28 BULLETIN 1410, U. S. DEPARTMENT OF AGRICULTURE 



SUMMARY 



Brown root rot is a name used to designate a condition on the roots 

 of tobacco and other plants which is characterized by a brown dis- 

 coloration and decay of the root system, resulting in a stunting of the 

 affected plants. 



This disease is especially common in the tobacco soils of the Con- 

 necticut Valley, where it causes large losses to the tobacco industry. 

 Brown root rot also apparently occurs to a serious extent in other 

 tobacco-growing districts, but it has not been established that the 

 causal agency is the same in all cases. 



In many respects brown root rot has the appearance of a disease 

 due to a parasitic organism. Its behavior with respect to soil sterili- 

 zation, infection, dilution of soil, and relation to environmental con- 

 ditions indicates a parasitic origin. No causal organism, however, 

 has been demonstrated to be definitely associated with the disease. 



The behavior of brown root-rot soils when exposed to desiccation 

 and aeration is not favorable to a parasitic hypothesis. The results 

 of crop-rotation experiments as well as the information gained from 

 a survey of the disease in the field indicate a crop relationship which 

 is contradictory to an explanation based on parasitism. A non- 

 parasitic or chemical explanation of the disease, however, has not 

 yet been found, and the actual nature of the causal agency therefore 

 remains unsolved. 



In addition to tobacco, several other crop plants may be affected, 

 notably tomatoes, potatoes, and certain legumes. No lesions are 

 apparent on such crops as corn, onions, cabbage, and beets. Whether 

 timothy and cereals are attacked was not satisfactorily determined. 



In a crop-rotation system with tobacco, certain of the crops ap- 

 parently not affected at all or not seriously affected by brown root 

 rot favor the development or persistence of the disease in the soil, 

 whereas the commonly affected plants, like tobacco and tomato, seem 

 to favor the disappearance of the disease from the soil. 



Whatever scientific explanation of brown root rot may eventually 

 be developed, the observations of the disease over a period of years 

 and the experimental results reported in this bulletin point toward 

 a possible means of control by attention to the rotation or cropping 

 system practiced. Final conclusions as to the best practices in this 

 respect need further verification, however, before the}' can be gen- 

 erally recommended. 



