274 
A preliminary account of the hypotheses upon which this 
remarkable theory was based was communicated by Dr. Shapo- 
valov to Mr. A. D. Cotton in March 1922. The correspondence 
was very kindly forwarded by Mr. Cotton and, at Dr. Shapovalov’s 
suggestion, we started to investigate the theory. We did not 
know that Dr. Shapovalov had carried out any direct experi- 
mental work of his own on the subject beyond the preliminary 
examination of a number of pustules, and we were, therefore, 
very surprised to learn from his paper that, no doubt as the 
outcome of this work, his theory had crystallised into a statement 
of proven fact. We venture to quote three paragraphs which 
show how very definite are the conclusions at which he has 
arrived. 
(1) ‘Neither Oospora pustulans nor any other filamentous 
fungus or environmental conditions mentioned by 
previous authors in connection with Skin spot are 
proved to be the primary cause of the disease.” 
(2) ‘The fungi associated with the spots in question are 
mainly secondary invaders developing during the 
: storage period.”’ 
(3) “. . . Skin spot pustules are essentially and primarily 
the closed or immature sori of powdery scab.” 
Before describing our own experiments, we wish briefly to discuss 
the salient points of evidence upon which these conclusions are 
ased. 
Shapovalov(!*) calls attention to the variety of organisms 
which have been isolated and given causative rank by different 
investigators. These have already been noted, but the author 
swells the number by including O’Brien’s work on Rhizoctonia 
Scab (Rhizoctonia crocorum)(*). In addition he himself has 
found the predominant organism in pustules stated to be Skin 
spot to be Colletotrichum atramentarium. In no case does he 
regard the causative nature of any of these organisms to have 
been proven by inoculation experiments, and, in view of these 
facts, he forms the opinion that such organisms are accidental 
or secondary invaders of the pustules—the particular species 
found in any place being dependent on climatic or environmental 
conditions. In this connection he notes the prevalence of 
O. pustulans in English material as compared with Colletotrichum 
atramentarium in American. A little unbiassed examination 
of this argument will show that it lacks the support of sub- 
stantiated facts. It is only too easy to name plant diseases 
which have passed under a category of misnomers before receiving 
their legitimate baptism. In the present instance neither Pethy- 
bridge(*) nor Milburn & Bessey(*) claim to have established the 
pathogenicity of the organisms they isolated. é 
Again, the work of Melhus, Rosenbaum & Schultz(4) shows very 
clearly that the Dry Rot which they found associated with 
Corky Scab produces lesions which are quite distinctive and 
unlike Skin spot pustules. In fact, in the colored plate referred 
