1889.] PUBLIC DOCUMENT — No. 33. 133 



of iron oxide in the soil ; and Sorauer thinks that ammonia set 

 free from the soil may sometimes have a similar influence. 

 Another authority, W. G. Smith,* considers that the chief 

 cause is mechanical irritation, from the presence in the soil 

 of corrosive substances ; and states that a difference may 

 often be noticed in the degree of scabbiness of potatoes from 

 different parts of the same field, depending on the relative 

 proportions of refuse in the soil of the different parts. Smith 

 also says that one form of the disease may be caused either 

 by long drought or by excess of moisture. All authors agree 

 that the scab-like crusts, which characterize the disease in its 

 complete development, originate from the natural effort of 

 the plant to repair the injury to the tuber by a secondary 

 formation of cork. Sorauer differs from the others quoted, 

 in rejecting the theory of irritation or corrosion as a primary 

 cause of the trouble. He quotes at length several experi- 

 ments, conducted in German experiment stations and else- 

 where, whose results seem to be conclusive against the idea 

 that foreign substances in the soil can cause the disease by 

 mechanical or chemical action. 



We may now proceed to consider the bearing of some 

 recent American observations on the views already stated. 

 The only experiments undertaken in this country for the pur- 

 pose of testing current theories, with which I am acquainted, 

 are those of Arthur and Beckwith of the New York Experi- 

 ment Station. f Plats of potatoes were planted and kept 

 under identical conditions, except that half of the hills were 

 kept wet by irrigation, while the others were not artificially 

 watered. One-half of the hills of each class were planted 

 without manure, and the remainder were manured. In the 

 unmanured hills, abundant moisture had practically no in- 

 fluence, for the percentage of scabby potatoes was very nearly 

 the same in the irrigated and unirrigated portions. On the 

 other hand, the irrigated hills on the manured ground pro- 

 duced seventy-one per cent, of scabby tubers, against only 

 thirty per cent, from the unwatered hills. A general average 

 gives forty-eight per cent, of scabby tubers on the irrigated 



• Diseases of Field and Garden Crops, London, 1884, p. 37. 



t Sixth Annual Report of the New York Agricultural Experiment Station, 1888, 

 pp. 307 and 344. 



