814 PRINCIPLES OF SOIL MICROBIOLOGY 



to the growth of the host plant, as in the case of potato scab 61 - 62 and 

 wheat scab. 63,64 Some organisms have a certain acid minimum or alkali 

 maximum which permit methods of control. Act. scabies and most other 

 actinomyces, for example, not do thrive well at pH less than 4.8; 

 Plasm, brassicae is inhibited by an alkali reaction 65 obtained by the addi- 

 tion of lime. Bart, solanacearum causes serious infection in acid soils, 

 but seldom in neutral or alkaline soils. 66 F. lycopersici causes minimum 

 infection at pH 6.4 to 7.0 (Scott) ; it has both an acid and an alkaline 

 maximum. 



Method of control. To combat disease producing organisms, one has 

 to know not only the life history of the pathogen, but frequently, as in 

 the case of nemas, the host history of the organism. In this case crops 

 susceptible to the particular species, but not the particular population, 

 may be employed. 



The saturation of soil with formaldehyde to prevent spreading of 

 disease has often been practiced. In addition to formaldehyde, various 

 other soil fungicides and volatile antiseptics, like CS2 and toluol, have 

 been frequently employed for the destruction of the plant pathogenic 

 fungi. 67,68 CS 2 can be used with success against a number of fungi, 

 such as Demalophora necatrix, Rhizoctonia, Synchitrium 69 etc. Miege 70 

 obtained the best results with toluene in controlling Sclcrotinia libcrtiana t 



61 Gillespie, L. J. The growth of the potato scab organism at various hydro- 

 gen-ion concentrations as related to the comparative freedom of acid soils from 

 the potato scab. Phytopathol., 8: 257-269. 1918; Gillespie, L. J., and Hurst, 

 L. A. Hydrogen-ion concentration-soil type-common potato scab. Soil Sci., 

 6: 219-236. 1918. 



62 Waksman, S. A. The influence of soil reaction upon the growth of actino- 

 myces causing potato scab. Soil Sci., 14: 61-79. 1922. 



63 Hopkins, R. F. The hydrogen-ion concentration in its relation to wheat 

 scab. Amer. Jour. Bot., 9: 159-179. 1922. 



64 Mclnnes, J. The growth of wheat scab organism in relation to hydrogen- 

 ion concentration. Phytopathol., 12: 290-294. 1922. 



65 Atkins, W. R. G. Hydrogen-ion concentration and club root. Sci. Proc. 

 Roy. Dubl. Soc. N. S., 16: 369-413. 1922. 



66 Arrhenius, O. A factor influencing the growth of tobacco wilt disease. Ark. 

 Bot., 18: No. 1. 1922. 



67 Halsted, B. D. Soil fungicides for potato and turnip diseases. N. J. Agr. 

 Exp. Sta., Sp. Bui. S. 1900. 



68 Johnson, J. The control of damping off disease in plant beds. Wis. Agr. 

 Exp. Sta. Res. Bui. 31, 29-61. 1914. 



69 Gimingham, C. T., and Spinks, G. T. Soil sterilization. Jour. Bat. and 

 West and South Cont. Soc. 14, 126-130. 1920. 



T0 Miege, 1917 (p. 766). 



