Diseases of the Potato. 299 



the entire"destruction of the leaf may be accomplished. A stoma, 

 or breathing pore may also serve as a point of entrance. 



The rapidity with which the fungus advances within the leaf 

 tissues depends very largely upon external conditions, and the 

 appearances of the affected parts is also modified to a very consider- 

 able extent. Unfavorable conditions frequently render the identifi- 

 cation of the parasite a difficult matter without the aid of a glass, 

 but under such circumstances the disease may be fairly widespread 

 and still cause little injury. In serious attacks, however, many 

 characteristic symptoms may be easily recognized. 



The colored plate represents a leaf which has been entered in 

 several places by the fungus causing late blight, or potato rot. The 

 growth of the parasite has been rapid, and the illustration may be 

 considered as a typical example in which the normal development 

 of the disease has taken place. The following points should be 

 noted : 



The diseased areas are of considerable extent, and possess a rich 

 brown color. They may be situated in any part of the leaf, but the 

 edges appear to sufl'er more from new infection than the more cen- 

 tral portions of the leaflets. This is probably due to the fact that 

 in 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 gain an entrance more easily than in drier places. The de- 

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

 of rapid invasions, 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 affected leaf is very sug- 

 gestive. 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 examined it will be found that the green and the 

 brown areas are not directly in contact with each other; they are 



