578 SPECIAL PLANT PATHOLOGY 



Experimental treatment indicates that high temperature and humidity 

 at the time of inoculation favor infection. 



Mosaic Disease of Tobacco} — This disease is one of the most serious 

 which attacks the tobacco plant. It is known locally as "calico," 

 "gray-top," "mottled-top," "mottling" and "foxy" tobacco. The 

 term "frenching" is used in southern tobacco sections to designate 

 abnormal, sickly plants with stringy, very thick and leathery leaves 

 which may be mottled, or not. It is not known whether this disease is 

 distinct from mosaic. Chlorosis has also been used for mosaic, as well 

 as the terms" brindle" or "mongrel." Allard states that the mosaic dis- 

 ease of tobacco is attended with various physiologic and morphologic 

 changes in the leaves, branches and sometimes flowers of all affected 

 plants. The character and the intensity of these symptoms vary 

 greatly, depending upon the age, habits of growth, species of plants 

 affected and external conditions. Allard classifies the characteristic 

 symptoms of mosaic, as follows: 



1. Partial or complete chlorosis. 



2. Curling of the leaves. 



3. Dwarfing and distortion of the leaves. 



4. Blistered or "savoyed " appearance of the leaves. 



5. Mottling of the leaves with different shades of green. 



6. Dwarfing of the entire plant. 



7. Dwarfing and distortion of the blossoms. 



8. Blotched or bleached corollas (in Nicotiana tahacum only). 



9. Mosaic sucker growths. 



10. Death of tissues (sometimes very marked in Nicotiana rustica). 

 The first visible symptom of mosaic in very young plants appears as a 



slight downward curling and distortion of the smallest innermost leaves, 

 which at the same time become more or less chlorotic. Small abnor- 

 mally dark-green spots and areas appear as these leaves increase in size 

 and if the plants are not crowded these spots develop rapidly into large, 

 irregular, crumpled swellings or blisters of a "savoyed" appearance. 

 The leaves of these young plants may grow to a disproportionate size, 



1 Woods, Albert F.: Observations on the Mosaic Disease of Tobacco, Bull. 18, 

 U. S. Bureau of Plant Industry, 1902; Chapman, G. H. : Mosaic and Allied Diseases, 

 Report of Botanist in 25th Annual Report Massachu ^etts Agricultural Experiment 

 Station, 1913; Allard, H. A.: The Mosaic Disease of Tobacco, Bull. U. S. Depart- 

 ment Agriculture, 40, 1914. 



