GENERAL CONSIDERATION OF PLANT DISEASES 



275 



matic and meteorologic conditions may be influential, when these dis- 

 turb the normal life of the plant. Light, heat, cold, rain, dew, hail, 

 frost, wind and Ughtning play an important role. The gaseous emana- 

 tions from gas pipes, smelter works, smokestacks, including soot, dust 

 from cement works, acids, poisons, and dye stuffs, which pollute streams, 

 all are determining causes of disease. Traumatism or mechanic injury 

 may be of various sorts and the effects are dependent upon the form and 

 severity of the injury, or wound. 



Fig. III. — Rose-chafer {Macrodaclylis subspinosus). a. Adult or beetle; b, 

 larva; c, d, mouth parts of same; e, pupa, /, injury to leaves and blossoms of 

 grape with heetles at work. {From Marlalt in Quaintance, A. L. and Shear, C. L., 

 U. S. Farmers' Bull. 284, 1907.) 



Living organisms, whether animal or vegetal, may be the cause of 

 disease. All groups of animals may be considered, but the mammals, 

 worms and insects (Fig. iii) are of most importance and interest. 

 Insect depredations of plants are notorious and insects occupy first 

 place in their destructive effects on plants (Fig. 112). Various para- 

 sitic flowering plants are known, as well, as the bacteria and fungi, 

 for their disease-producing powers. 



