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 
1 Cf. Melhus, I. E., Powdery scab (Spongospora subterranea) of potatoes, U. S. Department of Agriculture, 
Bulletin 82, 1914. This publication contains a full description of the disease and the causal parasite. 
