172 PRINCIPLES OF FARM PRACTICE 



Canker. Some tree diseases produce large dead areas 

 in the bark which enlarge more rapidly than the tree can 

 produce callus, and thus eventually girdle the trunk or limb 

 to cause the death of the tree. Such cankers are produced by 

 the fire-blight diseases of apple, by the Illinois apple-tree 

 canker, by the chestnut-blight disease, and by the citrus- 

 canker disease. 



Apple canker. Several diseases cause cankers on apple 

 trees. Among the most destructive of these is the Illinois 

 canker, the black-rot disease, and fire blight. The apple- 

 blotch disease also produces destructive cankers on young 

 twigs. Not only are these cankers destructive in themselves, 

 but they afford means for the parasites to live over winter. 



Smut. Smut diseases are so named because of the char- 

 acteristic masses of black powdery spores which are produced 

 on the host, often in the place of the normal grain which would 

 otherwise have been produced. These diseases are most com- 

 mon on the cereal crops such as corn, wheat, oats and barley, 

 although there is a destructive smut of onions. 



Rusts. The rust diseases are characterized by the yellow- 

 ish or reddish appearance which the small powdery cushions 

 of spores give to the surface of the host plant. The most 

 destructive rusts are probably those which affect our cereal 

 crops such as the black stem rust of wheat or other cereals. 

 The leaf rust, also found on cereals, is caused by an entirely 

 different fungus from that producing the black stem rust. 

 Other examples of destructive rusts are the rusts of the 

 apple, of the bean, of the raspberry, and of asparagus. 



The fungi which cause the rusts can live only as parasites 

 and do not grow when removed from their special host. This 

 is not true of many of the other diseases that have been 

 mentioned. 



