98 Journal of Agricultural Research \o\. v, no. s 



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 191 3 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 resembfing 

 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. 



