s 



BULLETIN 1410, 17. S. DEPARTMENT OF AGRICULTURE 



(2) Diseased roots: If diseased roots are washed free of soil, cut up, and 

 mixed with healthy soil, typical infection can be obtained. Apparently, the 

 roots are proportionally more effective in this respect than diseased soil. 



Fig. 5. — Cross section of a root of tobacco showing early stages of brown root rot. 

 The cells usually seem to contain a granular material which, together with the 

 cell walls, is discolored 



(3) Steam sterilization: Steam sterilization of the soil effectively destroys 

 the power of diseased soils to produce symptoms of the disease. (Figs. 7 

 and 8.) 



Fig. G. — A longitudinal section of a root of tobacco affected with brown root rot. 

 Note how sharply the affected cells are delimited from the healthy ones 



(4) Formalin sterilization: Treatment of the soil with dilute solutions of 

 formalin (1-50 or 1-100) destroys the ability of the soil to produce symptoms. 



