Diseases of the Potato. 303 



the doop of the fungus Macrosporium Solani, yet later investiga- 

 tions have shown that the matter is not so simple as at first ap- 

 peared. It is true that this fungus is almost invariably found in 

 plants affected by this blight, and that the life of the plants is 

 shortened perhaps to a considerable extent by the fungus, yet it 

 may be doubted whether the whole trouble should be ascribed to 

 the one organism. But, before entering into detailed discussion 

 concerning the cause of the early blight of potatoes, it may be 

 well to define as clearly as possible the trouble which is generally 

 designated by this term. 



Upon referring to the plate it will be seen that the leaf upon the 

 right differs considerably from the one which has thus far been 

 considered. This illustration also was made from a typical leaf 

 in order that the more essential features of the trouble might be 

 the more clearly brought out. It will of course be understood 

 that many variations occur, and that these are so great that fre- 

 quently it is impossible to distinguish with the naked eye whether 

 the phytophthora or the macrosporium is present. Such cases 

 are by no means rare, and I have seen a single leaflet suffering 

 from the attacks of both fungi, as was proved by cultures, yet the 

 two diseased areas were practically indistinguishable. Neverthe- 

 less, the following characters will be of assistance in determining 

 which of the two fungi is responsible for the trouble. 



Perhaps the most striking differences between the two leaves lie 

 in the size, form and position of the diseased areas. In the leaf- 

 lets affected with the early blight, it will be seen that these areas 

 are small as a rule, and that they are almost circular in outline 

 except where several have coalesced, in which case the entire area 

 is of irregular outline; but all inequalities have rounded outlines. 

 It appears as if infection occurred at a great number of points, 

 instead of in a few, as shown in the other figure. And it is also 

 interesting to notice that these many points of infection are, with 

 but comparatively very few exceptions, placed along the outer 

 edges or periphery of the leaflets. Although the same is to a cer- 

 tain extent also true with the other disease, still the fungus caus- 

 ing the potato rot does not confine itself nearly so persistently to 

 the edges, but as soon as it is established it extends rapidly to all 

 the softer tissues of the leaflets regardless of whether these are in 



