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COMMON SCAB OF POTATOESi 

 PART I 



By W. a. MILLARD, B.Sc. 



{Lecturer in Agricultural Botany and Adviser in Mycology, 

 University of Leeds.) 



(With Plates VIII, IX.) 



Common Scab has long been one of the most widespread of potato 

 diseases and is recorded as early as 1825. In spite of this, it has hitherto 

 received little attention from mycologists in this country. This neglect 

 may be attributed possibly to the fact that, although the disease occurred 

 in all parts of the country, it was only in certain localities that it appeared 

 with sufficient virulence to make it a serious problem. In such places, 

 as for example, in certain districts in the great potato-growing county 

 of Yorkshire, the crop is sometimes rendered so unsightly by the Scab 

 that it is practically unsaleable. In consequence, many farmers have 

 been obliged to omit potatoes from their rotation on land which, from a 

 yield point of view, was eminently suitable for them. 



The distribution and virulence of the disease appear to be closely 

 related to the type of soil on which the crop is grown, occurring chiefly 

 on soil of a light sandy or gravelly nature, to a much lesser degree on 

 heavy soil, and rarely, if at all, on true peaty soils. 



Accounts of Common Scab are numerous in American literature, but, 

 in many respects, these appear to be incomplete when applied to the 

 disease in this country and a short description will therefore be given 

 here. 



Description of Common Scab. 



The disease first appears in the form of small brown spots on the skin 

 of the tuber. These increase in size and, at the same time, the tissue of 

 the potato immediately below them becomes brown and pulpy. At this 

 early stage, the dark brown surface of the spot remains smooth and 

 unbroken and on it a delicate greyish white mycelium is produced. This 

 rapidly disappears when the potato is lifted and exposed to light. 



^ A grant in aid of publication has been received for this communication. 



