1913.] PUBLIC DOCUMENT — No. 31. 47 



without change or rest, and thus the soil has become inoculated 

 with the causative enzymes ; or roots and diseased plants may 

 have been left inadvertently in the soil, and on their decay the 

 enzymes persisted. This condition may he remedied l)y 

 thorough sterilization with steam, or by using beds which have 

 not grown tobacco before, -or by letting the beds grow to grass 

 for a year or two before using again, two years being better 

 than one. The activity of the noxious enzymes appears to les- 

 sen during this time, and finally ceases. Although we have 

 not verified the following in our experiments, it is stated that 

 too high a nitrogenous soil content will sometimes cause the 

 disease in the seed bed. 



Improper sterilization of the seed bed will sometimes in- 

 crease the disease for the following reasons : the soil which is 

 improperly steam-treated renders certain of the plant foods 

 more available, and yet does not render the enzymes inactive, 

 and thus for a time accelerates plant growth by excessive forc- 

 ing, rendering the plant more liable to attacks of disease dur- 

 ing the period of forcing. This condition has been observed 

 occasionally by us and can be explained on no other grounds. 

 As a result of the improper sterilization the temperature of the 

 soil is not raised sufficiently to kill the enzymes, and therefore 

 they remain active in the soil. Thorough sterilization will kill 

 enzymes, or at least two sterilizations will kill them, thus get- 

 ting rid of the residuary zymogen or revivified enzyme. 



Ix^ THE Field. 



As has been pointed out by other investigators, there are 

 many ways of contracting the disease in the field, but it has Ijeen 

 definitely proved that excess or lack of mineral fertilizer alone 

 v/ill not cause the trouble. This idea was first advanced by 

 Meyers (G) and Beijerinck (7) who showed that the disease 

 could not be brought on by excessive use of mineral fertilizers, 

 although a leaf distortion due to overfeeding is quite often 

 observed. These results were verified in our work in the labo- 

 ratory and field. 



One of the chief causes of the production of the disease is, 

 without doubt, — as has been pointed out by Woods (8), Sturgis 

 (9) and others, — improper methods or carelessness in trans- 



