PLANT MICRO-ORGANISMS 



393 



carried. Soil from infected fields may be carried on 

 implements, plants, rubbish of any kind, in soil used 

 for inoculation of leguminous crops, or even in stable 

 manure containing infected plants, or in the feces 

 resulting from the feeding of infected plants. Flooding 

 of land by which soil is washed from one field to another 

 may be a means of infection. 



Prevention is the best defense from diseases pro- 

 duced by these soil organisms. Once disease has pro- 

 cured a foothold, it is practically impossible to eradi- 

 cate all its organisms. Rotation of crops is effective 

 for some diseases, but entire absence of the host crop 

 is more often necessary. The use of lime is beneficial 

 in the case of certain diseases. Chemicals of various 

 kinds have been tried with little success. Steam- 

 sterilization is a practical method of treating green- 

 house soils for a number of diseases. The breeding of 

 plants immune to the disease affecting its particular 

 species has been successfully carried out in the case of 

 the cowpea and cotton plants and can doubtless be 

 accomplished with others. 



264. Plant micro-organisms not injurious to higher 

 plants. The vegetable micro-organisms of the soil 

 all take an active part in removing dead plants and 

 animals from the surface of the soil, and in bringing 

 about the other operations that are necessary for the 

 production of plants. The first step in the preparation 

 for plant growth is to remove the remains of plants and 

 animals that would otherwise accumulate, to the ex- 

 clusion of other plants. These are decomposed through 

 the action of organisms of various kinds, the inter- 



