624 Journal of Agricultural Research voi.x. No. 12 



system which allows the virus to gain an entrance at these points. Ex- 

 periments have shown that the promptness and certainty of infection 

 are greater when the green mosaic material is thoroughly mixed with 

 the entire mass of soil just before transplanting than when it is buried 

 carefully in the lower layers. The explanation seems to be that in the 

 former case the injured roots of plants during transplanting are more 

 likely to come directly in contact with infective material. In the latter 

 case the injured roots are given an opportunity to heal before any por- 

 tion of the extensive root system reaches the layers of mosaic material, 

 and for this reason infection is greatly delayed, should it take place at all. 

 Many healthy plants have been grown to maturity under these condi- 

 tions, although an examination has shown that the roots have penetrated 

 every portion of the layer of mosaic material. In fact, owing to the 

 availability of additional plant food through the decomposition of the 

 mosaic material, the plants have generally shown greater vigor and a 

 darker green color than control plants which received no vegetable 

 material. 



In a long series of experiments the effect of replacing mosaic plants 

 grown in 12-quart pails by young, healthy plants was studied. The 

 results were rather uncertain and showed that healthy plants do not 

 necessarily become mosaic, even though they immediately replace mosaic 

 plants. In some instances several successive plants have developed the 

 mosaic disease shortly after transplanting, followed by one or more plants 

 which remained healthy. Here, also, immediate infection probably de- 

 pends upon the fact that the injured roots of healthy transplanted 

 seedlings in some instances happen to come directly in contact with some 

 portion of the broken mosaic root system of the diseased plant just 

 removed. 



It is quite possible that injury due to underground parasites may be a 

 factor in facilitating root infection in the mosaic disease of tobacco. Dur- 

 ing a series of pail experiments with plants grov/n in soil, to the lowermost 

 layers of which chopped up mosaic leaves and stems had been added, 

 the root systems of some of the plants became badly infested with 

 nematodes. It is believed that their attacks were a factor in producing 

 some of the infection which followed in these instances. Since it is 

 known that the larval stages of tobacco flea-beetles feed upon the finer 

 roots of tobacco and other solanaceous plants, it would be of considerable 

 interest to know whether or not these parasites are able also to com- 

 municate the disease from the root systems of mosaic to healthy plants 

 in the field. 



BEHAVIOR OF NICOTIANA GLAUCA AFFECTED WITH THE MOSAIC 



DISEASE 



It was at one time thought that Nicotiaria glatica is immune to the 

 mosaic disease of tobacco. It is now known that this species does 

 acquire the disease by artificial inoculation, although perhaps less readily 



