IHE BROWN ROOT ROT OF TOBACCO AND OTHER PLANTS 



27 



agent is particularly injurious to the growth and normal develop- 

 ment of these crops. Although root lesions apparently occur on 

 clover, the writers can find no evidence that such is the case on 

 corn. That these crops do not produce the injurious action in all 

 soils is evident from their behavior on other soils. 



The relation between the problem described in this bulletin as 

 brown root rot and that described by various authors (i, 5, 6) as 

 crop effects remains to be ascertained. From present indications 

 these may or may not be separate problems. The senior writer 

 has frequently noted cases of the injurious effect of sod on tobacco 

 where brown root-rot symptoms were not present to a sufficient 

 extent to account for the reduced yield. Where brown root rot 

 does occur it seems logical to attribute all or part of the reduced 

 yield to the decay of the root system, although it is conceivable 

 that this may not always be the case. Until more is known, however, 

 about the production of toxic substances in the soil and their action 



Fig. 20. — Tobacco growing after tobacco on plat 9 at Whatelv, Mass., August 15, 

 1924. Compare with Figure 19 



on plants it will be difficult to distinguish between cause and effect 

 in their relation to plant growth. 



The results from the crop -rotation plats at Whately are believed 

 to be applicable to a large number of soils in the Connecticut Valley ; 

 that is, the disease, which has been studied and observed in a large 

 number of different soils, is believed to be due to the same cause 

 and consequently would respond similarly to the same treatment. 

 The results should therefore have an important practical bearing 

 on the control of the trouble. The logical system of crop rotation, 

 from the standpoint of general agriculture, is not apparently a 

 desirable one on soils likely to be affected with brown root rot, so 

 far as tobacco, tomatoes, and certain other crops are concerned. 

 Continuous culture is probably much more desirable on these soils. 

 This system will naturally lead to other difficulties, but according 

 to present indications, these can more readily be controlled than 

 brown root rot. The most serious objection to the continuous cul- 

 ture of tobacco on the same land is the frequent occurrence of 

 Thielavia root rot. The possibility of controlling this disease by the 

 development of disease-resistant strains is, however, very promising. 



