Feb. 12 , 1917 
Studies of the Genus Phytophthora 
259 
other members of the Peronosporineae. For example, on its receipt 
from Dr. Coleman P. arecae had produced and for some time afterwards 
continued to produce an abundance of oospores in culture, but now, after 
remaining in culture for three years, it produces scarcely a single oospore. 
Dastur (10) observed this to be the case also in P. parasitica . He says: 
“There is as much variability and uncertainty in the production of these 
oospores as Clinton found in the production of P. infestans Cultures 
of this species received by the writer from Dastur in 1914 and grown on 
media on which sexual bodies were previously reported by him failed to 
show a single oospore. 
What actually determines the production of sexual bodies in Phy¬ 
tophthora spp. has not been solved. Some of the species produce these 
Fig. 12.—Graph showing the variation in the diameters of oospores of Phytophthora cactorum (Phyllocactus 
sp.), P. cactorum (Panax sp.), P.fagi, and P, syringae. 
bodies readily and in great abundance on artificial media, while others 
produce few or none. 
Oogonia and the subsequent development of oospores have been 
studied by the writer from cultures in P. cactorum from both Phyllocactus 
sp. and Panax quinquefoliay P. fagi, P. syringae, P. phaseoli, P. arecae , P. 
erythrosepUca, and P. infestans. In addition P. parasitica Dastur (10), 
P. nicotianae von Breda de Haan (3), and P. colocasiae Butler and Itul- 
kami (6) are known to produce oospores in culture, but in the case of 
these three species oospores have not been seen by the writer. 
As a result of four years of intermittent work with the various species 
of Phytophthora some general statements may be made on the behavior 
of these cultures as regards the development of the sexual bodies. The 
species P. cactorum , P. fagi, P. phaseoliy and P. erythroseptica produced 
oospores in great abundance on a number of media. In the case of P. 
syringae, P. infestans , and P. arecae their production requires special 
