Diseases op the Potato. 305 



slowly, and that the change shall take place from the outer por- 

 tions toward the center. As already stated, such conditions may 

 or may not appear during an invasion of the late blight, and for 

 this reason the curling of leaflets affected by early blight is of 

 considerable value in identifying the present disease. 



In addition to the characters above mentioned, there are sev- 

 eral other factors which appear to be clearly connected with the 

 early blight of potatoes. As the popular name of the disease 

 implies, its appearance may be expected earlier in the year than 

 the late blight; but from this it does not follow that later attacks 

 may not take place as well. The growth of the fungus does not 

 seem to depend so. much upon the season as it does upon the con- 

 dition of the plants exposed to infection. The writer has occa- 

 sionally seen potatoes of the same variety growing side by side, 

 but which were planted at different times, but were unequally 

 affected by disease. The earlier plantings invariably showed 

 much more injury than the later ones. In some cases the differ- 

 ence was so marked that it would scarcely be exaggerating to say 

 that the younger plants were entirely free from disease, while 

 the older plants, or those first set out, had lost about 50 per cent, 

 of their foliage area. Other modifying conditions were sought, 

 but no other conclusions could be drawn than that in these cases 

 at least, the entrance of the fungus depended upon the plants 

 having reached a certain age. 



A similar circumstance has frequently been noted in various 

 parts of the state, with this difference, however, that the plants 

 growing side by side were not of the same varieties. The effect 

 was especially marked when late and §arly varieties were grown 

 in the same field. The earlier the potato the sooner did it show 

 the effects of disease, the later varieties remaining free for a long 

 time; or, in case of late plantings, the foliage may have escaped 

 the trouble to a marked extent. A large number of fields have 

 been examined with these points in mind, and such observations 

 have led to the conclusion that young, vigorously growing plants 

 are practically free from the disease, while those which have 

 almost completed their growth of foliage, and are rapidly forming 

 tubers, are much more subject to attack. 



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