4 BULLETIN 40, U. S. DEPARTMENT OP AGRICULTURE. 



gate above the uppermost leaves. At this time the cluster of flower 

 heads is very small, compact, and less than 1 inch across. After an 

 incubation period of 10 to 15 days, the first symptoms of the disease 

 become manifest as a very noticeable wilting of the upper leaves, 

 especially in a hot sun. These leaves, especially in transmitted sun- 

 light, show a very fine mottling, which very rapidly develops into 

 characteristic " mottled-top." 



If the leaves are examined closely, it will be seen that this appear- 

 ance is due to the distribution of the dark-green shades along the 

 fine anastomosing veins, while the lighter shades occupy the small 

 inclosed areas. The earlier manifestations of " mottled-top " in 

 some respects simulate certain color changes which accompany the 

 normal ripening of fully matured leaves. The " mottled-top " phase 

 of the disease in nearly mature plants produces little or no distor- 

 tion in the leaves which exhibit this appearance. Practical growers 

 do not consider this phase particularly serious. 



If plants become infected just prior to the appearance of the 

 blossoms, the leaves of the main stalk, all of which at this time 

 have practically completed growth, are little, if at all, changed in 

 normal appearance. The characteristic symptoms, however, such as 

 distortion, etc., are reproduced in the secondary growths (suckers) 

 in much the same manner that young plants are affected. 



ROOTS OF DISEASED PLANTS. 



The roots of mosaic plants superficially appear quite as normal 

 in many instances as those of healthy plants. It is probable, how- 

 ever, that the roots as well as the leaves may be more or less seriously 

 impaired in form and function, depending upon the age and condi- 

 tion of the plant when affected with the disease. 



LEAVES OF DISEASED PLANTS. 



The development of the mosaic disease in tobacco plants produces 

 a great variety of changes in the normal color and form of the 

 leaves. The more evident changes are confined to the actively grow- 

 ing portions, such as the immature leaves of young plants and the 

 vigorous secondary growth of more mature plants. A character- 

 istic feature of the true mosaic disease, which seems to distinguish 

 it from nearly all unhealthy appearances of purely physiological 

 origin, is the distinctive blotched and mottled appearance of the 

 leaves, sometimes accompanied by distortion, especially if the leaves 

 are immature. In very immature plants the leaves often show 

 marked distortion and irregularities of growth, from which they may 

 almost recover as they become larger and more mature. 



Fantastic leaf forms frequently arise in mosaic plants. In some 

 instances the development of the lamina is almost entirely suppressed, 



