BLACK ROOT-ROT OF TOBACCO 129 



effective than the heavier ai)j)lication; pH values were lowered by it in 1926 

 and 1927 but not in the following years. 



On this heavily Ihiied soil, 400 pounds of sulfur rather than 200 pounds 

 l>er aere were needed to lower the pH value of tlie soil effectively. Sulfur, 

 400 jxiunds, applied once was effective for two years. Sulfur, 400 pounds, 

 applied in two successive years, 1926 and 1927, remained effective through 

 1930. In 1930 limed soil not acidified had a pH value of 6.0, and limed soil 

 to which 400 pounds of sulfur had been applied in both 1926 and 1927 had 

 a pH value of 5.7 or a little below the usually accepted danger point for 

 black root-rot. 



On Black Eooi-rvt 



The severity of black root-rot on limed plots in 1926 was reduced by all 

 acidification treatments but by none of them was it eliminated. The disease 

 was present as a trace on plots not limed, severe on plots limed and not 

 acidified, and moderate or mild to moderate on limed plots acidified, without 

 very pronounced dift'erences between the several acidification treatments. 



In 1927 the degree of infection of tobacco by Thielavia basicoJa was 

 greatly reduced by sulfuric and orthophosphoric acids applied in 1926 and 

 19'27. On one of the limed plots so treated, there was no black root-rot and 

 on the other there was only a trace. Black root-rot on the check plots, limed 

 and not acidified, was less severe than the year before, but it was at least 

 moderate in intensity. Infection of tobacco on limed plots was reduced more 

 in 1927 by the application of sulfur in the two successive years than by its 

 application in 1926 only. 



In 1928 black root-rot of tobacco was again severe on limed plots and 

 absent, as in every year, from plots not limed. Its severity on limed plots 

 was reduced most this year on those plots to which the acids or sulfur, 400 

 pounds per acre, was applied in 1926 and 1927; but sulfuric and orthophos- 

 phoric acids permitted more infection by Thielavia in 1928 than in 1927, the 

 last year they were applied. 



There was black root-rot in 1929 on all tobacco plants on limed plots not 

 acidified, but the infection was no more than moderate. On limed plots, there 

 was least black root-rot on those to which sulfuric and orthophosphoric acids 

 were applied in 1926 and 1927. On these pilots there was only a trace of the 

 disease on many of the plants and some of the plants were entirely free from 

 it. The next most nearly disease-free of the limed plots were those to which 

 these acids were applied in 1926. The applications of sulfur, 200 or 400 

 poimds per acre, in 1926 and 1927 resulted in less black root-rot in 1929 than 

 did the application of these amounts of sulfur in one year (1926) only. 



In 1930 black root-rot Mas no worse than moderate, even on limed plots 

 not acidified. The acidification treatment of limed plots which resulted in 

 least black root-rot this year was the application of 400 pounds of sulfur per 

 acre in 1926 and 1927 or in )926 only. 



On Yields 



The percentage increases in yields of limed and acidified plots, for ex- 

 ample plots 9a and 21a (duplicates), over yield of check plots 12 and 24, 

 limed hut not acidified, may be seen by reference to Table 6. In each case the 

 yield of a limed plot acidified was com]>ared with the yield of the most nearly 

 adjacent limed plot not acidified. 



