32 



Diseases of Economic Plants 



at first soft and wet, in later stages corky, more or less dry, 

 and brown, tan-colored, or black. This spot under favorable 

 conditions increases in diameter at a rate vaiying from 1- 

 10 mm. each day. Single or numerous lesions may appear 

 upon the fruit, depending upon the abundance of the infect- 

 ing spores and their ability to gain access through the cuticle. 

 When a spot becomes quite large, it is usually somewhat de- 

 pressed, with a shriveled, 

 wrinkled surface, due to 

 loss of turgor of the un- 

 derlying tissue caused by 

 evaporation. As the spot 

 ages minute spore-bear- 

 ing pustules of the causal 

 fungus, barely visible to 

 the naked eye, appear, 

 first at or near the center. 

 The tissue of the fruit is 

 affected to some depth, 

 although the progress of 

 the disease is not so rapid 

 downward as laterally. 

 The pulp of the apple at 

 the margin of the rotten 

 region is usually bitter, thus giving rise to the common 

 name of the disease. 



Fruit of any age after its formation is liable to infection 

 if suitable climatic conditions and infective material obtain, 

 though the disease is most noticeable and does most damage 

 as the fruit approaches maturity. A series of hot, wet days 

 favors a destructive attack, while cold checks it. The fruit 

 as soon as it is badly rotted usually falls from the tree, 

 covering the ground beneath, while apples less advanced in 

 disease still hang upon the limbs. 



This same disease occasionally affects twigs and young 

 branches, causing cankers or rough spots in the bark. Young 

 cankers are slightly rough, owing to the dead bark, and ex- 



FiG. 9. — Apple in advanced stage of 

 bitter-rot. After Scott. 



