SYMPTOMS OF DISEASE 



15 



local or general, and 

 may or may not be fol- 

 lowed by rot. 



Wilt of stems or leaves is 

 caused usually by either 

 a plugging, or other in- 

 terruption, of the sap- 

 carrying veins, or by 

 some injury to the ab- 

 sorbing root system. 



Spots of leaves, bark, or fruit 

 may be caused by local 

 blight, rot of tissue, im- 

 perfect coloration, or by 

 local over-development 

 or under-development 

 of color. 



Scab in fruit, leaves, or bark 

 is due to the growth of 

 the causal parasite upon 

 or near the surface, or to thickening of the outer layer 

 or layers of tissue caused by the irritation of a parasite. 



- Sori of rust of asparagus. 

 Original. 



Fig. 5, — Hyphae in a leaf spot much enlarged. Redrawn after Gannan. 



