TOBACCO WILDFIRE IN 1922. 9 



Occurrence of Lesions on Midribs. 



In the process of "stemming" tobacco, the midribs are stripped from 

 the leaf and are sold as fertilizer (incorrectly called tobacco stems) . The 

 question has frequently been raised as to whether the land may become 

 infested by the use of "stems" from diseased tobacco. Observations as 

 to the occurence of lesions on midribs were made at various times in fields 

 during the summer. Frequently lesions were found running along both 

 sides and encroaching on the midrib and often extending directly across 

 the midrib. When the leaf was stripped from the midrib, parts of the 

 lesion remained with the "stem." Bacterium tabacuni was isolated di- 

 rectly from such denuded stems. This does not prove that the disease 

 may be carried back to the land by using stems, since it has still to be 

 demonstrated that the bacteria can survive the sweating process, but 

 there can be no doubt that they occur in the midribs and may survive the 

 winter thus in the tobacco shed. Clinton and McCormick (2: 416) pro- 

 duced lesions similar to those described above by inoculating the midribs 

 with pure cultures of the bacteria. 



Relation of the Condition of the Plant to Infection. 



No set of experiments has been planned to determine the relation of 

 the growiih and vigor of the plant to susceptibility, but incidental to other 

 experiments a number of observations have been made which indicate 

 that a rapidly growing plant is much more susceptible than one which is 

 growing slowly. During the fall of 1921 two beds were planted in the 

 greenhouse at Amherst, — one on very poor soil and one on soil rich in 

 rotted compost. Both were inoculated at various times and the rapidly 

 growing plants of the fertile bed became infected, but all inoculations 

 failed in the other bed until late in the spring, when the plants suddenly 

 began to grow rapidly. In the course of some experiments at the Massa- 

 chusetts Station during the summer of 1922 numerous unsuccessful 

 attempts were made to inoculate a bed of vorj^ slow-growing plants which 

 had received no fertiUzer. During the same time other rapidly growing 

 beds in the greenhouse were very readily infected. These experiments are 

 not accurate, but certainly give some strong indications. Also the fact 

 that infection is difficult to secure during the winter-months points to the 

 same conclusion. The relation of fertilizers to infection can probably be 

 interpreted by their influence in producing a rapid, succulent growiih or 

 the reverse. Other investigators of the disease have made similar obser- 

 vations. Chnton and McCormick (2: 390) state that "the use of any 

 fertilizer that favors rapid growth is more likeh' to help infection . . . 

 than where the fertilization is such that slower or less satisfactory growth 

 takes place." Fromme and Wingard (3: 27) express essentially the same 

 opinion. 



