192 



Potato Diseases 



may be obtained by scanning diseased tubers in the field when they are 

 lifted. In such cases the following points furnish a guide : (1) the stalk- 

 end of the tuber when cut across may be entirely without discolouration, 

 (2) the brown marks in the flesh of the tuber caused by the fungus, 

 frequently do not extend from the skin as far inward as the principal 

 cylinder of vascular bundles, (3) the brown shallow areas of diseased 

 tissue near the skin are often quite isolated and sometimes limited to 

 the neighbourhood of an eye. 



The subterranean attack might be occasioned by : 



(1) Mycelium, if it exists. 



(2) Oospores, if they exist. 



(3) Zoospores. 



Fig. 1. External view of tuber attacked by Phytophthora from the soil. 

 Fig. 2. Healthy tuber with clean skin for comparison with Fig. 1. 



As already stated no evidence has yet been obtained relating to the 

 presence of mycelium or oospores in the soil. With regard to zoospores, 

 it may be fairly reasoned that if the foliage is attacked, the conidia 

 formed on the leaves can be carried by the wind and distributed not 

 only to neighbouring plants but on the surface of the soil. During 

 a period of heavy rain the conidia or zoospores would be carried into 

 the soil and eventually reach the surface of the tuber and penetrate it. 

 This matter still needs adequate experimental proof. 



One of the chief puzzles afforded by the Phytophthora problem is 

 afforded by the case of field crops which are badly diseased when lifted 



