270 



THE ACTINOMYCETES, Vol. I 



soil became infested with competing organ- 

 isms. 



Further tests of the effect of additions of 

 steriHzed and nonsteriUzed organic matter 



Table 76 



The effect of the competition of soil microorganisnts 



upon the occurrence of scab in 



inoculated soils (Goss) 



Treatment of soil* 





Num- 

 ber of 

 tubers 



Tubers in various classes, 

 determined by percent- 

 age of scabby surface 



Sterilized soil, not inoculated 



11. No treatment 20 85 100 



Unsterilized soil, inoculated with S. scabies 



12. No treatment 50 211 17 25 39 19 



* All treatments were made at the time of inocu- 

 lation. All inoculations were made at the rate of 

 200 ml of inoculum per pot, equal to IK Petri dish 

 cultures. Saprophytic organisms were added at the 

 same rate. The filtrates were obtained by soaking 

 4 parts of soil or manure in 6 parts of water over- 

 night, filtering through cheesecloth, and adding 

 200 cc per pot. The manure in Sets 4 and 6 was 

 added at the rate of 200 gm per ])<)t . 



and of the filtrates of both to steriHzed inocu- 

 lated soil confirmed the above observations. 

 Three types of organisms were isolated from 

 the unsterilized soil, cultivated, and added 

 to the soil in amounts approximately equal 

 to the S. scabies content of the soil. This was 

 for the purpose of determining whether anj^ 

 one of these particular groups was an effec- 

 tive competitor of the scab organisms in the 

 soil. These tests were made during different 

 years in the greenhouse, but the results were 

 of the same general nature (Table 76). 



Goss stated that in emphasizing the num- 

 bers of actinomycetes and their relation to 

 soil moisture and other factors, one must 

 consider the fact that these organisms must 

 be growing vegetatively if they are to cause 

 infection. High plate counts often indicate 

 sporulation or fragmentation of the myce- 

 lium, whereas low counts may indicate vege- 

 tative development, and this in turn may be 

 correlated with high infection. Dippenaar 

 suggested that germination, growth, and 

 sporulation may all occur at their maximum 

 imder similar conditions. Conditions favor- 

 able for sporulation would tend to increase 

 the scab-producing power of the soil. If 

 this were followed by conditions fa\'orable 

 for vegetative development, more infection 

 might result. The effect of moisture and ster- 

 ilization upon scab development is brought 

 out further in Table 77. 



Numerous other investigations ha\'e been 

 carried out throughout the world on the ef- 

 fect of various environmental factors on 

 growth of the potato scab organisms and 

 on the cau.sation of scab (Shapo^'alov, Jan- 

 chen, Cocchi, Blodgett and Howe). The effect 

 of the variety of potato on scab develop- 

 ment also has received considerabk^ atten- 

 tion (Umbreit, 1938). 



Relntionship of Host to Parasite 



Schaal and his collaborators mad(^ an 

 extensive study of the effect of phenolic sub- 

 stances U})()n the degree of infection of i)ota- 



