BLACK ROOT-ROT OF TOBACCO 127 



The results of tlie observations now to be described indicate that alfalfa 

 is not sufficiently susceptible to black root-rot caused by Thielavia to pre- 

 clude, for that reason, the growth of alfalfa in rotation with or preceding 

 tobacco. 



Tlie soil of the limed jjlots on which alfalfa was grown following four 

 years of tobacco \\ as very completely infested with Thielavia, as evidenced by 

 the severe infection of tobacco in the preceding and the succeeding years. The 

 relative susceptibility or resistance of alfalfa to infection by this fungus was, 

 therefore, jDut to a real test. 



Roots of alfalfa were examined each year. Tlie roots showed a kind of 

 pitting very similar in appearance to an injury of undertermined cause which 

 has been described by other investigators (31, 29) ; but no infection by Thie- 

 lavia basicola was foimd, although soil conditions and tlie abundant presence 

 of the fungus were most favorable to it. 



As far as black root-rot is concerned, tiierefore, there seems to be no 

 reason why alfalfa cannot l)e grown in a rotation preceding tobacco. 

 Ou Black Root-Eot, Broicti Root-Rot, and Yields of Tobacco 



In l.')28, 1929, and 1930, the first, second, and third years of tobacco after 

 the liay crops, black root-rot of tobacco on limed plots was slightly less severe 

 fa little less conspicuous in its eifects on the roots) than on plots on which 

 tobacco had been grown every year. This eifect was too small, however, to 

 oft'set the brown root-rot effect of the hay crops on the tobacco which fol- 

 lowed them, at least in the first year or two. 



Brown root-rot of tobacco following timothy was equally severe on plots 

 with and without lime in 1928, the first year of tobacco. It was much less 

 se\ere in 1929, the second year after the hay crops. In 1930, the second or 

 third year of tobacco following the hay crops, there was little and in most 

 cases no brown root-rot; that is, the brown root-rot eft'ect of these predispos- 

 ing crops on the tobacco which followed them disappeared after the first one 

 or two years of tobacco. 



In the very rainy season of 1928, lowered yields and poor quality of 

 tobacco were associated with a preceding alfalfa crop only in the lowest part 

 of the field, where all tobacco grew poorly. Tobacco yielded as well after al- 

 falfa as after tobacco in the highest part of the field, and here the tobacco 

 was of as good quality following alfalfa as on plots where only tobacco had 

 been grown. In 1929 limed plots on which alfalfa was grown in 1927 and 1928 

 yielded 4 to 17 per cent less tobacco than did plots in continuous tobacco; but 

 limed plots on which tobacco was grown no later than 1927 yielded 15 to 30 

 per cent more tobacco than did limed plots on which tobacco was grown 

 every year. In 1930 the yield of tobacco following alfalfa, grown in 1926 

 and 1927 or in 1926, 1927, and 1928, was practically the same but no more 

 than tlie yield on plots in continuous tobacco. The ])ad effect of brown 

 root-rot on yields of tobacco following alfalfa was, therefore, confined to the 

 first year of tobacco. In the second year of tobacco after alfalfa there was 

 some increase in yield as compared with yield of plots cropped continuously 

 with tobacco; and in the third year, yields were about the same following 

 alfalfa as on plots on which tobacco was grown every year. 



With brown root-rot severe, the bad effects of timothy on the yield of 

 tobacco which followed it in 1928 were essentially the same on plots not limed 

 (on which the depressions in yield were 16 to 27 per cent) as on plots limed 

 (on which the depressions in yield were 17 to 33 per cent). In 1929 yields 



