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obtained in the spring of 1922. The tubers were well washed, 
dried, and sterilised in alcohol. Thin sections from a pustule were 
then taken from well below the surface and plated on Nutrient 
Potato Agar (pH @ 6-6) in Petri dishes. Very little bacterial 
contamination occurred, but fungus threads, apparently belonging 
to the same species, grew out in all directions from the sections. 
After plating a number of pustules from different tubers in this 
way, plates were obtained showing nothing but the fungus, and 
from these several pure cultures were made. The latter pre- 
caution was taken since it was thought, in spite of apparent 
similarity, that different species of fungi might be present. The 
cultures made, however, eventually turned out to be the same. 
We had not seen O. pustulans in culture before and Owen’s(’) 
account of the fungus was not to hand at the time. From our 
own cultures, however, we made single spore cultures in hangi 
drops of media by means of which we were able to trace the 
development of the species from the germination of the spore 
to the full development of the conidiophores and spore chains. 
Careful drawings were made throughout, and a comparison 
later with Owen’s(’) figures and description left no doubt whatever 
in our minds that we were dealing with O. pustulans. We were, 
moreover, interested to find that we had passed through the 
same difficulties in the choice of the most suitable media and of 
the optimum temperature for growth of the fungus which 
Miss Owen describes. All the cultures subsequently used for 
inoculation purposes were grown on potato ha 
Inoculation experiments. 
It has already been stated in the introduction to this paper 
that we were aware of the hypothesis which Shapovalov(!®) had 
tentatively advanced to account for Skin spot. We considered 
the idea with an open mind, and our experiments were designed 
to test it as far as possible. At the same time, we were much im- 
pressed by the abundance of the fungus in the skin spot pustules, 
and found it very difficult to suppose that this was but a secondary 
organism. We thought it possible, however, that a double infection 
of Spongospora and of Oospora might occur, and that, supposing 
an antagonism existed between these two, such as is well known 
to exist between certain species of Bacteria, this might account 
for the non-development of the plasmodium of Spongospora, 
and perhaps also for the subsequent death of the filaments of 
Oospora as recorded by Owen(?). This hypothesis was, no doubt, 
even more hypothetical than Shapavalov’s, but, at the same time, 
it appeared to us feasible and more in accord with the fact that 
the general appearance of Skin spot was so essentially different 
from Corky Scab. The following series of inoculations were, 
therefore, planned with these duplicate hypotheses in mind. 
Series 1. Inoculation with O. pustulans only. 
z. Corky Scab only. 
O. pustulans and Corky Seab. 
a? 2? 39 
2? 
bed 3. Ye 
., 4. Control. 
