Waksman — 168 — Actinomycetes 



of S. scabies. To this action, Millard and Taylor also ascribed the 

 effects of green manure in controlling potato scab. Species of Tricho- 

 dernia were found to have a marked effect upon the growth of the 

 scab organism in artificial culture media; in the soil, however, the toxic 

 substance produced by the fungus may be rapidlv destroyed, and the 

 nihibitive action was, therefore, questioned (86). 



Certain interesting conclusions bearing upon the above theories can 

 be drawn from the results presented in Table 43. When grown in 



Table 43 : Ejfect of competitioti of soil microorganisms upon occurrence of scab (299) : — 



Number Number Percentage of scabbv surface 

 of of ^ 



Treatment of soil* plants tubers 0-2 2-25 25-100 



Sterilized soil, inoculated with S. scabies 



Sterilized soil, not inoculated 

 No supplementary treatment 20 85 100 



Unsterilized soil, inoculated with S. scabies 

 No supplementary treatment 50 211 17 25 



* All treatments were made at the time of inoculation. The filtrates were obtained by soaking 4 parts of soil or 

 I parts of water over night, filtering through cheesecloth, and adding 200 ml per pot. The manure was added 

 at the rate of 200 gm per pot. 



sterilized soil, all tubers remained uninfected. When the same soil 

 was infected with S. scabies, only 2 per cent of the tubers remained 

 uninfected. The additional introduction of pure cultures of fungi 

 (^Penicillium sp.), of bacteria, and of actinomycetes CStre'ptomyces sp.) 

 and even of mixtures of microorganisms had comparatively little effect 

 in reducing the scabbiness of potatoes. Filtrates of fresh soil and of 

 sterile manure also had comparatively little effect. However, the addi- 

 tion of fresh stable manure and, to a somewhat lesser degree, of steri- 

 lized manure and of filtrate of fresh manure resulted in a considerable 

 reduction in the percentage and in the degree of scabbiness. This may 

 possibly be due to the fact that the manure stimulates the development 

 of microorganisms which are antagonistic to the scab organism. 



Cultural methods of control.— Various methods are used for the 



