72 



rubbed off the surface of the tuber without causing any 

 evident injury. The spongy or powdery scab, caused by 

 Sfongospora 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 arises 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. 



t See Leaflets 171^ 137 and 232. 



