THE SPREAD OF DISEASE 



215 



spores are destroyed. On the other hand, in the case of 

 barley, infection takes place through the flower ; hence 

 treatment of the seed is of no avail. 



Potato growers have for some time past expressed the 

 opinion that infection by spores alone appeared inadequate 

 to account for sudden waves of potato disease appearing 

 simultaneously over a wide area. 



Various experiments have been conducted at Kew with 

 the object of determining the different modes of infection 

 of potato tubers. The following, among others, is considered 

 as affording definite evidence as to the existence of hyber- 

 nating mycelium of potato blight (Phytophthora infestans), 

 and potato leaf-curl (Macrosporium so/ani) in potato 

 tubers, capable of imparting the disease to the offspring of 

 the infected tuber. 



Three potato tubers showing the rusty stains indicative 

 of the presence of Phytophthora mycelium in the flesh 

 were each cut into two equal parts. Each half tuber was 

 planted separately in a plant pot. The soil and manure 

 used was of the same kind for all, and was sterilised by 

 steam. Three of the pots were placed in a house having 

 a temperature ranging between 70 and 80 F., and very 

 often the moisture was at saturation point. Each pot was 

 covered by a bell-jar. The three remaining pots were 

 placed in a house without any artificial heat, and having 

 the air exceptionally dry. These pots were not placed 

 under bell-jars. An equal amount of water was supplied 

 to each of the six pots. The three plants grown under 

 conditions of high temperature and much moisture grew 

 quickly and were much * drawn.' The Phytophthora first 

 appeared when the shoots were six weeks old, and a fort- 

 night later all three plants were blackened and destroyed 



