SECTION OP VEGETABLE PATHOLOGY 



POTATO SCAB. 



During the pas.t year the Section has received frequent inquiries from 

 different parts of the country relative to this disease, and while it is not 

 our purpose in tbis paper to fully answer these, it is hoped that what 

 is here stated may lead to a better understanding of the nature of the 

 malady among those who are particularly interested. 



Potato scab is not a new disease by any meaus, nor is it peculiar to 

 America, for we hud frequent mention of its occurrence in Europe; in 

 fact, most of the important papers on the subject are to be found in 

 German works on plant diseases. 1 



The cause of scab is a question tbat has long been discussed. Some 

 of tbe older writers held that it was due to the attacks of fungi, while 

 otbers attributed it to the depredations of animal parasites. At pres- 

 ent, however, it is the general belief of those who bave given the mat- 

 ter careful study that it is not, as a general thing, due to either of the 

 foregoing causes, but is the result of certain physiological changes tbat 

 take place in the tuber when the latter is grown under certain condi- 

 tions. 



The potato is in reality an underground stem, and, like similar parts 

 of other plants, its outer covering consists of a thin, tough membrane, 

 which serves as a protection to the more tender parts within, This en- 

 veloping coat is made up of minute cells, the walls of which consist of 

 a substance known as suberiue, or cork. The corky membrane is pro- 

 vided with numerous minute structures known as lenticels, by means of 

 which an exchange of gases takes place between the interior and exterior 

 of the tuber. 2 In the presence of au excess of moisture the lenticels 

 greatly increase in size, and often appear as small, woolly tufts, scattered 

 here and there over the surface. At the same time the cork layer be- 

 comes thickened at these points, and as a final result of this process 

 small wart-like projections are formed. Where these occur the skin is 

 weakened, and, if the conditions which favor this development continue, 

 decay soon sets in. In its efforts to heal the wound thus produced the 

 tuber gives rise to new layers of cork cells beneath the diseased parts, 

 and, as a result of this continued dying of the outer cells and the form- 

 ation of a new growth beueatb, a scab is produced. 



Wherever the corky membrane is wounded the tuber immediately 

 makes an effort to repair the injury. It would seem reasonable, there- 



1 See Bibliography. - Stahl, Bot.. Zeit. 1873. 



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