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



the disease in a plant produces in the blossoms a pink-purple color 

 several shades darker than that which occurs in normal blossoms of 

 a healthy plant. 



The development of the mosaic disease in tobacco plants is not 

 invariably indicated in the blossoms by all the color changes pre- 

 viously described. With regard to the typically blotched appear- 

 ance of the blossoms, however, the writer has failed to find an in- 

 stance when this appearance was not associated with the disease in 

 a plant. Experiments with healthy tobacco plants grown in green- 

 houses at Arlington, Va., during the winter of 1912, indicate, how- 

 ever, that high temperatures together with high humidity may pro- 

 duce very pale or almost white blossoms, indistinguishable from the 

 white or pale blossoms sometimes produced by mosaic plants. 



Now and then tobacco plants show the most pronounced symptoms 

 of the disease in the upper leaves and suckers and at the same time 

 produce blossoms entirely normal in color. At other times the 

 disease appears on a single sucker, all the blossoms of which are 

 distinctly mottled, although the blossoms of the main flower head 

 and other suckers are perfectly normal in color. In other plants the 

 blossoms of the main stalk are mottled, while the suckers at the same 

 time produce normal pink blossoms. Even in the same flower head 

 some blossoms may show the most pronounced phases of mottling, 

 while others retain the uniform pink coloration of a healthy blossom. 

 In local manifestations of the disease the immature leaves as well 

 as the blossoms of affected branches usually indicate the disease, 

 although this is not an invariable rule. 



By carefully timed inoculation the writer has caused the disease 

 to develop in healthy plants subsequent to the apperance of the 

 first open blossoms. Blossoms appearing in advance of the earliest 

 symptoms of the disease displayed the uniform pink coloration of 

 the normal bloom, while later blossoms becoming affected by the 

 disease displayed the characteristic blotched colorations. The first 

 observable symptoms have sometimes made their appearance in 

 the blossoms alone, and not until suckers had developed was the 

 disease manifest elsewhere in the plant. 



In particularly malignant phases of the mosaic disease the plants 

 often produce depauperate and misshapen blossoms. In such in- 

 stances the corollas remain small and imperfectly formed, so that 

 the distorted, immature stigma and stamens extrude long before the 

 blossom has approached normal maturhry. Likewise, on such plants 

 the seed pods are abnormally small and shrunken and may contain 

 very few viable seeds. (PI. Ill, fig. 2; PL IV, fig. 1.) 



The irregular distribution of color in the blossoms of plants 

 affected with the mosaic disease seem to be confined entirely to the 

 pink-flowered varieties of Nicotiana tabacum. None of the greenish, 



