1889.] PUBLIC DOCUMENT — No. 33. 131 



POTATO SCAB. 



BY JAS. ELLIS HUMPHREY, PROFESSOR OF VEGETABLE PHYSIOLOGY. 



The value of the potato crop in Massachusetts exceeds 

 that of any other pLinted crop ; consequently, the loss by 

 any widespread and serious disease of this crop must be an 

 important item. The commonest and most constant disease 

 which attacks the potato in the field is that commonly 

 known as the " scab." It is well known in both Europe and 

 America, and attacks the tubers, giving little or no evidence 

 of its presence in those parts of the plant above ground. 

 The cause of this trouble is not at all understood, though 

 various theories are held as to its nature. It is proposed in 

 the present paper to discuss briefly the present state of our 

 knowledge of the potato scab, by way of introduction to a 

 series of investigations of the disease which the writer ex- 

 pects to carry on during the coming year. 



The disease first manifests itself in the form of small cor- 

 roded spots or pustules on the surface of the potato. Writers 

 on the subject generally agree that these spots replace the 

 " lenticels " of the tubers. 



If a smooth potato tuber be closely examined, there will 

 be seen spots of the size of a pin's head or smaller, of a 

 slightly different shade, and somewhat roughened or granular 

 in appearance. These breaks in the continuity of the tissue 

 of tabular cork-cells which form the so-called "skin" of 

 the potato, are filled with loose, globular cork-cells, through 

 whose intercellular spaces an interchange of gases can take 

 place between the interior of the potato and the outer air. 

 They are then, so to speak, the ventilators of the tuber, 

 and are known as "lenticels." (The normal structure of 

 the potato tuber is shown in the accompanying Fig. 2.) 

 It is in these lenticels that the scab originates or first shows 

 itself. 



From these spots the disease rapidly spreads, until some- 

 times almost the whole exterior of the tuber becomes in- 

 volved in the decay and breaking down of the surface tissue. 

 In many cases, at least, there are developed over these 

 patches, rough, brittle scales or crusts of corky tissue, which 

 peel readily from the surface, and which render the name 



