BLACK ROOT-ROT OF TOBACCO 



121 



Two related problems concerned (1) the effect of lime on tobacco in the 

 absence of root-rot and (2) the eftect of the timothy cover crop. 



In regard to the first, it is safe co assume that the growers considered lime 

 beneficial, else it would not have come into such general use. Briggs found 

 tluit 1 per cent of lime in disease-free soil increased growth. Hartwell and 

 Damon (3:437) include tobacco among the lime loving plants. McCall (9) 

 reports that, on heavy loam soil in Maryland, lime increased the yield of to- 

 bacco, but that direct applications of lime tended to lower the quality of the 

 leaf. Hutcheson and Berger (4:7) found that, in Virginia, lime at first gave 

 a larger yield but the tobacco was coarse and "bony." In the later years of 

 the experiments, however, they found the tobacco reduced both in quantity 

 and quality by lime. It is (piite possible tliat in this case the later effects 

 were due to root-rot rather than to the direct effect of lime. This is true in 

 other similar experiments where the first effects were good but in later years 

 there was a reduction. Since in most cases root-rot is not mentioned, one 

 cannot be sure whether the experimenter was measuring the effect of the 

 lime or of root-rot. This difficulty renders worthless most of the literature 

 on the effect of lime on tobacco. 



During the last decade, the use of timothy as a cover crop has come into 

 considerable favor with tobacco growers of the Connecticut Valley. It re- 

 duces washing and l)lowing of ti)e light soils during the long period between 

 tobacco crops and also adds considerable humus to the soil. S',>me growers 

 also have the impression that it reduces black root-rot by neutralizing in some 

 way the ill effect of lime. When this experiment was begun tiiere were no 

 published records of experiments to determine the effect of its use on the suc- 

 ceeding crops of tobacco. It became desirable, tiierefore, to test by accurate 

 experiments its immediate and cimiulative effects. 



The Five- Year Experiment 



In 1921 a series of experiments was, therefore, started for the purpose of 

 answering these three questions: 



1. What effect does application of lime have on growth of toliacco in the 

 absence of root- rot? 



2. What is the relation between severity of infection and acidity as mea- 

 sured by hydrogen ion concentration ?4 



3. What is the effect of a timothy cover crop on the yield and quality of 

 tobacco and on the severity of root-rot? 



During the last five seasons Havana tobacco has been grown continuously 

 on a field of 1.2 acres on the experiment station farm where no tobacco has 

 been previously raised -for at least 8 years. The soil is apparently of the 

 Gloucester series. The field was divided into 24 plots of 1-20 acre each 

 according to the diagram presented in figure I. It will be noted from this 

 diagram that limed strips of plots alternated with uniimed strips in one direc- 

 tion; while at right angles, there were duplicate strips of plots (1) without 



4 In measuring reaction in terms of hydrogen ion concentration method, a neutral 

 soil or other substance is designated by the number pH 7, acid substances by the 

 numbers below 7, alkaline substances by those above 7 up to 14. Thus a soil desig- 

 nated by 6 is slightly acid; 5, much m.ore so; 4, more than 5; etc. We are not con- 

 cerned here with the alkaline end of the scale since all our tobacco soils are naturally 

 acid, and fall within the range from 4 to 7. Even when a large amount of lime is 

 applied, they do not remain above 7 for any considerable time. The degrees of acid- 

 ity are usually written pH 5, pll 4.5, etc. 



