SECTION XXXVIII. THE CAUSES OF 

 DISEASES OF PLANTS 1 



MOST plants have a green color and thrive in the light. 

 Certain very small plants, called fun'gl, however, have no 

 green color. Among them are the tiny plants that cause 

 rotting of fruit, spotting and dying of leaves, rust, and smut 

 of grain. Since they have no green substance enabling 

 them to use the carbon from the air, they cannot make their 

 own living. Instead they draw their nourishment from the 

 sap and substances already made by green plants. There- 

 fore they rob the plants on which they grow and cause 

 various diseases, which may affect the leaves, stems, or 

 fruits of useful plants. 



Molds. Among the fungi are certain molds. Fruit 

 mold, or bread mold, is made up of a mass of fine white 

 threads, some of the short branches of which bear tiny 

 black heads. These contain the spores, dust-like bodies 

 from which another crop of fungi grows. Spores are to 

 fungi what seeds are to plants. The spores of mold and 

 of most fungi are so small and light that they are blown 

 everywhere by the wind. This explains why plant diseases 

 are so "catching," or contagious. 



Some fungi cannot grow through the skin of fruits, but 

 need to have their spores planted in cuts or bruises. 

 Other kinds are able to force their way through the skin. 

 Very often they push in through the " gateways " or 



1 The five sections on plant diseases were written by Dr. B. M. Duggar of 

 the faculty of Cornell University. 



Q 225 



