96 Journal of Agricultural Research voi. v. No. a 



This method of perpetuation readil}^ explains how P. infestans has 

 spread from its native home in South America to every corner of the globe. 

 As pointed out by Jensen (14), it was probably brought to Europe in the 

 mycelial stage in seed potatoes. Likewise, it may well have gone to 

 Australia, New Zealand, North America, and other parts of the world. 



MYCELIUM OF PHYTOPHTHORA INFESTANS IN THE SOIL 



That the mycelium might live over winter in the soil was possibly first 

 suggested by Kiihn (18), who arrived at this assumption because he was 

 unable to grow infected plants from diseased tubers, combined with the 

 fact that the potato fungus occurred year after year. This theory 

 received support later at the hands of Brefeld (7) in connection with his 

 excellent cultural studies of the smuts. He devoted some attention to 

 P. infestans also and was probably the first to grow this fungus sapro- 

 phytically in semipure cultures. It was this significant achievement that 

 led him to support Kiihn's theory. 



Darnell-Smith (10) has studied the possibihty of P. infestans living 

 over in the soil. A large number of experiments were made by mincing 

 infected tubers in the soil and planting it to potatoes. He also smeared 

 spores on the tubers when planted, but in no case did he get any infection 

 of P. infestans. Some recent experiments by Stewart (33) also bear 

 directly on Brefeld's theory (7, p. 26). He planted healthy tubers in soil 

 mixed with blighted vines and tubers and made conditions highly favor- 

 able for the infection of the growing potato plants. No infection of 

 P. infestans was obtained. 



According to the writer's studies, under certain conditions of moisture 

 and temperature the fungus may grow and sporulate on the surface of 

 the soil to a very limited extent, as described in an earlier part of this 

 paper, but no evidence was obtained showing that it remains alive in the 

 soil for extended periods of time. Jones, Giddings, and Lutman (17) 

 have also recorded the fact that the fungus may spread from infected 

 tissue out over the surface of the soil to a limited extent. Our increased 

 knowledge of culturing parasitic fungi on artificial media, and especially 

 of P. infestans, does not permit such deductions at the present time as 

 were made earlier by Brefeld (7). 



MASSEE'S LATENT-MYCELIUM THEORY 



The early literature on P. infestans, then known as the "potato mur- 

 rain," is full of interesting theories as to its origin. The literature is in 

 every case naturally tinted with spontaneous generation and lack of infor- 

 mation as to the life history of the fungus. Fully as interesting is a theory 

 more recently advanced by Massee (20). He maintains that the usual 

 explanation for the sudden appearance of P. infestans over wide areas 

 by the dissemination of conidia is inadequate and that the fungus is 



