36 



FUXGI AXD FUXGICIDES 



bish. Then the Bordeaux mixture treatment recom- 

 mended for the prevention of apj^le scab will have a 

 decided influence in lesseninor the amount of damasre. 



An account of experiments with this disease may be 

 found in Bulletin Xo. 44 of the Kentucky Agricultural 

 Experiment Station ; and of its nature in the 18S9 rei^ort 

 of the same station. It is also discussed in the 1887 

 Eeport of the U. S. Department of Agriculture, and 

 more fully in the Journal of Mycology, v. YI, p. 1G4. 



The Apple Rust 



Gymnosporangium and Roestelia 



A j^eculiarity of mauy parasitic fungi is that the 

 complete cycle of their existence is not passed upon a 

 single host-plant, but that, instead, one phase of devel- 



FIG. 19. .1, surface view of pustule; i?, mycelium thread; C, ^'eriuiuating spore. 



May:nified. 



opment occurs upon one plant, while another quite differ- 

 ent stage of existence is passed on an entirely different 

 host. A striking illustration of this is seen in the case of 

 the rusts which sometimes affect apple trees, causing the 

 leaves to become more or less blistered with orange-yel- 

 low spots. In this disease the fungus on the apple is an 

 alternating form arising from sj^ores produced by the so- 

 called ^' cedar-apples," or *^ cedar-balls" of cedar or juni- 



