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to by Shapovalov, the only figures (Nos. 1 and 6), in which the 
pustules show any resemblance whatever to Skin spot, are of 
tubers from which Phoma tuberosa was not isolated by these 
writers. The present writers are familiar with Phoma Dry Rot 
accompanying Corky Scab on stored potatoes, but fail to under- 
stand how this could possibly be mistaken for Skin spot 
Wollenweber’s("!) work on Phoma eupyrena is very unconvincing 
and his attempt to bring his work into line with that of Melhus, 
Rosenbaum & Schultz(+) weakens his case so much as to make it 
negligible in any serious consideration of the disease. In the 
reference to Rhizoctonia Scab, a similarity is found between 
Skin spot and Plate v of O’Brien’s (*)paper, but apparently this 
is an error, and Plate vu is that meant. The truth of this com- 
parison is admitted, but, on the other hand, the inclusion of this 
photograph by O’Brien as a specimen of Rhizoctonia Scab seems 
to be based on general observation rather than on experimental 
fact, and the only reference to it in the text is entirely descriptive. 
On this account, it cannot be properly used in any critical survey 
of the literature. 
The only previous work, therefore, which is relevant to the 
discussion is that of Owen(’), and the only organism which is at all 
authenticated as causative of Skin spot is Oospora pustulans. 
We must admit, however, that the inoculation experiments carried 
out by Miss Owen were inconclusive and, to this extent, 
Shapovalov’s criticism is justifiable. 
Turning now to Shapovalov’s own experimental work we 
see that the main results are as follows :— 
(1) The predominant organism found in the pustules of a 
batch of potatoes from Pennsylvania was Colletotrichum 
atramentarium.. Other Fungi and Bacteria were 
isolated but Oospora pustulans was not found at all. 
The latter organism is stated to be rare or entirely 
absent in Skin spot material grown in America. 
(2) A similarity was observed in the appearance of Skin spot 
pustules and the primary or “ Halo” stage of Corky 
Seab described by Kunkel.() _ Furthermore, sections of 
Skin spot pustules were thought to reveal the same 
structure and characteristics as Kunkel describes in 
this “‘ Halo” stage of Corky Scab. 
(3) Inoculation experiments carried out with cultures of 
O. pustulans failed to reproduce Skin spot. 
With regard to the first of these points, it is unfortunate that no 
photograph of the potatoes from which the isolations were made 
is given. Although we are not acquainted with the particular 
species of Colletotrichum mentioned, we were of the opinion that 
members of this genus commonly produced minute, dark-coloured, 
erumpent acervuli easily seen with the naked eye on the surface 
of their host plant. On these acervuli are borne the conidia 
and certain spine-like setae typical of the genus. We have 
certainly never seen anything of the kind on a Skin spot pustule. 
A2 
