188 Potato Diseases 



3. By planting not-diseased tubers that have been in contact 

 with diseased or not-diseased tubers from an infected soil. 



4. By transference of soil from an infected area by some means 

 or other, even though actual disease be not present in that area. 



Every possible precaution should of course be taken to prevent the 

 dissemination of these infective bodies, and the destruction of all 

 diseased material is a reasonable and sound requirement. 



Questions naturally arise as to how the disease can be fought in situ 

 and the following issues need urgent attention : 



1. The possibility of devising some treatment for sound tubers 

 raised on infected soils. 



2. The experimental treatment of infected soils. 



3. Extended experimental work with varieties possibly immune. 

 The subject of resistance to Chrysophlyctis is an extremely important 



one, A high degree of resistance for certain varieties is claimed in the 

 Report issued by the Intelligence Division and it is stated therein that 

 the varieties when planted on infected land in several districts proved 

 highly resistant. This matter needs keen attention under more searching 

 experimental conditions in different localities and seasons. 



The Phytophthora problem. 



Although Phytophthora infestans is still the cause of the greatest 

 financial loss through actual damage to the potato harvest in Great 

 Britain, and in spite of the mass of literature that has arisen in connection 

 with this subject, several important matters still need urgent attention. 

 Of these, the manner in which the infection of the potato crop is 

 occasioned each year, will be first considered. 



Marshall Ward^ held the view, by analogy with the behaviour of 

 many rust fungi, that the mycelium of the fungus could remain dormant 

 in the tuber through the winter and bring about infection in the following 

 season : 



" More commonly however the tubers are beginning to ripen when 

 the hyphae reach them and the mycelium goes to sleep — passes into a 

 dormant state — between the cells of the ripening tuber." 



The expression " dormant mycelium " is not necessarily restricted 

 to mean mycelium hibernating in tubers showing disease, but admits 

 the presence of mycelium in healthy tubers. Hence Marshall Ward 



i H. Maishall W'anl in DIsaisrs of Pl'inh. p. 75. 



