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



In the "mottled-top" or "gray-top" phase of the disease the top- 

 most growing leaves and suckers first manifest the disease. Leaves 

 lower down on the main stalk usually do not show symptoms of the 

 disease, since these completed their growth in advance of its develop- 

 ment. 



This phenomenon, coupled with the fact that the disease in young 

 plants affects successively younger and later leaves as they appear 

 on the elongating main stalk, has led to the erroneous belief that the 

 course of the disease is upward and never downward. 



By inoculating large, nearly mature plants at various points the 

 writer has brought out some very interesting details concerning the 

 occurrence of local and general manifestations. If the plants are 

 inoculated while young or at a period much in advance of maturity, 

 the mosaic disease in tobacco plants very rarely manifests itself as a 

 local trouble, since practically all suckers, which appear at a consid- 

 erably later period, become affected. On the other hand, large, well- 

 nourished plants at maturity usually produce an abundance of long, 

 leafy, vigorous suckers. Properly topping the plants at this period 

 stimulates a yet more vigorous sucker development. By introduc- 

 ing the virus into the top of one of these, very pronounced local 

 sjonptoms of the disease sometimes precede the general phase which 

 invariably follows. 



Several weeks prior to blooming, when the immature flower heads 

 were only one-half inch to 1 inch across and suckers were only feebly 

 developed, several series of plants were inoculated with mosaic virus. 

 One series was inoculated by puncturing the main stalk deeply with 

 a long needle just above ground. A second series was inoculated by 

 puncturing the main stalk deeply at several points, beginning at the 

 ground and extending along about two-thirds of its length. A third 

 group of plants was inoculated at the very apex of the immature 

 flower head of the main stalk. Other healthy plants were allowed 

 to mature fully in order to obtain long, vigorous suckers, which, in 

 many instances, arose near the base of the plants and nearly or quite 

 equaled the main stalk in length. One of these was then inoculated 

 with mosaic virus at or near the apex of the immature buds. 



When the virus was introduced into the main stalk at its base or 

 at several points along its length all immature parts of the plants, 

 including the top leaves and suckers, usually developed pronounced 

 symptoms of mosaic simultaneously. When the virus was introduced 

 at the very apex of the immature flower heads of the main stalk a 

 pronounced mottling of the corollas of the main flower head in 

 several instances was the only noticeable evidence of the disease, 

 although at this time long vigorous suckers were not infrequently 

 present. Later, however, these also showed the usual disease symp- 

 toms. In one plant the writer, by antiseptic methods, carefully cut 



