38 DISEASES OF FIELD & GARDEN CROPS. [CH.VII. 



insects and fungi light on the injured parts, repeated 

 efforts are made to repair the damage, and so the bark is 

 brought into a scabbed and cracked condition. Lime 

 rubbish, builders' refuse, refuse from ashpits, and other 

 materials of the same class, are said to cause corrosion, 

 scab, and cracking of the bark of potato tubers by contact. 

 When the bark is carefully removed small depressions 

 are clearly seen, answering to the scabs removed with the 

 bark. In bad cases the pits beneath the scabs are exca- 

 vated deeply into the substance of the potato, and when 

 the bark is removed the substance of the tuber, though 

 frequently slightly discoloured, is left intact. 



It generally happens that a portion only of a crop of 

 potatoes is scabbed, and this portion can be often dis- 

 tinctly traced to one part of the field whence the potatoes 

 were derived. On visiting this position the irritating 

 substances in the soil will usually be seen. When scab 

 and cracking can be thus traced the remedy is obvious. 



