THE MOSAIC DISEASE OF TOBACCO. 29 



spring, when the expression of the disease is coincident with the 

 growth of new shoots. On several occasions the writer has observed 

 on these plants the occurrence in large numbers of aphides, which 

 may serve as an important agency in the dissemination of this 

 disease. 



OCCURRENCE OF THE DISEASE UNDER PRACTICAL FIELD CONDITIONS. 



So far as is known, the mosaic disease of tobacco occurs to a greater 

 or less extent in all countries where tobacco is grown. Mosaic plants 

 may first appear in the seed bed, or at any stage of growth after the 

 plants have been transplanted to the field. The extent of injury 

 caused by the disease depends very largely upon the age of the plants 

 at the time the disease manifests itself. 



Not infrequently plants which appear healthy when cut down at 

 maturity give rise to mosaic sucker growths from the stubs through- 

 out a field. This late development and widespread occurrence of the 

 disease might be explained as due to the fact that the instrument 

 used to cut the plants carried infection from mosaic to healthy plants 

 during the process of cutting the stalks. Priming the leaves also 

 would serve to cause a widespread development of the disease in 

 sucker growths appearing very late in the season after the period of 

 harvesting. 



The greatest injury is caused when the disease develops in young 

 plants, since these are as a result usually much dwarfed in all their 

 parts. The development of the disease in plants that are nearly 

 mature may in itself result in little or no loss to the grower, on account 

 of normal leaf growth having been already attained. It is a common 

 impression among many growers, however, that mosaic plants are 

 much more susceptible to rust, so that in the end this secondary effect 

 may sometimes occasion great loss. 



In the field, the relative distribution of mosaic and healthy plants 

 is sometimes very peculiar. It is claimed that only alternate plants 

 in the rows throughout a field sometimes develop symptoms. It is 

 very evident that field conditions can not account for this peculiar 

 occurrence of the disease. Such uniformity in the distribution of 

 mosaic plants can be explained in only two ways. Since two men 

 riding on a setting machine drop plants alternately in the row, it 

 may be conceived that one man's hands have become badly infected 

 in some way, so that the alternate plants, being the ones which he 

 has dropped, later develop the disease; but it is more reasonable to 

 believe that all the plants in the lot dropped by the one man have 

 been pulled from an infected portion of the seed bed. 



Although the opinion seems to be quite generally held that the 

 refuse of mosaic plants plowed into the soil may be responsible for 



