296 THE ESSENTIALS OF AGRICULTURE 



reddish-brown, or black spots on stems (Fig. 145) and leaves of 

 the affected plants. There are several species of rusts, each 

 species being confined more or less closely to its own particular 

 host plant. When the spores are scattered and fall on new 

 wheat or oat plants and the weather conditions are favorable, 

 the infection of new host plants takes place. 



The prevention of rusts is very difficult. Individual treatment 

 of plants cannot be undertaken. Since the grain fields are ex- 

 tremely extensive, spraying is out of the question. Since the 

 rust is not transmitted through the seed, it cannot be controlled 



FIG. 146. Cowpeas on wilt-infected soil 



The variety known as " The Iron " at the left is resistant to the disease ; that at the 

 right-is nonresistant. (After Orton) 



by seed treatment, and crop rotation is of little help. The only 

 method of controlling grain rusts is to grow varieties of grains 

 which are resistant to the disease. Some are already known, 

 such as the durum wheats, common on the great plains of 

 the middle Northwest. Crossing with them is being carried 

 on, and there is promise that valuable resistant varieties may 

 be obtained. The use of varieties of plants resistant to disease 

 is well illustrated by the growing of rust-resistant asparagus, 

 recently developed by the United States Department of Agricul- 

 ture, wilt-resistant cowpeas (Fig. 146), and wilt-resistant cotton. 

 Even the chestnut-bark disease (Fig. 147), which has recently 

 destroyed whole forests of chestnuts in the East, probably will 

 not injure the Japanese chestnut or its hybrids. 



