BLACK ROOT-ROT OF TOBACCO ^29 



infested as to render prclitabJe tobacco growing impossible. The all import- 

 ant question now is: Just where is the dividing line between safety and 

 certain loss? Just what degree of acidity must a soil possess in order to be 

 safe from root rot? 



The data on the soil and crop of 1924 are valuable in determining the loca- 

 tion of the dividing line, because at that time the soil must have more nearly 

 reached an equilibrium after the treatment of tiie three preceding years, and 

 because this equilibrium was not disturbed by artificial inoculation or applica- 

 tion of lime in 1924.. Correlating for that year the root inspection records, 

 t!ie yield and sorting records and tiie soil reaction records, we find that root- 

 rot was not serious on any of the twelve ]ilots wliich had an average reaction 

 of slightly over 5; but that it caused serious damage on all the inoculated 

 and most of the uninoculated plots with an average of 5.9. By using indi 

 vidual plot records it may be confined more closely. Especially instructive is 

 the record of plot 13 which kept a reaction of 5.5-5.6 throughout the three 

 years and which was inoculated as thoroughly as any other plot in the field 

 for three years; but on the 4th year only an occasional lesion on the roots 

 could be found and it was the best plot in the field. Similarly, an inspection 

 of the individual limed plots indicates pretty severe loss at 5.8. 



In 1925, soil reactions were determined in April and in October. The pH 

 values were found to have changed considerably during the growth of the 

 tobacco in 1925. This was especially conspicuous in the limed plots, in which 

 the soil became more acid, probably because of the removal of some of the 

 lime by the crop. The pH values determined in April are indicative of the 

 environment in which the crop grew. The reaction of plots without lime was 

 between pH 4.5 and 5.0, and in these plots black root-rot caused no apparent 

 loss. Limed plots had pH values between 6.4 and 6.9, and in all of them black 

 root-rot caused serious damage. There were this year no plots having pH 

 values between 5.0 and 6.4, and so tiie critical point cannot be located any 

 more closely than has been done above on the basis of the results in 1924. 



Experiments in pots in the greenhouse, which are to be described in a later 

 paper, have led us to the conclusion that loss from black root-rot occurs 

 whenever the pH value of the soil is 5.9 or higher, and that loss from black 

 root-rot does not occur at pH 5.6 or below, except in exceptionally cold soils. 



We may therefore conclude that the critical point is somezehere between 

 b.6 and 5.9. It is apparent however that there cannot be in reality a critical 

 point but rather a critical region, below which damage from root-rot may be 

 expected to be little if any, and above which root-rot is almost certain to 

 cause trouble. There are at lea.st three reasons why this region cannot be 

 defined more accurately than the limits given above: 



1. The transition from a soil reaction unfavorable for growth of Thielavia 

 to a reaction which favors is a matter of degree — not of plus and minus. 



2. During a very warm summer the critical region would probably be 

 higher since a higher soil temperature reduces the effect of root-rot, as has 

 been shown experimentally by Johnson and Hartman (7:60). A cold summer 

 on the other hand would probably depress the critical region toward the acid 

 range. 



3. On compact, poorly drained soil black root-rot is commonly worse than 

 in looser soil in the same field, due probably to the difference in the rate of 

 leaching and to lower soil temperature. This also is in accord with the experi- 

 mental results of Johnson and Hartman (7:80). It is not improbable that 



