THE POTATO QUARANTINE. 5 



POWDERY SCAB. 



Powdery scab is a tuber trouble, differing from the common scab 

 mainly in the following particulars :* The scab spots, or sori, are more 

 often circular and not usually as great in diameter as those of the 

 common (Oospora) scab. They first appear as discolored, slightly 

 raised spots covered by the epidermis, which later breaks away, leav- 

 ing a pit, filled at maturity with a brownish dust, the spore balls of the 

 parasite. With powdery scab there is less of a corky layer formed 

 under the spot than is the case with common scab. For this reason 

 there is a loss of moisture in storage and the eventual formation of a 

 depressed spot. In severe attacks of powdery scab there is a can- 

 kerous stage or eating away of the tuber, which nearly or quite de- 

 stroys its value. Finally, there is a great difference between the 

 organisms which cause the two kinds of scab. Common scab is due 

 to a parasite (Oospora scabies) of very minute, threadlike form, now 

 considered to be more related to the bacteria than to the filamentous 

 fungi. Powdery scab is due to a slime mold (Spongospora subter- 

 ranea), a relative of the cabbage clubroot organism. Its spore balls 

 appear under the microscope as large balls characteristically marked 

 and easily recognized. 



Osborn holds that the soil moisture determines to a great extent 

 the damage done by the disease. Under dry conditions of the soil the 

 external appearance is limited to small circular patches about 5 mm. 

 across. Under wet conditions the damage is more serious and the 

 scabs may be as large as 3 to 4 cm. in diameter and as much as 2 cm. 

 in depth. 



Powdery scab is common in northern Europe, where it has been 

 known for many years. In Canada it occurs in the provinces of New 

 Brunswick, Prince Edward Island, Nova Scotia, and Quebec, not 

 universally but rather generally distributed in many sections. The 

 disease appears not to be established in the United States except in 

 isolated cases, mostly near the Canadian border, where further sur- 

 veys are now being made. There is need for the continuance of 

 careful surveys in all States where any imported potatoes may have 

 been planted, to insure the stamping out of any infection that may be 

 present. 



POWDERY SCAB IN IMPORTED POTATOES. 



Very little is known of the extent to which powdery scab was pres- 

 ent in potatoes brought from Europe prior to 1912. In October, 1913, 

 in response to market demands, large shipments of potatoes began to 

 come in from the Netherlands, Belgium, and Denmark, as well as 

 from Canada. Examinations of these potatoes at the ports of New 



i Cf. Melhus, I. E., Powdery scab (Spongospora subterranea) of potatoes, IT. S. Department of Agriculture, 

 Bulletin 82, 191-1. This publication contains a full description of the disease and the causal parasite. 



