46 Journal of Agricultural Research voi. xvn. No. a 



GENERAL CONSIDERATION OF FACTORS CONCERNED 



It is evident at the outset that any attempt at a separate analysis of 

 each factor concerned in disease occurrence is practically impossible. 

 Varying one single factor of the environment to the total exclusion of 

 variability in all others is an ideal to be kept in mind in experimental 

 work of the nature to be described. Failure to reach this ideal in prac- 

 tice, however, need not necessarily reduce the value of the result, pro- 

 vided the effect of other variables on such a result is properly considered. 

 As an illustration of a complication of factors of this sort, there may be 

 cited the maintenance of two pots of soil at two different temperatures, 

 say 30° and 10° C, respectively, in order to compare the effects of these 

 temperatures on the occurrence of T. hasicola on the roots of tobacco. 

 By means of proper controls in uninfested soil the influence of many 

 factors involved may be eliminated, but it does not seem possible to sepa- 

 rate clearly the factor of soil temperature from that of soil moisture. 

 The soil and plants at 30° will require several times as much water as will 

 the soil and plants at 10° because of the increased evaporation and trans- 

 piration at the higher temperature. The correct replacement of this 

 water for maintaining like moisture relations for the host and parasite is 

 uncertain no matter how frequently and carefully it may be done either 

 by weight or by the use of an auto-irrigator. If, however, the moisture 

 relations have previously been studied and the range of the effects to be 

 expected from this factor are known, it may be possible to carry on 

 soil-temperature studies with only moderate attention to the moisture 

 relations. 



A study of the factors concerned in the development of the Thielavia 

 rootrot has served to bring out clearly the fact that all the factors con- 

 cerned are inseparably connected with one another, and that the amount 

 of disease occurring is the product of a number of plus and minus factors, 

 but that, nevertheless, in an analytical study of this nature, it is possible 

 to arrive at the relative importance of these various influences. 



It is important, furthermore, that not only the true environmental 

 factors be taken into account, but also that such inheritable factors as the 

 relative degree of susceptibility of the host plants used and the virulence 

 of the parasite concerned receive proper consideration. In addition, the 

 amount of infection and the time of its' occurrence may greatly modify 

 the results both as regards the readiness with which the host may become 

 infected and the effect of a "mass action" upon the measurable end result 

 of disease. 



The purely environmental conditions to which the roots of the host or 

 the parasite harbored by the soil are subjected may be conveniently con- 

 sidered under the following subjects: (i) Amount of infestation present; 

 (2) percentage of moisture; (3) temperature; (4) soil reaction; (5) physical 



