32 BULLETIN 40, U. S. DEPARTMENT OF AGRICULTURE. 



the process of curing, the diastatic activity of tobacco leaves is greatly 

 increased, since under these conditions the leaf cells are undergoing 

 gradual starvation. 



As a matter of fact, Bunzel, 1 from a biochemical study of the 

 curly-top of sugar beets, a malady in many respects similar to the 

 mosaic disease of tobacco, has recently shown that an abnormally 

 high oxidase content is not necessarily a function of the disease, since 

 his investigations show that plants subjected to various abnormal 

 growing conditions, such as drought, overwatering, etc., develop a 

 higher oxidase content than normal plants growing under favorable 

 conditions. It is apparent, then, that a high oxidase content simply 

 indicates disturbed metabolism arising from any one of a variety of 

 unrelated causes. 



Although purely physiological mosaic symptoms attended by 

 chlorosis and various morphological changes in the leaves have been 

 induced in tobacco plants by submitting them to sudden temperature 

 changes, by severely cutting them back, etc., such disturbances are 

 never infectious and do not usually persist under favorable condi- 

 tions of growth. In the light of recent facts brought out in our ex- 

 periments it is hard to see how the mosaic disease of tobacco can be 

 logically placed in the category of purely physiological disturbances. 

 The theory of parasitism accounts for the primary origin of the dis- 

 ease more consistently than the enzymatic hypothesis. 



SUMMARY. 



The mosaic disease of tobacco is communicable to a great variety, 

 of solanaceous plants. A number of species appear to be immune to 

 the disease. In the susceptible group some species show a far greater 

 degree of resistance to infection than others. The disease does not 

 seem to be communicable to plants of other families. Although quite 

 similar in many respects, the mosaic disease of pokeweed appears to 

 be distinct from the mosaic disease of tobacco. Both diseases are 

 readily communicable to plants of their kind. 



The incubation period of the mosaic disease is variable, depending 

 upon conditions favorable or unfavorable to the growth of the plants. 

 The mosaic virus permeates all parts of the plant, including the roots 

 and corollas as well as the foliage. The virus does not infect the 

 embryos of seed produced by mosaic mother plants; therefore, such 

 seeds produce healthy plants. 



By artificial inoculation the mosaic disease can be induced to mani- 

 fest itself, at first locally. In the course of time, however, the disease 

 always becomes generalized. 



1 Bunzel, II. II. Biorhcmlcal study of the curly-top of sugar beets. U. S. Depart- 

 ment of Agriculture, Bureau of Plant Industry, Bulletin 277, 28 p., 1913. 



