May IS, 1919 Infitience of Soil Environment on Rootrot of Tobacco 49 



rate experiments of this nature were carried out. In a third experi- 

 ment, steam-sterilized tobacco-field soil was used for mixing with the 

 untreated tobacco soil in the same way as before. A gradual falling off 

 in growth will be noted with the increase of amount of infested soil 

 (PI. I, I). In the case of No. 4, in experiment 3, the lowered yield, as 

 compared with all infested soil is no doubt due to the frequently observed 

 fact that reinfested sterilized soil favors the growth of fungi and conse- 

 quently the increased development of disease in the soil. In a mixture 

 of three-fourths infested and one-fourth steamed soil the balance of 

 infestation and conditions apparently was such as to cause greater 

 infection than in all-infested soil. 



Table I. — Influence of amount of infestation of soil by Thielavia basicola on the yield 



of tobacco 



Pot 

 No. 



Soil mixture. 



Average air-dry weight of duplicates. 



Infested. 



Umnfested. 



Experi- 

 ments." 



None 



One-fourth. . 



One-half 



Three-fourths 

 All 



All.... 



Three-foturths 



One-half 



One-fourth . . . 

 None 



Gm. 



6-93 

 4. 20 

 2. 65 

 a. SI 

 ^■33 



Steam-sterilized soil was used as uniafested soil in Experiment 3. The low yield in pot 4 was probably 

 due to steamed infested soil favoring disease more than unsteamed infested soil. 



The results obtained in Table I are considered to be due only to the 

 fact that more spores exist per unit of soil in the higher proportion of 

 infested soil, and therefore the roots are more likely to come in contact 

 with infecting material. This fact has an important bearing upon 

 results obtained in a study of environmental conditions. However, since 

 any deleterious or beneficial action to which the parasite is subjected 

 may merely reduce or increase the amount of infestation, the results 

 will be proportional in some measure to the amount of infestation present. 



INFLUENCE OF MOISTURE CONTENT OF SOIL 



A review of the literature concerning factors influencing the severity 

 of the rootrot of tobacco shows that soil water has been considered the 

 primary limiting factor by practically all observers and investigators of 

 this disease. Most of the conclusions drawn in regard to this, however, 

 have been based on observations in the greenhouse or in plant beds, 

 where artificial watering is resorted to and where it is relatively common 

 in many instances to overwater the soil. A study of the disease under 

 field conditions with reference to moisture, though equally indefinite and 

 inconclusive owing to the number of other variable factors, is at any 

 rate convincing that an oversupply of moisture is not necessary for heavy 

 infection and severe attacks by the parasite. 

 108122°— 19 2 



