PART II 



DISEASES OF CULTIVATED PLANTS. 



INTRODUCTION. 



THE idea of disease is not a simple one, though it may seem so 

 before trying to define it. In reality the term disease as ap- 

 plied to plants means any change in that plant toward re- 

 duced vigor, etc., from the ordinary or average behavior. To 

 put it another way, a plant is said to be diseased when it shows any 

 deviation from the ordinary or average behavior of that plant in 

 respect to appearance, growth, color of bark, foliage, fruitfulness, 

 time of dropping leaves or length of life ; in short, when the plant 

 fails to conform to those averages which we have established by ex- 

 tended observation for the species and variety in question, we say it 

 is diseased. Under such a general definition, variegated or purple 

 hued spots would be included, although potentially rather than ac- 

 tually in diminished vigor. Variegated spots succumb to parasitic 

 attack and, as later investigations show, are really suffering from 

 enzymatic troubles. 



The more usual symptoms of disease are marked by evident dif- 

 ferences in the plant. The leaves become spotted, curled or discol- 

 ored, or may even drop prematurely ; the fruit may develop unevenly 

 or be marked by decayed spots, or the twigs may blight, wilt or die. 

 In all such cases we have a manifest loss of vigor and reduced profit. 

 Yet we may not attribute all these to parasitic fungi or to parasitic 

 insects; purely physical or chemical agencies may be at the bottom of 

 certain troubles. Plants may be asphyxiated by too much water 

 which excludes the air supply; they may likewise be strangled by 

 escaping gases, especially in the case of city shade trees, or their 

 protoplasm may be attacked by chemical agents such as strong acids 

 and alkalis. Quick growing plants appear to fall in drought, as with 

 cucumbers when started during a period of excessive rains. Plants, 

 and especially trees, may be locally injured by winter freezing, by 

 hail, by overbearing with exhaustion of water supply, and by a 

 variety of causes. 



While we must keep our minds open to these varying causes of 

 impaired vigor, by far the larger number of the diseases described in 

 this bulletin are directly attributable to parasitic fungi which attack 

 the plant or host in some vital part and rob it of its substance. The 

 conditions of injury arising from the attacks of insects alone are not 

 included. These fungus parasites of particular plants are of differ- 

 ing sorts, which produce, each, its more or less particular effects. It 

 must follow, therefore, that the diseases produced differ in nature 



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