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soil temperature of potato growing areas in the United States is 
unfavourable for the development of the disease. 
SUMMARY. 
. In a recent paper by Shapovalov'™ the generalisation has 
oe made that Skin spot, as such, does not exist, but that the 
pustules called by that name are the immature sori of Corky 
Scab. Other fungi, such as Oospora pustulans, found in the 
pustules are stated to be secondary invaders of the Corky Scab 
sori. 
2. An analysis of Shapovalov’s argument shows that his 
conclusions are drawn from evidence which is for the greater 
part irrelevant, and from experimental results which are mainly 
negative. The most outstanding of these experimental results 
is the failure to reproduce Skin spot with cultures of Oospora 
pustulans. This fact, combined with the somewhat doubtful 
inoculation experiments previously recorded by Owen'?, raised 
some doubt as to the pathogenicity of Oospora pustulans in 
connection with the disease. 
3. A series of inoculation experiments with pure cultures of 
O. pustulans was carried out by the present writers and was 
successful in reproducing an abundance of typical pustules of Skin 
t. The infection occurred mainly in and about the “ eyes’ 
of the tubers. 
4. Parallel series of inoculation experiments with Spongospora 
subterranea produced no pustules bearing any resemblance to 
Skin spot, but discrete Skin spot pustules and Corky Scab sori 
were produced in a third set of inoculations with inoculums o 
Oospora pustulans and Spongospora subterranea together. These 
Siete afford confirmatory evidence of the individual and separate 
nature of the two diseases. 
5. The external appearance and the internal anatomy of a 
Skin spot pustule are extremely constant and characteristic. 
Sections of the pustules compared with Kunkel’s(*) figures of 
Corky Scab sori show that there are few points of resemblance 
but many of striking difference between the two diseases. 
6. The anatomical features of Skin spot are sufficient in 
themselves to distinguish the disease from any other. 
7. There is no relation whatever between the two diseases, 
Skin Spot and Corky Scab, and the previous work of Owen who 
named Oospora pustulans as the causative organism of the former 
disease is completely confirmed by this work. 
In conclusion, we tender our thanks to Mr. A. D. Cotton for 
his courtesy in putting the problem here discussed before us and 
to Dr. Shapovalov for his generosity in supplying us with a - 
preliminary account of his hypothesis which ate thus enabled us 
to reply to him so soon after the publication of his own paper. 
