10 BULLETIN 81, U. S. DEPARTMENT OF AGRICULTURE. 



It is certain that our common scab is much more common and 

 disfiguring throughout the United States than it is in Europe, and 

 the injuries caused b} 7 the fungus Rhizoctonia to potatoes in the West 

 are greater than any reported from Europe. 



COMPARISON OF POWDERY-SCAB INJURIES. 



Common scab produces a roughened spot or pockmark on the 

 tuber, which in its worst stage may cover the whole potato. Under- 

 neath the scab spots, however, a cork layer is formed and the potato 

 remains sound. It is not more subject to decay than other potatoes, 

 and the actual injury from a food standpoint is due to the greater 

 loss in peeling before cooking. 



Powdery scab in its milder form causes no greater outward disfigu- 

 ration than common scab, but there is less of a cork layer formed 

 and a progressive decay frequently follows. The cankerous stage of 

 powdery scab is more objectionable than any phase of common scab 

 and is as bad as the wart disease. Finally, no means of controlling 

 powdery scab through the disinfection of seed potatoes, as practiced 

 for common scab, has proved wholly satisfactory. 



All these considerations led the Department of Agriculture to the 

 conclusion that measures for preventing the introduction of powdery 

 scab into the United States were not only fully justified, but were 

 demanded by every rule of prudence and precaution. 



Most foreign countries have long since wisely adopted a similar 

 procedure with reference to American potatoes, mainly on account 

 of the Colorado potato beetle. Canada maintains a complete em- 

 bargo against all European countries, and most of the English 

 colonies restrict the importation of potatoes to a greater or less extent 

 on account of the wart disease and other troubles. 



OTHER REASONS FOR POTATO REGULATIONS. 



Experience gained in the enforcement of the potato quarantine 

 order of September 20, 1912, and further investigations of potato 

 diseases and insect enemies have shown that more efficient and log- 

 ical means are required for the adequate protection of this country 

 against the potato parasites of the world. 



Where a quarantine is laid against a whole country on account of 

 an infection limited to a small portion of that country, the justice 

 of the act is questioned by residents of the disease-free districts, yet 

 there has been no means of limiting quarantines by other than national 

 boundaries except through the active cooperation of the foreign 

 government. 



Where a quarantine is laid against one country and not against 

 another concerning a commodity like potatoes, which is a staple ar- 



