46 



Diseases of Economic Plants 



leaves show symptoms of disease, and the fruit fails to grow 

 to full size. With complete girdling the limb dies. 



Since all infection seems to occur through wounds, pre- 

 vention must look largely toward the avoidance of wounds 



Fig. 



20. — Blister-canker. Some of the discs have been re- 

 moved to show the underlying marking. 



by tools, machinery, harnesses, pickers' ladders, boots, etc. 

 It is also an excellent sanitary measure to cut out and burn 

 all infective material, and in incipient cases excision of 

 diseased tissue in a limb may be practiced with profit. 



