cj26 Journal of Agricultural Research voi.xx.No. 7 



CONDITIONS INFLUENCING THE DISEASE 



A thorough study of the environmental conditions influencing the 

 occurrence and extent of injury by the Fusarium-wilt disease has not 

 been undertaken. Some evidence has been obtained, however, through 

 experimental work and observation which is of interest in this con- 

 nection. The progress of experimental work along this line was inter- 

 fered with by the difficulty of obtaining a high percentage of infection 

 even under favorable conditions, so that a considerable number of plants 

 would have to be grown to obtain good data. This very fact should in 

 itself stimulate further research along this line, since it is evidence that 

 the environmental conditions most conducive to parasitism are not fully 

 understood. Where a number of factors are involved, however, this 

 subject becomes exceedingly complex. The introduction of such a factor 

 as wounding, which may occur "naturally" or may vary in considerable 

 degree when produced artificially, is a complicating factor in the tobacco- 

 wilt disease, which in some respects renders it unfavorable for such a 

 study. 



The evidence for the conclusions presented in this paper will not be 

 given in detail. The methods of investigation were essentially the same 

 as those which were used in a study of the influence of soil environment on 

 the rootrot of tobacco as described by Johnson and Hartman (5). The 

 soil used was, however, artificially infested from pure cultures following 

 steam sterilization. The object of the soil sterilization has been partly 

 to secure better infestation of the soil following inoculation. In practi- 

 callv all cases the inoculum has been grown on 100 parts sand, 10 parts 

 of corn meal, and 1 part of glucose to 50 parts of water. A heavy 

 growth of mycelium and an abundance of spores on this medium un- 

 doubtedly suffice to inoculate the soil thoroughly, as is shown by instances 

 in which "100 per cent infection" occurs (Pi. 63, B. C). Where a uni- 

 form infestation of the soil is not required, rapid infection can be secured 

 by using conidia and mycelium from ordinary cultures placed in the soil 

 about the wounded stems. 



As will be shown later, the White Burley variety of tobacco was 

 found to be the most susceptible to the Fusarium-wilt disease; therefore, 

 this variety was used in all cases in the environmental studies. The 

 use of other more resistant varieties would have rendered the securing 

 of results far more difficult. It is assumed, however, that the same 

 relative results would have been secured with the more resistant 

 varieties. 



The seedlings were in all cases transplanted into the infested soil from 

 steam-sterilized soil. The root systems especially were therefore in all 

 cases more or less wounded in their removal from the soil. Although 

 infection has been observed in seedlings not transplanted, it is quite 

 certain that the tobacco-wilt organism is largely dependent upon wounded 



