the slightest growth 
on sterilized slabs of carrot and sweet potato, whereas the writer's pro¬ 
duced copious growth on both. This proved later not to be a legitimate 
comparison, however, for a strain of P . parasitica obtained from Dr. C. D. 
Sherbakoff, of the Tennessee Experiment Station, grew practically as 
well on these media in the form of steamed plugs as did the rhubarb 
species. P. parasitica, as reported by Dastur, would not affect tomato 
fruits, even when they were wounded, while the writer's spread very 
rapidly into the interior of such wounded fruits. Oospores failed to 
