40 



THE MICROBE AND ITS ACTIVITIES 



Some nematodes are predacious in nature, feeding upon plant 

 parasites. 



Soil Organisms Causing Plant and Animal Diseases. — 

 The soil also harbors a number of organisms which are causative 

 agents of disease, either in plants or in animals. Some of them 

 find in the soil only a temporary habitat, while others persist in 

 the soil for long intervals. The animal pathogens are repre- 

 sented in the soil by such important organisms as Clostridium 

 tetani, causing tetanus, Clostridium hotulinus, causing food poison- 

 ing, and various organisms causing sporotrichosis and actinomy- 

 cosis in man and animals. The number of plant pathogens which 



, ^/^ 



Fig. 30.— Flagellate. Fig. 31.— Ciliate, Boiari- Fig. 32.— Ciliate, CoZpoda 

 Bodo caudatus (from tiophorus elongatiis (from steinil (from Goodey). 

 Martin and Lewin). Goodey). 



find a temporary or permanent habitat in the soil is quite large. 

 This group includes organisms causing such bacterial diseases as 

 various rots, wilts, leaf spots, and galls; such fungus diseases as 

 numerous root rots, dam ping-off fungi and blights; such actin- 

 omycotic diseases as scab in potatoes and sugar beets and pox 

 in sweet potatoes; various nematodes causing diseases which 

 appear as swellings on the roots of a number of plants. 



The soil microorganisms are frequently classified into bene- 

 ficial and injurious forms, on the basis of their relation to man and 

 his cultivated plants and domestic animals. The beneficial 



