MOSAIC DISEASE OF TOBACCO. 79 



of the disease ■will be given so that no misunderstanding may arise, as 

 several other leaf troubles more or less chlorotic in character have often 

 been confounded with the true "mosaic." The disease may show on the 

 leaves at all stages of the growth, from the seedling to the mature plant. 

 It is often difficult in seedlings to diagnose the trouble definitely, as 

 the slight mottling and curl of the leaves may be due to other factors. 

 As a rule, in young plants the leaf is rougher and a permanent mottling is 

 observed, very slight in character, however, and not to be confounded with 

 the mottling due to normal metabolic processes which occurs under certain 

 conditions of growth. As the disease progresses, however, the leaf is 

 found to be divided into light and dark green areas; in mild cases there 

 does not appear to be any marked leaf distortion, and the light green areas 

 sometimes verge on the yellow in color. The dark green areas apparently 

 deepen in color with the intensity of the disease, and in extreme cases the 

 leaf is much distorted and the dark portions appear blister-like, due to 

 their more rapid growth. The leaves, as a rule, are much stiffer and 

 thicker to the touch than are the normal healthy leaves. Sometimes in 

 the later stages of the disease there are found dry, dead, brown patches or 

 spots on the leaves, sometimes where the dark green areas were originally, 

 but more often the light green portions show this extreme condition. Both 

 the light and dark areas show abnormalities in structure; nevertheless, 

 the light green areas are the more truly diseased ones, the dark green areas 

 presenting different characteristics, and although showing changes in 

 cell arrangement, etc., function more normally in many respects. Most 

 investigators have held that the light green areas are the diseased portions 

 of a leaf, but some have been of the opinion that the dark green areas are 

 the diseased portions. As will be seen from the writer's experiments the 

 former is the more correct view, as the increase in color intensity and the 

 blistering of the dark green areas is due to the necessarily increased func- 

 tioning thrown on these portions of the leaf. 



Occasionally a leaf may be distorted in such a manner as to present the 

 appearance of being little more than a long filament consisting principally 

 of midrib, with but very little leaf surface. This condition has been 

 observed by the writer in some instances, but should not be confounded 

 with a similar trouble occurring on tobacco in certain regions, which is of 

 an unknown character but which is not the true mosaic as it is not infec- 

 tious. This latter trouble has been noted particularly in Java, etc., as is 

 reported by Peters ^ in his work on the diseases of tobacco. It has not 

 been observed in tobacco fields in this region by the writer. 



It is thought that soil and moisture conditions are responsible at least 

 partially for this disease. 



> Peters, L.: Krankheiten und Beschadigung des Tabaks. Mitteil. aus der Kaiser. Anstalt 

 F. Land- u. Forstwirtschaft. Heft, 13: 64 (1912). 



