26. 



BULLETIN 1410, U. S. DEPARTMENT OF AGRICULTURE 



or bacterium known to behave like the causal agency as regards 

 exposure to desiccation or aeration in the absence of sunlight. On 

 the other hand, this behavior is not in all cases typical of toxic soil 

 constituents which may have arisen from the decomposition or dis- 

 integration of organic or mineral matter. Toxins to be harmful 

 must be appreciably soluble and attain a toxic concentration. These 

 experiments indicate that the causal agency in brown root rot ap- 

 parently is not soluble in water and furthermore that it is not 

 destroyed, although greatly reduced in activity, by dilution. The 

 behavior in these diluted soils is more like that which might be 

 expected in diluting organisms, except that, so far as the writers have 

 been able to observe, no apparent marked increase of the injurious 

 agent follows when affected plants are repeatedly grown in the 

 diluted soil. It is believed, however, that the logical mode of at- 

 tacking the problem would now be from a chemical point of view, 

 with the idea of determining more definitely than the writers have 



Fig. 19. — Tobacco growing after clover on plat 10— A at Whately, Mass., August 15, 



1924 



been able to do the relationship between brown root rot and some 

 of the soil phenomena of a more or less obscure chemical origin. 



The behavior of brown root rot in the cropping experiments at 

 Whately, although contradictory to a parasitic explanation of the 

 disease, does not necessarily eliminate it. The writers are inclined 

 to believe that the disappearance of brown root rot from the soil 

 when grown to tobacco and certain other crops is in some way related 

 to its disappearance when the soil is dried and aerated. There is 

 some practical basis for this hypothesis also when the relative extent 

 of summer cultivation received by the various crops is considered. 

 That tobacco, tomato, and similar crops destroy the injurious agent 

 by oxidation or by other means seems disproved by the fact that 

 fallow plats recover equally well. 



The development of the brown root rot is just as remarkable as 

 its disappearance. Such crops as timothy, corn, and clover are appar- 

 ently excellent examples of agencies either producing or contributing 

 to the cause. There seems to be no evidence, however, that the causal 



