Second Report on Potato Cuf.TURfi. 63 



in any part of the leaf, but the edges appear to suffer more from new 

 infection than the more central portions of the leaflets. This is proba- 

 bly due to the fact that in the case of rains these portions remain moist 

 for a longer period than the center, since the water drains to the lower 

 parts of the leaflets, and collects there in the form of drops of greater 

 or less size. It is to be expected that under such conditions a fungus 

 could giin an entrance more easily than in drier places. The decayed 

 portions are inclined to droop ; this is especially true in cases of rapid 

 invasion's, for at such times the parts do not dry so fast as the parasite 

 advances. The rapid decay also prevents the edges of the leaflets 

 from curling, although this takes place when the air becomes warm 

 and dry. 



The distribution of colors over the afTected leaf is very suggestive. 

 Under normal conditions, the unaffected parts retain a deep green 

 color, while the diseased area may be yellowish brown, dark brown, or 

 nearly black. But whatever the color each area is sharply outlined. 

 There is no gradual merging of one into the other, but a distinct 

 change of color marks the progress of the disease. Occasionally 

 another peculiarity may be noticed. If the leaves are closely exam- 

 ined it will be found that the green and the brown areas are not 

 directly in contact with each other ; they are separated by a narrow 

 strip in which the green has been destroyed, and the brown has not 

 yet appeared. It consists of a colorless or at most a very pale yellow 

 line in which the growth of the fungus is probably very active^ But 

 during periods which are unfavorable to the development of the para- 

 site this line cannot be discerned, and the green and brown tissues are 

 apparently in contact. Under such circumstances the identification 

 of the disease without the aid of a microscope is an exceedingly diffi- 

 cult matter. Let us suppose that the fungus has succeeded in gaining 

 an entrance, and that it has advanced' a limited distance in the leaf 

 tissues. If at this time the weather should turn dry and hot, the 

 development of the parasite would be checked, and the result would 

 be the formation of a small brown spot or area, perhaps near the edge 

 of the leaflet, and if several such spots exist the injury might be 

 ascribed, without careful examination, to what is commonly known as 

 the early blight fungus. 



The name " downy mildew " has been given to the potato rot dis- 

 ease trom the fact that there appears, under favorable circumstances, a 



