52 Journal of Agricultural Research voi. xvii, no. a 



saturation, which usually wilted during the daytime owing to lack of 

 moisture. By July 21 marked signs of heavy infection of all the plants 

 in the infested soils was shown by reduced growth and yellowing of the 

 lower leaves; this condition was most marked at full saturation. The 

 uninfested soil showed the optimum growth at three-fourths satura- 

 tion. On August 14 the experiment was discontinued, the plants photo- 

 graphed (Pi. I, II-III), and then cut and dried. The air-dry weights 

 are given in Table II under experiment 4. 



While the results of this experiment as shown by air-dry weight in 

 comparison with the leaf area given in experiment 2 do not conform in 

 all details with those of experiment 2 , they are believed to agree in general 

 in that the greatest amount of injury from disease resulted in the satu- 

 rated soil; the ratio of the yield on infested soil to that on uninfested 

 soil was again considerably greater than in the other cases. 



It is realized that further experimental evidence could be profitably 

 obtained as to the relation of moisture to the disease. The difficulties 

 already referred to, however, together with complication of other factors 

 such as temperature, and some of the more obscure factors such as 

 aeration and compactness of the soil, have rather discouraged further 

 tests until more accurate technic can be devised. It is certain, how- 

 ever, that T. bascicola has a wide range of action as regards actual 

 percentage of moisture present in the soil. It has been found, for 

 instance, that in water culture containing a spore suspension, good 

 infection occurs on roots and that it will occur in soils too dry to permit 

 anything like normal growth of tobacco. Whether there is a direct 

 increase in amount of infection and severity of the disease with per- 

 centage increase of moisture in the soil may not be exactly clear from 

 the data here presented. From the majority of the data obtained, 

 however, some of which is not given here, it seems fairly certain that 

 such direct proportionality does not exist, but that a fairly constant 

 relation is maintained in soils v/ith moisture content ranging from those 

 sufficient only for poor plant growth to those approaching saturation, 

 followed by a very rapid increase of disease from this latter point up 

 to full saturation. 



At any rate it may be said that poorly drained infested soils which 

 are likely to remain saturated for a period of three or four days, or 

 any inlested soil kept near saturation for a period of days due to excessive 

 rainfall will undoubtedly show higher infection than well drained soils 

 or soils not affected by an excessively wet season. On the other hand 

 it appears that, as a rule, soil moisture is not an important controlling 

 factor in the prevalence of the rootrot of tobacco. Relatively dry or 

 relatively wet seasons, in so far as they affect soil moisture alone, are 

 not especially to be feared nor to be relied upon for holding the disease 

 in check. For the same reason a careful check has not been kept upon 

 the moisture content of the soils in the various experiments described 



