1909.] Varieties of Scab in Potatoes. 



749 



VARIETIES OF SCAB IN POTATOES. 



Scab due to Mechanical Injury. — This type of scab is recog- 

 nised by the very superficial nature of the scars, which often 

 cover the greater portion of the surface of the tuber, and are 

 entirely due to mechanical irritation of the skin of the potato 

 during the early stage of growth. The most general cause of 

 irritation is the presence of ashes in the soil, consequently it 

 is very prevalent in the neighbourhood of towns, where night- 

 soil is used as a manure. In an experiment conducted at 

 Kew, for the purpose of proving the correctness of the above 

 statement, twelve potatoes free from scab of any kind were 

 planted in soil mixed with ashes. A second row parallel 

 with the first, planted in soil containing no ashes, acted as a 

 control. When the crop was lifted, all the potatoes grown 

 in the land containing ashes were badly scabbed, whereas 

 those grown in the land not containing ashes were free from 

 scab. This experiment has been repeated on several occa- 

 sions with similar results. 



In addition to ashes, certain climatic conditions favour the 

 formation of the scab under consideration. During a dry 

 period the surface of the quickly-growing tubers presses 

 against the comparatively unyielding soil, with the result that 

 the skin is injured. As such injured tubers continue to 

 increase in size, the original minute scars are stretched wider 

 and wider, and become bounded by a somewhat raised border 

 of wound-cork, the result being scab. This type of scab is 

 often very prevalent on potatoes grown in sharp sandy soil 

 during a dry season. 



Unless complications follow, the injury caused is slight, as 

 the wounds are only "skin deep," and such scabbed potatoes 

 may be used for "sets." In many instances, however, 

 mechanical injuries to the skin enable eelworms, bacteria, 

 fungi, &c, to gain an entrance into the tuber, which may 

 result in its complete destruction. 



Wart Disease or Black Scab of Potatoes.* — This disease 

 is recognised by the presence of one or more irregularly 

 shaped, corrugated masses or warts projecting from the sur- 

 face of the tuber. These warts are pale when young, but 



* A memorandum on this disease appeared in the Journal, Dec, 1908. 



