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Pot D. All the tubers were absolutely smooth and clean 
as when they were inspected after harvesting. 
Reference was made earlier in this paper to the fact that half 
the pots in the infection experiments were mulched with grass 
in order to keep them constantly damper than the other pots. 
It was hoped thereby to ascertain what relative effect, if any, 
such dry and wet conditions would have on the infections made. 
The season, however, was so uniformly wet that the mulching 
made little difference, and it was impossible to ascribe the greater 
or lesser infection in any pot to this treatment. We are not, 
therefore, in a position to say whether Skin spot is favoured by 
wet or dry seasons, or whether indeed it is affected at all by 
climatic conditions in this country. The deductions to be drawn 
from results of the experiments are obvious, and in our opinion 
they constitute conclusive proof that Oospora pustulans is the 
causative agent of Skin spot. The ample reproduction of the 
disease in typical form by means of pure cultures of the fungus 
under the conditions imposed is in itself sufficient evidence for 
this conclusion. If, however, further proof were needed, it is 
found in the fact that no pustules in any way reminiscent of 
Skin spot were reproduced by Corky Scab infection alone, but 
when, as in Series 3, Oospora pustulans was added to this, the 
two diseases Corky Scab and Skin spot appeared side by side. 
Identification of Skin spot pustules, 
So good a general description of Skin spot has already been 
given by Owen(’) that we do not wish to duplicate it. There are, 
however, a few points connected with the external and internal 
appearance of the pustules that we should like to comment on. 
In the first place, Owen differentiates between two types of 
pustule, a “ pimple ” form, and a form which she calls “ flat”’, 
with a “ crater-like”’ depression. She regards the former type 
as occurring on the coarser skinned varieties, such as Arran Chief, 
and the latter as being more common on the thinner skinned 
varieties such as King Edward. We cannot quite agree with 
her on this point. The potatoes used in our experiments were 
King Edwards, and here the smaller pustules were definitely 
pimple-like. As, however, the age and size of the pimple 
increased, so a dark depression gradually formed around it 
producing a crater-like appearance. It is easy to see how with 
the further development of this depression, and the natural 
shrinkage of the tuber, the crater-like impression would be 
intensified. We are strongly inclined to believe, therefore, that 
in all cases the pimple form is merely the early stage of the disease. 
The diameter of the artificially reproduced pustules (including 
the depression when present) varied from 0-5 to 2 mm. in diameter, 
the great majority having a diameter of 1 mm. 
In the next place, Owen(’) describes the colour of the pustules as 
brown when wetted, whilst Carruthers() called them bluish black. 
This colour is most difficult to define on account of the curious and 
