SOIL AS HABITAT FOR PATHOGENIC MICROORGANISMS 809 



so-called "ring rot." 33 The common soil organism R.hizopus nigricans 

 is the cause of soft rot of sweet potatoes. 



Plant and animal diseases caused by species of actinomyces. Several 

 plant pathogenic actinomyces have been found in the soil, including 

 Act. scabies, the organism causing the common scab disease of potatoes 

 and sugar beet. 



On comparing a large number of organisms isolated from scabby 

 potatoes, we can readily recognize that we are dealing here not with 

 one species, but with a whole group which can be readily subdivided 

 into several sub-groups, not only on the basis of physiological charac- 

 teristics, but also on the basis of morphology. As a matter of fact, 

 as many as 30 species of actinomyces have been described, 34 which are 

 supposed to be causative agents of potato scab, the type of lesion being 

 influenced by the species. 



The formation of pox on sweet potatoes may be due, to some extent at 

 least, to an actinomyces 35 found in the soil. 



The causative agents of human and animal actinomycotic diseases are 

 often claimed to be brought about by soil organisms or forms harbored 

 upon plants. 36 Klinger 37 called attention to the fact that the aerobic 

 actinomyces commonly found on grasses and in straw infusions (also 

 in soil) have never been isolated by him in any actinomycotic case. 

 Only anaerobic forms were obtained from the latter; these develop on 

 most media at temperatures above 30°C, and only seldom were cultures 

 obtained which make a scant growth under aerobic conditions. Mixed 

 infections consisting of anaerobes growing at body temperature together 

 with aerobes are often obtained. We have to do here with species 

 which have adapted themselves to a symbiosis with warm blooded 

 animals, and which have almost nothing in common with aerobic sap- 

 rophytes. However, there is no doubt that some aerobic actino- 

 myces are capable of causing infections of men and animals. 

 The actinomyces capable of causing plant diseases, such as potato 



33 Cook, M. T., and Taubenhaus, J. J. Trichoderma koningi the cause of 

 a disease of sweet potatoes. Phytopathol., 1: 184-189. 1911. 



34 Wollenweber, H. W. Der Kartoffelschorf. Arb. Forsch. Inst. Kartoffelbau. 

 H. 2. 1920; Millard, W. A., and Burr, S. Ann. Appl. Biol. 13: 580-644. 1926. 



35 Taubenhaus, J. J. Pox, or pit (soil rot), of the sweet potatoes. Jour. 

 Agr. Res., 13: 437-450. 1918. 



36 Odermatt, W. Aetiologisches zur Aktinomykoseerkrankung. Schweiz. 

 Med. Wochnschr., 50: 26-28. 1920. 



37 Klinger, R. Zur Oetiologie der Aktinomykose. Centrbl. Bakt., I, Or., 

 85: 357-359. 1921. 



