76 MASS. EXPERIMENT STATION BULLETIN 175. 



The disease is not due to parasites of any kind, but is the result of defective 

 nutrition of the young dividing and rapidly growing cells, due to a lack of elabo- 

 rated nitrogenous reserve food accompanied by an abnormal increase in the 

 activity of oxidizing enzymes in the diseased cells. The unusual activity of the 

 enzyme prevents the proper elaboration of the reserve food, so that a plant once 

 diseased seldom recovers. On the decay of the roots, leaves and stems of both 

 healthy and diseased plants, the enzyme in question is liberated and remains active 

 in the soil. The enzyme is very soluble in water and appears to pass readily 

 through plant membranes. If the young plants take it up in sufficient quantity 

 to reach the terminal bud, they become diseased in the characteristic way. Under 

 field conditions there is little danger from infection in this manner, but in the 

 seed bed the danger is much greater on account of the greater susceptibility of the 

 young plants to the disease, and the greater amount of free oxidizing enzymes 

 likely to be in the soil due to the decay of the roots and plants. New or steam 

 sterilized soil should therefore be used for the seed bed. 



I have shown that transplanting, especially when the roots are injured, may 

 produce the disease. Great care must, therefore, be taken not to injure the roots in 

 this process or in the subsequent cultivation, or to check the growth of the plants. 



There is evidence that rapid growth, caused by too much nitrogenous manure or 

 too high a temperature, is favorable to the disease. Why this should be the case 

 has not been determined. It is probably connected with the manufacture of 

 reserve nitrogen by the cells and its distribution to the rapidly growing parts. 



Plants grown under such conditions are less able to stand successfully marked 

 variations in temperature and moister conditions of soil and atmosphere. Varia- 

 tions of this kind favor the development of the disease in the less resistant plants. 



Close, clayey soils, packing hard after rains and requiring constant tillage, are 

 not favorable to the even growth of either the tops or roots of tobacco plants. In 

 moist, cloudy weather the plants will grow too fast, and in hot, dry weather the 

 soil is likely to bake, checking growth and making probable injury to the roots in 

 cultivation. Such soils are very favorable to the development of the mosaic 

 disease, as pointed out by Thaxter. i He found that loosening the soil by liming 

 and giving partial shade, thus causing a more even condition of growth, very 

 greatly reduced the disease. 



Crops grown under cheesecloth covers protected at the side are said to be re- 

 markably free from the disease. The plants make a steady rapid growth, much 

 greater than in ordinary field culture. . . . 



The disease is not, so far as observed, produced by a lack of soil nutrients, though 

 from its nature we would expect that a deficiency of nitrogen, phosphoric acid, 

 lime and magnesia might favor its development. Koning^ says that manuring 

 with kainit and Thomas slag diminishes the extent of the disease. Mayer, Beijer- 

 inck and other investigators, however, agree that the trouble is not caused by the 

 lack of any soil nutrients. It appears, so far as my own investigations go, that the 

 trouble cannot be cured by giving the plants additional food of any kind. Over- 

 feeding with nitrogen favors the development of the disease, and there is some 

 evidence that excess of nitrates in the cells may cause an excessive development 

 of the ferments that cause the disease. Very slight attacks of the disease known 

 as "mottled top" are said not to injure the quality of the leaf to a sufficient extent 

 to be noticeable commercially, though they may be less elastic and have a poorer 

 burn and aroma than healthy leaves. 



Hunger, ^ in his work on the mosaic of Deli tobacco, verified much of 

 the work of previous investigators, and later, in carefully planned and 



« Thaxter: Conn. Agr. Exp. Sta. Rept., Ill, 253 (1899). 

 ' Koning, C. J., loc. cit. 



> Hunger, F. W. T.: De Mozaiek-ziekte bij deli Tabak. Med. s'Lands Plantentium, Batavaia. 

 Deel 1: 63 (1903). 



