CAUSATION OF 1M>A\T DISEASES 



269 



100 



pH 



Figure 105. Relation of soil reaction to the occurrence of potato scab (Reproduced from: Dippenaar 

 B. J. Union of South Africa Dept. Agr. Sci. Bull. 1.36: 52, 1933). 



found difficult if not imposs^ible to do this. 

 It has been obsen-ed in western Nebraska 

 that potato seal:) is always severe following 

 heavy, packing rains when the field cannot 

 be cultivated sul)se(iuently. Scab was also 

 commonly foinid in flooded areas, at the 

 lower ends of irrigated fields, and in poorly 

 drained portions of dry-land fields. This is 

 the reverse of what one might expect from 

 the work of Sanford and Dippenaar on soil 

 moistiu'e and of Sanford on the necessity of 

 abundant aeration for the best development 

 of the scab organism. 



A study was made to det(»rmine whether 

 the effect of soil sterili;cation on the occur- 

 rence of scab was due to chemical or physical 

 changes in the soil or to lack of competing 

 organisms. Sterilized soil was treated with 

 an extract of unsterilized soil. The effect of 

 the time of inoculation was determined ))y 

 adding the inoculum either at the time of 

 planting or at the begiiming of tuber forma- 

 tion. In sterilized soil thei-e was a greater 

 amount of scab. This was due to a lack of 



Figure 106. Corky scab of potatoes (Repro- 

 duced from: Millard, W. A. Common scab of pota- 

 toes. Univ. of Leeds Pam. Xo. 118, 1921, p. 2). 



competing soil microorganisms that would 

 antagonize the scab-producing forms. The 

 numl)ers of actinomycetes in the soil, as de- 

 termined by the plate method, checked well 

 with the incidence of scab in the various 

 tests. The largest number of actinomycetes, 

 82 days after sterilization, occurred in the 

 soil which had been inoculated for the long- 

 est time and in which the inoculum had ap- 

 parently- become well established before the 



