68 



Diseases of Economic Plants 



Scab ^^' "^^ {Venturia pyrina Aderh., Fusidadium) . — This 

 scab much resembles that of the apple and is often accom- 

 panied by a peculiar cracked appearance of the skin. This 

 symptom, however, is secondary and is not necessarily 

 present. Rough cankers are formed upon the twigs. Pear 



scab is generally distributed 

 throughout the United States 

 and Europe. 



The use of a dormant 

 spray followed by two appli- 

 cations of Bordeaux mixture, 

 while the buds are unfolding, 

 is recommended. 



Leaf-spot ^^ {Mycosphce- 

 rella sentina (Fr.) Schr., Sep- 

 toria). — Leaf -spot, while 

 rarely completely destructive 

 to foliage or crop, does inter- 

 fere with the general pro- 

 ductiveness and health of the 

 tree by diminishing its green 

 surface, often causing defoli- 

 ation. It is known over a 

 wide range and is common, 

 especially in the Eastern 

 United States and in Europe. 

 Leaf-spot may be distin- 

 guished from scab and blight 

 by the fact that the diseased area is rather sharply defined 

 and characteristically angular in outline. The center of the 

 spot, usually ashen in color, is surrounded by a narrow brown 

 zone, and this in turn by one of purplish hue. The ashen 

 center bears several minute, dark-colored pycnidia. 



The treatment recommended for pear scab is preventive. 



Leaf -blight'^ (Fabroea maculata (Lev.) Atk., Entomos- 



porium). — This leaf -blight, more serious than leaf -spot, is 



almost as widely distributed as is the pear itself, though 



Fig. 33. — Pear leaf-spot. 

 Anderson. 



After 



