36 



Diseases of Economic Plants 



both fruit and leaves, probably in all localities where apples 

 are grown, and is serious in Europe, Australia, Africa, and 

 New Zealand, as well as in Ainerica. 



Upon the fruit, the scab first causes the skin to take on 

 a dark olive-green color; later, as the outer skin breaks and 



Fici. 12. — Apple scab. After Craig. 



exposes the fungus underneath, the color changes to black. 

 An apple in this stage is shown in Fig. 12. Still later the 

 scab enlarges, and the spores and fungous cells at the center 

 of the spot may fall away, leaving a skin of rusty appearance 

 surrounded l^y a dark ring where the fungus has not yet 

 fallen off. Farther from the center of the spot may be an 

 olive-green or silvery ring, showing the still younger stage of 

 the disease. Cracking and distortion occur in cases of very 



