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others, but we have not yet come across any pustule 
in which it is entirely absent. 
5. The much septated hyphae of O. pustulans are found in 
large numbers in the pustule. They are present in both 
the ‘“ cuticularised ”’ and the thin walled cells, and in 
those of the latter which are invaded, one or more walls 
generally show the initial stages of ‘‘ cuticularisation.”’ 
In sections of fresh material cut during the months March to 
June, the hyphae were hyaline and we have only seen brown 
yphae on two occasions. The brown hyphae mentioned by 
Owen'”? may possibly be more numerous later in the season or on 
early maturing varieties of potatoes. 
A comparison of the features described with those seen in a 
young sorus of Corky Scab is sufficient in itself to show that the 
two diseases are distinct entities. In fact, if this were not the 
-case and the diseases were identical, we should have to believe 
that none of the characteristics of Skin spot described were ever 
noticed by Kunkel'® in his work on Corky Scab. This we cannot 
think possible. We are, however, prepared to admit that in 
seed potatoes where the characteristic appearance of Skin spot is 
masked by the roughness of the skin or by soil particles clinging 
to the tuber, a certain rough resemblance to the young, closed 
sori of Corky Scab exists and is only dispelled by washing the 
potatoes. The comparison may be made on King Edward 
potatoes which often show numbers of immature Corky Scab 
sori and are also very prone to Skin spot. The pustules of the 
two diseases may be found on the same tuber, but a little experience 
soon enables a trained ppserver readily to distinguish between 
them. 
In conclusion, we may say that, although there is no vilation 
between Skin spot and Corky Scab, we may hazard a guess as to 
how the idea of synonymity has arisen in America. 
In the first place, it appears that the superficial type of 
Corky Scab is more prevalent in America than the canker form 
and the immature sori associated with the former will frequently 
occur. An illustration of such sori is given by Shapovalov®® in 
his Plate 3a. On the other hand, an inspection of his plates 
illustrating Skin spot reveals the fact that all are photographs of 
material received from England, Scotland or Denmark. It was 
only from this foreign material that Oospora pustulans could be 
isolated with any degree of constancy, whilst, in the American 
cee it was almost entirely absent. With these facts in 
ind we cannot help but suspect that the American material 
whieh Shapovalov' describes as “undoubted Skin spot” was 
not Skin spot at all. This indeed appears to be the only 
interpretation to place on his work. We are not in a position 
_ to say whether Skin spot disease really exists in America or not, 
but, remembering the preference of O. pustulans for a low range 
of temperature, we make the suggestion that the relatively high 
