SOIL AS HABITAT FOR PATHOGENIC MICROORGANISMS 803 



In other words a soil condition may be brought about, commonly 

 referred to as "sickness," which may not be a result of the activities of 

 microorganisms directly pathogenic to plants, but which is due to certain 

 processes brought about by saprophytic soil microorganisms. 



Saprophytism and parasitism among soil microorganisms. Parasitism 

 is a form of nutrition whereby an organism obtains its nutrients from 

 the tissues or cells of another living organism. Symbiosis or mutual 

 parasitism takes place when the host plant obtains nutrients from the 

 invading organism. Parasitism is a matter of degree in the case of a 

 number of microorganisms (especially fungi) found in the soil. (1) 

 Some organisms are strictly parasitic and are brought into the soil by 

 the growing plant, by wind or other agencies. They cannot grow in 

 the soil, but remain there alive and capable of causing the disease only 

 for a short time; this is true of most smuts and certain bacteria among 

 plant pathogens; Bad. typhosum is a case of an animal pathogen. (2) 

 Some organisms continue to live in the soil a season or more but, in the 

 absence of the host plant, they soon die out. They may, however, be 

 able to attack a number of hosts and thus become more or less perma- 

 nent in the soil, until all the host plants are completely removed ; this is 

 true of the club root organism, Plasmodiophora brassicae; (3) Some 

 organisms are able to grow saprophytically in the soil for many 

 years and become parasitic when the specific host plant is introduced. 

 In this group there are various species of Fusaria (F. oxysporum, F. 

 radicicola, F. lini, etc.), Act. scabies, certain Rhizoctonia (Rh. solani), 

 and many other plant pathogens. Bac. tetani, Bac. anthracis, Bac. 

 botulinus, numerous gas gangrene types as Bac. welchii, and Act. 

 bovis are instances of organisms capable of causing animal diseases, 

 which may find a more or less permanent habitat in the soil. (4) 

 Finally we find organisms primarily saprophytic in nature and abun- 

 dantly distributed in the soil. They are capable, however, of causing 

 diseases of the growing plant or various storage rots, when conditions 

 become favorable. This group includes Rh. nigricans, certain species 

 of Fusarium, Penicillium, etc. 5 



In the interaction of plants and microorganisms, various gradations 

 between strict parasitism and strict symbiosis are found, with mutual 

 parasitism as an intermediary phenomenon. 6 Mycorrhiza formation 



6 See Strong, R. P. The relationship of certain "free-living" and saprophytic 

 microorganisms to disease. Science N. S., 61: 97-107. 1925. 



6 Bernard, N. Involution dans la symbiose. Ann. Sci. Nat. Bot. 9 me Ser., 

 1909, 9; Caullery, M. La symbiose chez les animaux. Bull. Inst. Past., 19: 

 569-583, 617-627. 1921. 



