THE FUNGI WHICH CAUSE PLANT DISEASE 239 



On the leaves the first evidence of the spot is the slight blanching 

 of a single one of the smaller areola of the leaf. Soon the blanch- 

 ing extends to adjacent areolae, and if an areola is entered it is 

 usually entirely involved. The small veinlets form the margin 

 of the spot so that the outline is finely crenulate. By the time 

 the spot is .3 to .4 mm. in diameter it has a cinereous appearance. 

 The margin, while sharply defined, is not changed in color. By 

 the time the spot is 1 mm. in diameter, the margin appears as a 

 black line, while the remainder of the spot is grayish-brown. A 



i^ ®.SL. ^jy= Spates 



FiQ. 174. Diagrammatic section through a pycnidium, show- 

 ing how the spores are produced and how they germinate. 

 After Reddick. 



little later the margin is a brownish band and the brown gradually 

 extends inward until the whole spot is covered. As soon as the 

 brown band attains some width the blackish line on the margin 

 is to be seen again. A second wave of deeper brown may pass 

 across the spot but sometimes it does not get entirely across and 

 thus leaves a marginal band of a deeper brown than the central 

 disc. Spots vary in size from 1 mm. up to 8 mm. in diameter, but 

 in general are 3 to 5 mm. or larger. Occasionally the whole leaf is 

 destroyed but this is by the coalescence of many spots. When 

 the spot has attained full size pycnidia protrude from under the 

 cuticle and either dot the entire surface of the spot with minute 

 specks or are more often confined to a more or less concentric ring. 

 The different shades of color are apparent on the imder side of 



