98 
Journal of Agricultural Research 
Vol. V, No. a 
earlier part of this paper it is shown how an epidemic developed by 
artificially inoculating two plants in a plot of potatoes in a section of 
the country where P, infestans did not develop that year and how plants 
immediately surrounding the two initially infected ones succumbed 
before any of the others at a greater distance, thereby giving rise to infec¬ 
tion centers in the plot in which the vines were killed long before the 
rest and which increased until it included the whole plot. 
Other cases are cited where similar centers known to have originated 
from the spread of the mycelium up the stem were found and carefully 
watched under field conditions during the growing seasons of 1913 and 
1914. Furthermore, the development of P. infestans has been followed 
for the last three seasons, but no evidence has been obtained to show that 
it originates uniformly on the lower leaves throughout a whole field. 
In many cases, when observations are made early enough, the disease is 
found to originate at some one point and spread outward and radially. 
RESTING SPORES OF PHYTOPHTHORA INFESTANS 
Resting spores, or oospores, are produced by most of the species of 
Peronosporaceae. Their function, as is well known, is to bridge the 
fungus over periods unfavorable for its growth and development. 
Whether P. infestans has oospores has been a bone of contention for the 
last 60 years. Until recently, however, the prevailing opinion has been 
that oospores were not produced by this fungus. 
During the last decade bodies resembling oospores have been found in 
pure cultures by Jones (15), Clinton (9), and Pethybridge (26). This 
discovery has doubtless influenced Pethybridge (25, p. 343) in making 
the following statement: 
It appears to be practically certain that the primary attack of blight each season is 
due to spores , but where these spores come from is not known with certainty, and 
whether they are similar to those produced on the potato foliage in warm, moist 
weather in the summer after the primary infection of the crop has taken place, or are 
of the nature of the thick-walled resting spores produced by species of Phytophthora 
allied to Phytophthora infestans , can'not definitely be stated at present. 
This statement plainly discredits the perennial-mycelium theory and 
suggests that spores, either conidia or oospores, function in renewing 
infection. That the mycelium in diseased seed tubers may renew an 
epidemic of late-blight has been clearly shown in an earlier part of this 
paper and needs no further argument. 
Pethybridge (25) unfortunately does not define the spore that serves 
to perpetuate P. infestans . If he means conidia, there is little evidence 
to support his contention, as has already been pointed out. On the other 
hand, it must be conceded that the discovery of bodies resembling 
oospores in pure cultures of P, infestans must be seriously considered 
when discussing the overwintering of the fungus. At present, unfor¬ 
tunately, there is little positive evidence to support the oospore theory. 
