10 BULLETIN 82, U. S. DEPARTMENT OF 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 exisis 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 check its spread to 

 nearly every State in the Union. 



MACROSCOPIC DIFFERENCES BETWEEN SPONGOSPORA AND OOSPORA 



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 Chryso- 

 pldyctis 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 

 characteristics 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 



