CHAPTEE VII. 



SCAB AND CRACKING OF POTATOES. 



THE brownish scab on the skin or bark, and the cracking 

 of the bark in potatoes, are probably due to one and the 

 same cause, and that a mechanical one. Scab and crack- 

 ing are often confounded with smut in potatoes, but 

 whereas the latter ailment is invariably caused by a 

 fungus, there is seldom any fungus present (unless it be 

 one of the species which so commonly follow injuries) in 

 scab and cracking. Cracked and scabbed potatoes are 

 not marketable, and as the fissures in the bark expose the 

 inner substance of the potato to the earth it is generally 

 said that tubers so injured possess an earthy and dis- 

 agreeable taste foreign to uncracked and unscabbed ex- 

 amples. 



Scabbing and cracking begin at a very early stage of 

 growth in the tuber, and both are at first seen as small 

 corroded spots, or minute open pustules ; in bad cases the 

 spots and cracks become confluent, and the whole bark 

 of the potato presents an unsightly appearance. When 

 the inner substance of the potato is once exposed minute 

 insects, and sometimes fungi, add to the injury. If such 

 fungi as Peronospora or Fusisporium light on the exposed 

 places, destruction of the tuber is soon complete. 



Scab and cracking are said, in the first instance, to be 

 due to the presence of some irritating or corrosive sub- 

 stance in the soil. Continued drought, and possibly 

 sudden and superabundant moisture, will also cause one 

 form of scab. A natural effort is made by the potato to 

 repair the injury, and so a hard scab originates ; when 



