^58 AGRICULTURE OP MAINE. 



in size, somewhat circular or angular, and always stop at a vein 

 or midrib. Early blight never causes a decay of the tuber. Late 

 blight, as shown in the next two illustrations, produces large 

 blotches on the leaves rather than spots, and when the condi- 

 tions are right these spread very rapidly and kill the entire leaf. 

 They do not stop at the midrib or vein, and are surrounded by a 

 yellowish or lighter green color, fading off from the healthy 

 green of the unattacked portions of the leaf. The under side 

 of the diseased spots on the leaves, if examined early in the 

 morning after a heavy dew, or on a cloudy day following a rain, 

 will show a very delicate, faint, white fringe which is almost 

 invisible to the naked eye. This is made up largely of the fruit- 

 ing organs of the fungus which produces spores in great num- 

 bers in moist, cloudy weather. 



Occasionally the disease may attack the more succulent stems 

 as well as the leaves. This is well shown in the next two illus- 

 trations which were made from material collected in Van Buren 

 some two or three years ago. 



As is well known, epidemics of rot follow severe outbreaks 

 of late blight on the foliage. In its most destructive phases this 

 occurs as a soft, wet, stinking rot, but this is due to the 

 secondary invasion of bacteria and other fungi following the 

 attacks of the late blight fungus. The primary decay caused 

 by this fungus is a dry rot like that shown in the next illus- 

 tration. The next gives the appearance of such a tuber when 

 cut in two. The following two are also illustrations of cross 

 sections of tubers affected by dry rot. 



It is interesting to consider for a moment how the late blight 

 fungus is able to spread so rapidly and do so much damage 

 when conditions are right. The next illustration throws some 

 light upon this point. It is a diagrammatic drawing of a sec- 

 tion of a potato leaf showing the under side. It will be seen 

 that the leaf is made up of cells like all other plant tissues; that 

 the cells on the two surfaces of the leaf are so modified as to 

 form a protective layer to the spongy tissues between. The 

 long pointed bodies projecting from the lower side are leaf 

 hairs. The threads of the late blight fungus are light colored 

 or almost transparent but in the illustration they have been 

 colored so as to make them stand out more clearly. It will be 

 seen that when the tissues become diseased the threads of the 



