256 VEGETABLE FOES AND DISEASES 



taste, decay setting in, and the devastation is ac- 

 celerated by the action of minute animal and vege- 

 table pests, never slow to take advantage of feeding 

 on exposed vegetable tissues, hence scabbed and 

 cracked Potatoes are often wholly unfit for human 

 food and not marketable. 



This brownish scab and cracking are due to the 

 presence of some irritating or corrosive substance in the 

 soil. Lime rubbish, builders' refuse, ash-pit manure or 

 ashes, and the use of highly nitrogenous manures, 

 such as fowl and pigeon manure placed in direct con- 

 tact with the sets and thus ready for the young tubers 

 to come into contact with, also rank stable or farmyard 

 manure used at the time of setting below or over the 

 sets, are likely to cause the scabbing and cracking of 

 Potatoes. Continued drought when such substances 

 are used may extend the action in a corrosive direction. 

 The tuber strives to repair the injured part, the bark 

 becomes scabbed and cracked, and insects and fungi 

 take possession. 



It generally happens that a portion only of a crop 

 of Potatoes is scabbed, and this can often be traced to 

 a part of the garden or field where irritating or cor- 

 rosive substances have been applied. When scab and 

 cracking can thus be traced the remedy suggests 

 itself. 



Potato Scab or Smut (Sorosporium scabies) pro- 

 duces one form of scab in Potatoes. It grows beneath 

 the skin of the tuber, where it forms a thin, dark, 

 greenish-brown stratum, often extending over the 



