TOBACCO MOSAIC 
This is a disease not only of tobacco but also of many other solanaceous 
lants. 
Plants affected with the mosaic disease, or as it is often called, calico, 
mottled-top, grey-top, or frenching, do not usually die. There is pro- 
nounced hypoplasia, especially of young leaves. There is a reduction 
in size, and chlorophyl-development is halted, so that a true chlorosis 
results. 
SYMPTOMS 
The leaves, and sometimes the branches, are the only organs of the host 
which exhibit symptoms. The plants are often much dwarfed and, in the 
case of Nicotiana rustica L., are sometimes killed. In some of the so- 
called immune varieties, it has been found that a progressive rotting of the 
tissues follows inoculations with the virus, but in such cases there are none 
of the usual symptoms of the disease. (U.S. Agr. Dept. Bul. 40: 2.) 
The symptoms vary from small blisters on the leaves, or the distinct 
dark-green islands in large areas of yellow, to an excessive suppression 
of the development of the lamina of the leaves, in which only the midrib 
remains normal. 
On the entire plant. Study the diseased and healthy tobacco plants 
provided. OBSERVE :— 
1. The comparative size and appearance of the healthy and 
diseased plants. SKETCH or copy from U. S. Agr. Dept. Bul. 40, pl. VII. 
Maintain relative proportions between the healthy and diseased plants. 
On the leaves. Examine the plants provided and locate, but do not 
remove, leaves showing various symptoms. OBSERVE :— 
2. The characters of a healthy leaf, its regularity of surface 
and margin; its size. DRAW. 
3. The peculiar green and yellow blotched appearance of diseased 
leaves. Note the blister-like elevations on the upper surface. Are they 
swellings or are they archings of the leaf? In which areas do they occur? 
Is there any relation between the veins and the green areas? 
4. That the yellow areas of the leaf are apparently thinner 
than the green islands. 
5. The comparative size of healthy and diseased leaves of the 
same age. 
DRAW a diseased leaf on the same scale as the healthy leaf. Bring out 
in the drawing and labeling as many of the characters as possible. 
6. The distorted leaves. These may be long and ribban-like 
or the lamina of the leaf may be entirely undeveloped, although the mid- 
Tib is not affected. DRaw, or copy U.S. Agr. Dept. Bul. 40, pl. II, fig. 1. 
7. That certain upper leaves may show a mottling (green and 
yellow) but without the accompanying distortions of the surface. Note 
how the green areas follow the veins. This is the phase that gives the 
names, grey-top and mottled-top, and occurs when the plant becomes 
infected just as the immature flower-heads appear. Draw, or copy U. 
S. Agr. Dept. Bul. 40, pl. I, fig. 2. 
On the blossoms. Examine the flowers of the plant provided, or 
U.S. Agr. Dept. Bul. 40, pl. III and IV. Various species of Nicotiana 
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