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



are usually descriptive of the characteristic mottling or otherwise 

 sickly appearance of plants affected with the disease. It is true, 

 however, that they are in many instances somewhat loosely applied 

 to a variety of abnormal appearances and symptoms not at all asso- 

 ciated with the specific mosaic disease of tobacco. 



Both in Europe and America the mosaic disease of tobacco has 

 been the subject of wide inquiry. Hitherto no investigator has 

 been able to offer very conclusive evidence which would consistently 

 explain the baffling nature and mysterious origin of the disease. 

 Each in his own way, however, has emphasized a favorite opinion, 

 so that the literature of the disease is especially conspicuous for its 

 widely contrasted theories. 



The view seems to be generally accepted that the disease is a 

 physiological or functional disorder, although it has long been known 

 that it is more or less infectious. For this reason the writer's experi- 

 ments were at first planned along physiological lines. Facts soon 

 came to light, however, which led to the conclusion that the disease 

 must be parasitic in its origin rather than physiological or functional. 



The writer has found that particular aphides can induce mosaic 

 symptoms in plants infested by them, and much evidence has accumu- 

 lated to show that aphides are responsible for serious outbreaks of 

 the disease both in the greenhouse and in the field. Since it has been 

 found that insects may act as carriers of the disease, this phase of the 

 problem has been taken up in cooperation with the Bureau of Ento- 

 mology with the hope that further light may be thrown on this point. 



APPEARANCE OF AFFECTED PLANTS. 



The development of the mosaic disease of tobacco is attended with 

 various physiological and morphological changes in the leaves, 

 branches, and sometimes the flowers of all affected plants. The char- 

 acter and intensity of these symptoms vary greatly, depending upon 

 the age, habits of growth, species of plants affected, and external con- 

 ditions. The following symptoms are more or less characteristic of 

 different phases of the disease at one time or another* 



(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 taoacum only). 



(9) Mosaic sucker growths. 



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



The extent and character of the first observable symptoms of the 

 disease depend upon the age and vigor of the plants at the time of 



