MOSAIC DISEASE OF TOBACCO. 89 



have been many cases where eveiy plant for some distance in a row 

 has developed mosaic, but this might be explained if it is assumed that 

 both workmen had handled diseased seedlings, or if a number of plants in 

 the lot were diseased. In time, the causal agent becomes so attenuated 

 that infection ceases, and the remainder of the row remains healthy. 

 Experimentally, this method of transmission has also been shown to be 

 possible, and a high percentage of infection has been obtained. In one 

 experiment, after thoroughly rubbing the hands with the tissue of a dis- 

 eased plant, and then pulling and transplanting healthy seedlings, over 80 

 per cent, of the transplants became mosaicked within a month. Only a 

 relatively small number of seedlings in this instance were treated in 

 this way, however, the total being 28, of which 24 developed mosaic 

 symptoms within three weeks. 



Another manner of transmission is by cultivation. If some of the sap 

 from a diseased plant comes in contact with the tools, etc., employed, 

 there is a possibility that the infection might be carried to healthy plants 

 by this means, but the percentage of infection of this character is probably 

 very low in actual field practice. 



The workmen when budding and topping are very often carriers of 

 infection, as they are not as a rule careful to leave untouched the plants 

 showing mosaic symptoms but take the plants as they come, and thus 

 spread the disease to many healthy plants. This method of dissemina- 

 tion has been very often observed, and perhaps is the most fruitful source 

 of infection in the field. The subsequent new growth will almost in- 

 variably be mosaic in character, as will also the suckers developing later. 

 The amount of damage to the marketable leaves, however, providing the 

 suckers are removed, is very sHght, if any, and cannot be said to injure 

 the leaf in any way, at least in so far as our observations bear on this 

 point. If the suckers are left, however, the plants present a ragged ap- 

 pearance, and the mosaic on the suckers is quite noticeable, and might 

 injure the sale of the crop at the price it ought to command. 



Seed. 



The causal agent is not carried by the seed, and seed from mosaic 

 plants has never produced a larger percentage of mosaicked seedlings than 

 seed collected from healthy plants, when germinated and grown under 

 the same conditions. It is difficult to conceive of this, as it has been 

 shown by Allard (loc. cit.) that the tissues closely enveloping the seed in 

 the pod are capable of causing infection; but the writer has saved seed 

 from badly mosaicked plants for three successive years, and the seedlings 

 from such seed showed no signs of the disease, unless infection was pro- 

 duced artificially through some external agency. 



It should be pointed out, however, that there is the possibility that the 

 vigor of the seed from mosaicked plants may be less than that from healthy 

 ones, and consequently the plants developed from such seeds, being 

 weaker, might be more susceptible to the factors active in the product on of 



