538 BOARD OF AORICULTUKE. [Jnn. 



"scab" an appropriate one for the disease. In Fig. 1 is 

 shovvn the appearance of the fully developed scab, rcf)ro- 

 duced from photographs of potatoes raised on the plats of the 

 Station in 1888. This whole change goes on while the tuber 

 is still in the ground ; and after the crop is dug and stored, 

 no further change occurs. The disease aifects the tissue to a 

 depth of only a few cells, all below remaining in a normal, 

 healthy condition. The cells affected lose their starch, and 

 contain, in its place, according to some writers, globular 

 brown masses, usually regarded as disorganized cell-contents. 

 In so far as the starch, which gives its chief food value to the 

 potato, is destroyed, that value is lessened ; but the unsightly 

 appearance of " scabby" tubers causes a much greater pro- 

 portional decrease in their selling value, since, by paring 

 away the affected superficial tissue, the remainder is made 

 perfectly suitable for food. 



The cause of this disease has been discussed by several 

 writers. Most of the views expressed are based on the 

 first important discussion of the subject by Schacht, in a 

 work on the potato plant and its diseases.* This author 

 believes that the efficient cause of the scab is an excess of 

 moisture in the soil. It can readily be shown, that, when a 

 potato tuber is exposed to an abundance of moisture, the 

 lenticels become more prominent, in consequence of the 

 loosening and separation of the cells which fill them. This 

 affords, Schacht thinks, an easy opportunity for the water to 

 enter those tissues of the tuber bordering the lenticels. They 

 thus become water-soaked, and rapidly decay, assuming a 

 dark and muddy appearance . Two of the chief recent writers 

 on the diseases of plants, Frank and Sorauer, adopt this 

 view. Frank f regards the disease as a case of breaking 

 down of tissue, originating in what is practically a wound. 

 Sorauer | thinks the scab develops rapidly during short but 

 specially favorable periods, and instances, as such a period, 

 the time of a heavy rain following a drought. Each of the 

 above writers mentions as a possible cause, or at least an ag- 

 gravating condition, the presence of lime, marls, or especially 



• Bericht tiber die Kartoffelpflanze und deren Krankheiten, Berlin, 1854, p. 24. 



t Krankheiten der Pflanzen, Berlin, 1880, p. 140. 



t llandbuch der Pflanzenkrankheiten, Berlin, 1886, vol. i, p. 227. 



