May IS. 1919 Influence of Soil Environment on Rootrot of Tobacco 75 



left uninoculated as controls. All were then transplanted with the White 

 Burley variety from sterilized soil. It became evident at once that on 

 uninfested soil the rate of growth of the plants in the extremes of the 

 series was markedly different; the leaf mold was much more favorable 

 for growth than the sand with nutrient solution. Several tests were 

 run on these pots, and also on another series made up in a similar manner 

 (Experiment II, Table VIII). Most of the data concerning them exists 

 as notes and estimates of relative growth during the progress of the 

 experiment. Much reliance can not be placed on the weights, owing 

 to the large variation in fertility, although the ratios given of the growth 

 on infested to that on uninfested soil indicate the general trend of the 

 results. 



The experiments, of which there were a considerable number, can not 

 profitably be discussed here in detail. Ownng to the variation in results 

 obtained in growth, much reliance was placed on estimates of actual 

 infection on the roots themselves, estimates difficult to express in figures. 

 After summarizing the results of all the tests run (nine in number), it 

 can be stated with considerable confidence that the importance of organic 

 matter in the soil is relatively small, so far as infection and severity of 

 the disease are concerned. It seems, however, that heavy infestation 

 is more rapid, and is more likely to be maintained through unfavorable 

 periods for the parasites in soils high in organic matter rather than in 

 those low in organic matter. Given a uniformly heavy inoculation of 

 the soil with endoconidia of T. hasicola, the rate and severity of infection 

 is apparently practically the same in pure sand as in the pure leaf mold. 

 Massee's conclusion (jp) that T. hasicola is a weak parasite and unable to 

 infect the host except in the presence of organic matter seems entirely 

 unwarranted. This has been further shown by infections obtained from 

 spore suspensions in pure water or spores alone placed directly upon roots 

 grown in a moist atmosphere. After the lapse of a considerable period 

 of time from the date of inoculation, however, it seems certain that 

 T. hasicola becomes more finnly established in pure leaf mold than it 

 does in pure sand, although this is apparently a difference of amount of 

 infestation and not one of virulence (Pi. 4, V-VI). 



With regard to the various mixtures of sand and organic matter, the 

 conclusion seems justified that, so far as infection following inoculation 

 is concerned, it takes place wth equal ease in all (PI. 4, III-IV). The 

 development of infestation of the soil, however, has not given quite the 

 expected results. Doubling or tripling the content of organic matter 

 apparentl)^ has not increased infestation, and in some cases increasing 

 the ratio up to 80 parts of leaf mold seemed actually to reduce it. The 

 results, however, have not been sufficiently uniform in this respect to 

 warrant a final conclusion, and it is not certain that factors other than the 

 organic matter do not play a part here. Nevertheless, the fact that 

 increasing the organic content of the soil, two, four, six, and eight times, 



