10 BULLETIN 82 ? U. S. DEPABTMENT OE AGRICULTURE. 
been found on the American Continent, and the brief experience with 
it in eastern Canada gives no hint of what its behavior would be in 
the southern trucking districts, the central West, or the irrigated 
sections. The common scab is much worse in many parts of the West 
than in the East. 
Another reason for grave concern in the United States is that the 
disease exists in that portion of Canada adjoining the State of Maine, 
which is the chief source of seed potatoes for the Central Atlantic and 
Southern States. If powdery scab becomes generally distributed in 
Maine, only the most extraordinary efforts can cheek its spread to 
nearly every State in the Union. 
MACROSCOPIC DIFFERENCES BETWEEN SPONGOSPORA AND OQSPQEA 
SCAB. 
It should be made clear in discussing the similarity of and differ- 
ences between Spongospora and Oospora scab that the symptoms and 
ultimate effect on the tuber vary markedly in the case of both dis- 
eases, depending upon external influences. In spite of the wide 
variation of powdery scab, two characteristic stages of the disease 
may be recognized, namely, the scabby and the cankerous stages, 
shown in Plates II and III, respectively. It is only the former of 
these that can be easily confused with the Oospora scab, and there- 
fore the latter stage needs no further consideration in this connection. 
As pointed out by Home, the early stages of Spongospora resemble 
markedly the beginning stages of the wart disease caused by Cliryso- 
glilyctis endobiotica, in that wartlike excrescences appear on the 
tuber. Such symptoms are in no way like those of the early stages 
of Oospora scab, and this naturally leaves for comparison only the 
eh&racteristies of the two diseases as found on the mature tuber at 
harvest time and shortly thereafter. 
The scabby stage of Spongospora on the mature tuber, as illus-' 
trated in Plate II, usually differs essentially from Oospora scab in 
three ways : 
(1) The sori are more often circular and not usually as great in 
diameter as those of Oospora scab. 
(2) The periphery of each sorus is bordered by the upraised outer 
epidermal layer of the tuber, so that virtually small cups or pits are 
formed, as shown in Plate II, B and C. 
(3) These pits are usually deeper than those of common scab and 
are always filled at maturity with a brownish colored semicompacted 
dust or sediment, as shown in Plate II, C. The sori of Oospora are 
usually shallow and composed of corky material of a compact and 
interwoven nature. 
It should be remembered that it is extremely difficult, if not 
impossible, to define the difference between two diseases varying so 
