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The object of Series 1 is obvious. From Series 2 we expected 
to be able to observe Corky Scab in all its stages, whilst in 
Series 3 there would be every opportunity for the dual invasion 
of the tubers. Should, therefore, the inoculation in Series 1 
fail to produce Skin spot, a comparison of results from Series 2 
and 3 might give us a solution to the problem. The first three 
series each consisted of four 14-inch pots, and the control series 
of two similar pots. The pots were scrubbed with strong 
Formalin solution, put under a tap of superheated steam, and 
finally placed in a greenhouse which had been thoroughly 
sterilised with Formalin. 
The soil, a light loam off limestone, and apparently poor in 
humus, was autoclaved at 130° C. for 1 hour. 
The potato sets, “ King Edward,” were sterilised in } per 
cent. Formaldehyde solution for 3 hours, and, after sprouting 
in the greenhouse, one set was planted in each pot. When the 
plants were 6 inches high the inoculations were made in the 
following manner. 
Series 1. Six potato plug cultures were carefully shaken 
into a dish of sterile soil. The plugs were then cut up with a 
sterile knife and mixed with the soil. This infected soil was 
carefully spread around the base of one plant and the whole 
pot was then covered to a depth of an inch or more with sterile 
soil for which space had been left. Each of the four pots was 
given a similar inoculation, since previous experience had shown 
us that, in pot inogulations, the conditions are generally unsuitable 
for the development and penetration of aerobic organisms and 
heavy infections such as those described are advisable. 
Series 2. The Corky Scab inoculum was prepared by scraping 
out the spore balls from the open sori of a large number of 
scabbed potatoes. These potatoes werea part of a crop from 
some experiments on Corky Scab made the previous year. They 
had been washed and dried before being stored, and, consequently, 
the sori were relatively free from dirt. Obviously, however, 
some soil particles would be introduced with the inoculum an 
spores of other organisms (including possibly O. pustulans) would 
be present in it. The number of such spores would, however, be 
infinitesimally small compared with the masses introduced with 
the cultures in the other series. ; 
Series 3. Here, each pot was given an infection of O. 
pustulans and Corky Scab each equal in amount to that given in 
the previous series. 
Series 4. A quantity of sterile soil was added to each of the 
pots as in the other series, and it may be pointed out that this 
plan, forming as it does an analogy to “earthing up” in the 
field, is extremely useful in pot work in securing a good crop. 
All the pots were given a dressing of sterilised artificial manure, 
which was applied at the time the potatoes were planted. In 
order to ascertain whether wet or dry conditions of the soil 
might exert any influence on the infection produced, two of the 
