42 Journal of Agricultural Research voj. xvii, No. a 



the fungus is readily recognizable, and both the parasite and the host 

 are easily manipulated under widely varying environmental conditions. 

 With reference to this disease alone no problem was seemingly more in 

 need of investigation from a practical standpoint than the great varia- 

 bility in the occurrence of the disease observed both in general and 

 local areas, and the influence of external conditions on the application 

 of prophylactic measures. The literature, furthermore, abounds in state- 

 ments intended to explain the epidemics of this disease, which are greatly 

 in need of modification and correction. 



Accordingly, a study was undertaken with the view of covering prac- 

 tically all phases of the environmental conditions which might influence 

 the tobacco rootrot. Although it is felt that the problem is still in need 

 of further study, it is believed that the evidence here presented will serv^e 

 to show the relations of the more important factors concerned. 



SYMPTOMS OF ROOTROT 



The rootrot of tobacco and other plants, caused by Thielavia hasicola 

 (B. and Br.) Zopf, is the most serious disease with which the tobacco 

 growers in most producing sections have to contend. Its importance 

 is especially evident in Kentucky, Wisconsin, Ohio, Connecticut, and 

 Petmsylvania. The aboveground symptoms are much the same as those 

 produced by the usual unfavorable soil or weather condition which may 

 stunt the growth of tobacco; hence, as a rule, its effects are not recog- 

 nized by the growers as having a parasitic origin. Where infection is 

 abundant, however, the signs of the disease on the roots are sufficiently 

 specific to leave no doubt as to the causal organism. It is difficult, how- 

 ever, even for the pathologist to judge adequately the relative amount 

 of damage done by T. hasicola and by other causes which may reduce 

 yield, even when the roots are carefully removed from the soil and 

 washed before examination. The relative importance can be deter- 

 mined with considerable accuracy, nevertheless, by comparing the plants 

 especially the roots, which have been grown in infested and uninfested or 

 sterilized soil ; or by comparing both resistant and susceptible strains grown 

 on infested soil. In this way one may find what appears to be a compara- 

 tively unimportant amount of infection is in reality a controlling factor; 

 or, on the other hand, that a seemingly heavy infection is of compara- 

 tively small importance. In this way casual judgment may be replaced 

 by definite experimental evidence. 



The effects of the rootrot may range from a complete checking of the 

 plants, or even death when infection occurs in the early stages of growth, 

 to only slight signs of reduced yield. It is indeed highly probable that 

 under certain conditions considerable infection may be present without 

 appreciably affecting the yield. Furthermore, it appears to be equally 

 certain that in some instances infection by T. hasicola has markedly in- 

 creased yields as a result of temporarily delaying growth during a period un- 



