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THE ACTINOMYCETES, Vol. I 



12 



10 



7.0 



Figure 104. Influence of pH on the growtli of the potato scab organism (Reproduced from: Dip- 

 penaar, B. J. Union of South Africa Dept. Agr. Sci. Bull. 136: 68, 1933). 



vary with the soil moisture or the soil re- 

 action. Increasing soil moisture decreased 

 the amount of scab at all soil temperatures 

 and in all types of soil used, and increased 

 the yield of potatoes. Lower moisture was 

 required to control scab at 13 and at 25 °C 

 than at either 17 or 21 °C. The disease on 

 actively growing tubers was controlled by 

 increasing the soil moisture after the tubers 

 had developed scab. Relatively scab-free 

 tubers growing in a scab-infested soil with 

 a high soil moisture content will become 

 scabby if the soils are allowed to become dry. 

 In greenhouse experiments, Dippenaar 

 found that a reaction of pli 5.0 and lower 

 either controlled or reduced the disease in 

 severely scab-infested soils bul did not elimi- 

 nate scab entirely. A reaction of pH 4.8 and 

 lower, however, had an adverse effect on the 

 potato plant. 



Goss (1937) made a detailed study of the 

 environmental factors influencing the cau- 

 sation of potato scab by S. scabies. The soil 

 was found to be the major source of infec- 

 tion. Although crop rotation reduced con- 

 siderably the incidence of the disease, the 

 fact that tubers in soil never before planted 

 to potatoes could still show severe scab sug- 

 gested that various other factors were in- 

 volved which influenced the severity of the 

 disease. 



Among these factors, high temperature, 

 low moisture, alkalinity, abundant aeration, 

 and the addition of barnyard manure are 

 generally accepted as fa\'oring the incidence 

 of the disease. On reading the extensive lit- 

 eratiu'e, one would assume that it should be 

 possible to correlate the occurrence of the 

 disease in the field with one or more of these 

 factors, rnfortimatel}^ it has often been 



