THE BROWN ROOT ROT OF TOBACCO AND OTHER PLANTS 



(5) Temperature relations: The behavior of the disease on plants grown at 

 different soil temperatures is apparently more closely related to biological 

 than to chemical activity. (Fig. 9.) 



(6) Symptoms of disease: The lesions produced on the roots and the re- 

 sponse of the plant in general, together with a tendency to recover from the 

 disease under changing environmental conditions, are more or less typical of 

 parasitic root rots. 



For several years during the course of this investigation little 

 doubt was entertained that the disease was of a parasitic nature. Ex- 



Fig. 7. — Comparison of the growth of tobacco plants : A, In soil affected with brown 

 root rot, showing a brown and deficient root system and stunted growth ; B, in 

 soil like that used for A but sterilzed 



periments were conducted, moreover, which seemed to indicate that 

 various other factors of a nonparasitic nature were not concerned, 

 such as overfertilization, soil toxins, or soil-air relations. 



Two facts have developed, however, during the course of these 

 studies which seem inexplainable on a parasitic basis. Diseased 

 soils spread out in a thin layer and exposed to rapid drying will 

 almost completely lose their power of producing symptoms within as 

 short a time as one to six days. (Fig. 10.) This is true whether in 

 the presence or absence of direct sunlight. This observation seems 

 S7761— 26 2 



