376 



JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



balls fall away from the potato, hollows of various sizes remain or the 

 surface of the tuber appears to be corroded. Different effects will be 

 produced according to the manner and extent of the encroachments of 

 the parasite. Hence, the tuber in fig. 101 a in a later stage of the 

 disease would show hollow places where the spore-halls now rest ; the 

 tuber in fig. 101 d would show a cankerous band and so on. 



Tt is quite clear from a study of the symptoms of potato canker 

 due to Spongospora solani that it is of the nature of a canker 

 (canker = an eating, corroding or other noxious agency producing 

 ulceration, gangrene, rot, decay, etc.). The organism [Chrysophlyctis 

 endobioiica) causing potato tumour (black scab), on the other hand, 

 does not produce injuries of a cankerous nature, but induces the 

 formation of swellings or tumours — hence the name " potato tumour " 

 given to it in the first instance by Cooke.* The terms canker and 

 tumour express not only the trend of the disease, but the essential 

 difference between the characters of the injuries brought about in the 

 tuber in each case. 



The names commonly used for these diseases are as follows : — 



Disease due to Spongospora sola7ii. 



f Corky Scab (T. Johnson). 

 Powdery Scab (T. Johnson). 

 Spongospora Scab (G. H. Pethybridge). 

 Potato Canker (G. H. Pethybridge). 

 Corky End. 



Disease due to Chrysophlyctis endohiotica. 



Potato Tumour (M. C. Cooke). 



^ , I (W. Carruthers' Eeport, 1902). 



Potato Canker i .-tt rr~ n- \ 

 I (PI. T. Gussow). 



Warty Disease (M. C. Potter). 



Black Scab (G. Massee). 



During the progress of the disease, the tubers may become variously' 

 misshapen owing to the formation of tuberous outgrowths (fig. 102 e)| 

 Fig. 102 F shows one of these outgrowths which was cut througlj 

 immediately upon removal from the soil in order that the distributiorl 

 of the parasite in the tissue might be seen. The tissue of the brownislj 

 layer {h) contains the spore-balls of the parasite. Just below the browr 

 layer is one of milk-white appearance (c) : the tissue of this layer coni 

 tains the stages prior to spore-ball formation. Below the second layei 

 a fine dark line can be distinguished with a lens when looking at the cu! 

 surface of the tuber, but this is not clearly visible in the photographj 

 lliis line iiidicates the boundary between the diseased and healthi 

 tissues, and beyond it the parasite does not exist. These outgrowth 



I 



* M. C. Cooke, Jour. F.H.S.. xxvii. (1902), p. cxliv., and Card. Chro\ 

 xxxii. (1902), p. 124. j 



t The name " Corky Scab," given to this disease by Johnson, was su 

 sequently abandoned by him as nnsnitable. {S!ri Proc. Boy. Dublin. Soc, x" 

 (n.s.) (1909), p. 165.) 



