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ditions for their growth, and it is usually found that 

 crops rapidly show signs of disease, which becomes 

 more virulent as this kind of weather continues. It is 

 found that the temperature most favourable to the- 

 development of the disease is 73 deg. Fahr. ; above 

 78 deg. Fahr. no germination takes place ; below 

 50 deg. Fahr. there is practically no germination. 



As soon as the conidia form they are blown away, 

 and in favourable seasons the air about a potato field 

 is charged with incalculable millions of these spores, 

 all of which are capable of setting up a fresh attack of 

 disease. It is, therefore, not surprising that the 

 disease spreads rapidly, for in a fortnight each of the 

 spores which establishes itself is able to give off 

 millions of spores. The disease must have been 

 established in the plant for several days before it 

 becomes visible to the naked eye. As each of the 

 spores is capable of giving off so many spores, it is 

 obviously important that the plant should be rendered 

 as impervious as possible to attack in the first instance, 

 and it is because of this that the early spraying of 

 potato crops is so necessary. . 



Where so many of the spores are given off it is only 

 natural that many fall or are washed down upon the 

 soil, and this accounts for a large number of the very 

 frequent instances of diseased tubers on plants which 

 have not been struck in the leaf. It is noticeable that 

 those varieties on which the tubers form in clusters 

 about the stem are more often attacked than are those 

 which form singly at a distance from it. This is 



