MOSAIC DISEASE OF TOBACCO. 113 



If the metabolic processes are changed ever so sHghtly, due to any stunu- 

 lus, far-reaching effects may be induced throughout the organism, and 

 this we find to be the case in the mosaic disease, and the writer beUeves 

 that it is justifiable to look upon the matter in this light, as it is no more 

 h}T)othetical than the concept of an "ultramicroscopic" parasite, which, 

 if demonstrated (and no amount of concentration or methods of culture 

 have indicated in any way the presence of aggregates or colonies), certainly 

 would become visible if multiplication occurred. 



Theoretically is it possible to conceive of an organism, functioning as 

 such, to be made up of so few molecules of protein, fat and carbohydrate 

 that it would be impossible to demonstrate its presence? If so, our ideas 

 of relative size of molecules of protein, etc., must be changed. 



Prevention and Control. 



The question of the prevention and control of mosaic disease is of prime 

 importance to the grower, entirely aside from more technical considera- 

 tions as to the exact cause or causes of the disease, and it is believed that 

 with reasonable care it is possible for the grower to lessen materially the 

 amount of mosaic in the field. 



Many recommendations have been made regarding treatment of dis- 

 eased plants after they have once contracted the disease, but so far the 

 writer has never observed a plant which, once attacked by the disease, 

 recovered at any subsequent period of its growth. On the other hand, 

 it has never been observed that the disease killed a plant, at least in this 

 region. 



It is doubtful, owing to the character of the disease, if it can ever be 

 entirely eliminated on some soils and under certain unfavorable conditions 

 occurring during some seasons. As has been indicated previously there is 

 apparently Uttle or no relation to be found between excess or lack of food 

 materials and the prevalence of the mosaic. It has been in some instances 

 stated that favorable results have been obtained from the use of lime in 

 different forms, but this treatment cannot be recommended for various 

 reasons. Experimentally it has been shown that heavy liming has little 

 or no effect on the disease once a plant has contracted the disease, and 

 even when applied to soils from old beds no consistently favorable results 

 have been obtained (see page 91). Used in the larger quantities it might 

 be inferred from the results that the lime apparently did exert a beneficial 

 action, but to apply Hme generally in such amounts would be folly, as 

 it would in many cases bring the soil to a comparatively neutral or alka- 

 hne condition, which reaction would favor the development of root rot, 

 caused by the fungus, Thielavia, and this, once thoroughly established, 

 in a field or seed bed, is much more injurious to tobacco than is the mosaic 

 disease. 



As has been pointed out, the writer, from his observations, is strongly 

 of the opinion that much of the field infection may be traced to the seed 



