264 PLANT DISEASES 



siderable loss in plants. Gnawing and browsing ani- 

 mals may injure many plants, and insects cause an 

 enormous amount of damage to plants and their prod- 

 ucts, many millions of dollars' worth of plant products 

 being destroyed by them every year in the United 

 States. These insect injuries may be merely me- 

 chanical, as when the foliage is eaten by caterpillars ; 

 galls or other deformities may be formed on the plants; 

 or the life of the plant may be so interfered with by 

 attack on a root system, for instance, that the whole 

 plant dies. 



A third class of plant injuries may be said to be 

 physiological. When the conditions of weather, soil, or 

 other natural surrounding factors are not quite normal, 

 the plants may become sickly and even die. There are 

 many such diseases which are clearly marked and which 

 can, at the present time at least, be attributed only to 

 some serious default in the environment of the plant. 



The plants known as fungi constitute the fourth 

 class of causes of injury and ordinarily the term plant 

 disease is limited to this and the preceding class. Fun- 

 gous diseases of plants are exceedingly common al- 

 most everywhere and are of enormous economic im- 

 portance. One epidemic of rust caused a damage of 

 probably more than ten million dollars in one year in 

 three states of the United States. It is with the fun- 

 gous diseases of plants that this chapter particularly 

 deals. 



Fungi. First of all, it must be understood that the 

 fungi are plants. Some idea may be obtained of the 

 many kinds of fungous plants by enumerating a few 

 conspicuous examples : the mushrooms, common mold 

 of bread, mold of cheese, puff balls, the woody shelves 



