Waksman 



162- 



Actinomycetes 



From his use of the contact sHde method, Dippenaar (90) con- 

 cluded that soils rich in organic matter, and containing water equiva- 

 lent to between one-half and two-thirds of its water-holding capacity, 

 permit the vegetative growth of the scab organism. 



In general, the nature and abundance of organic matter, the reac- 

 tion of the soil, and the presence of antagonistic organisms— all greatly 

 influence development of the scab-producing organism and the amount 

 of scab. 



-TUbers scabbe6 



pHoj: soi/ 



Fig. 33.— The relation of soil reaction to the occurrence of potato scab Cfrom 

 Dippenaar, 90). 



Influence of soil and environment tifon scab develo-pment.— Aside 

 from virulence of the infecting organism and susceptibility of the potato 

 variety to scab, the nature of the medium in which the potato is grown, 

 that is, the soil, and the environment plav the greatest role in the pro- 

 duction of scab. The more stable factors include the organic matter 

 content of the soil, its degree of decomposition and its reaction. The 

 fluctuating environmental conditions comprise three important factors, 

 namely moisture, temperature, and aeration. 



