THE POTATO FUNGUS 211 



may penetrate very deeply. In other words, they may 

 cause deformities by purely mechanical means, pushing 

 the tissues of the host apart, although in almost every in- 

 stance it is evident that the excretions and secretions of 

 the fungi exert a poisonous influence on the adjacent or 

 even over remote parts of the plant. This may be illus- 

 trated in the following specific diseases: 



The Potato Fungus, the Vine Mildew, the Onion 

 Mold all grow in the tissues of their host, push the cells 

 of the leaf apart and injure or kill the cells, presently 

 emerging by breaking the epidermis and sending out 

 through the crack their reproductive bodies. 



The Rusts. — The rusts of wheat and many other 

 plants are due to minute fungi, the spores of which either 

 infect the grains or are scattered from the parent plant 

 and subsequently infect the young plants by entering the 

 leaf through the stomata, in either case attacking mainly 

 the tissue of the leaf and thereby interfering with the 

 processes of food manufacture upon which the profitable 

 cultivation of the plant must depend. Another rust is one 

 which forms the cedar balls on the juniper or cedar trees 

 of the eastern half of the continent. A similar one is to 

 be found on the leaves of the wild mallow of the Pacific 

 Coast. 



The Mildews. — Another group of fungous diseases is 

 known as the mildews. The one with which people gen- 

 erally are most familiar occurs on the foliage of the rose. 

 The varieties of rose differ very considerably among 

 themselves in their susceptibility to mildew. They are 

 all more or less susceptible, however, according to the 

 condition of the weather. It is perfectly possible 

 to control this disease by the simple expedient of 

 spraying. That the mildews cause very real injury is 

 proved by observation of the behavior of dahlias. These 

 may be blooming abundantly in the garden; but when 

 extensively attacked by mildew on their leaves, they cease 

 to bloom and remain sterile for the rest of the season. 



