72 
rubbed off the surface of the tuber without causing any 
evident injury. The spongy or powdery scab, caused by 
Spongospora solani, a fungus akin to the Wart Disease 
organism, is easily recognised by the light brown colour 
of the rather powdery warts, or cankers, it produces. 
Brown Scab, or Ordinary Scab, of the tuber also shows as 
light brown warts, but usually evenly distributed over the 
surface of the potato. It may be due to a variety of 
causes, in which mechanical irritation by gritty 
particles of soil and infection by definite parasitic 
fungi probably play an important part. None of these 
diseases, however, are so destructive as the Wart Disease; 
further particulars of them may be obtained from the 
leaflets issued by the Board of Agriculture.t It is quite 
certain that these scab-like injuries are caused by different 
fungi, but it is not surprising that the resulting diseases 
present certain superficial similarities. In each case the 
presence of a parasitic fungus irritates the cells of a 
potato, causing them to actively divide and thus give rise 
to the warts. Even in the Fingers-and-Toes disease 
the result is similar, and in all the cases with which we 
have dealt the attack arisés from the presence of the 
organism in the soil. By, therefore, adopting precautions 
to prevent the infection of the soil, and by appropriate 
treatment similar to that advised for the Club-Root 
disease, such diseases can, in some measure, be controlled. 
+ See Leaflets 171, 137 and 232. 
