TOBACCO DISEASES AND THEIR CONTROL. 



35 



ordinarily feared by growers, since it often occurs without doing ap- 

 preciable damage, although in some years, in certain districts par- 

 ticularly, the losses from the 

 disease are very large both in 

 yield and quality. 



The symptoms of mosaic in 

 tobacco are numerous, varying 

 from only faint signs to those 

 that are most distinct and pro- 

 nounced. The most common 

 and characteristic symptom is 

 mottling of the leaves — that is, 

 alternate dark green and yel- 

 lowish (chlorotic) areas irregu- 

 larly scattered over the leaf 

 surface, usually being most dis- 

 tinctive in the top or youngest 

 leaves, particularly in the early 

 stages of the disease (fig. 19). 

 The more pronounced symp- 

 toms are blistering, curling, and 

 distortion of the leaves, fol- 

 lowed by dwarfing of the entire 

 plant (PL XIII, fig. 1). In 

 older plants the flowers may 

 also become dwarfed, distorted, 

 and bleached. Under certain 

 weather conditions it is not un- 

 common for mosaic plants, even • 

 though showing only faint 

 symptoms in the leaves, to de- 

 velop more or less spotting, or- 

 dinarily referred to as " rust " 

 or " firing," but such cases are 

 by no means always a conse- 

 quence of mosaic (PI. XIV). 

 On plants where the suckers 

 show characteristic mosaic 

 symptoms and the spotting is 

 not due to any other evident 

 cause, mosaic is usually respon- 

 sible. The losses from mosaic 

 follow as a consequence of the 

 reduced yield due to dwarfing 

 of the plants and the mottling 

 or abnormality of the leaves. 

 which render them unsuitable 

 for use for certain purposes of 

 manufacture. 



Cause. — From the standpoint of science mosaic is one of the most 

 puzzling diseases of plants. Kepeated efforts to demonstrate the 

 actual cause of this disease have so far failed. It is well known, 

 however, that the disease is infectious, being readily transmitted from 

 diseased to healthy plants in various ways. It practically follows 



Fig. 19. — Tobacco mosaic. The mottled ap- 

 pearance is the most characteristic sign of 

 this disease. 



