630 Journal of Agricultural Research voi. x, no. 12 



Although Preissecker casually observed that there was some relation 

 between the presence of plant lice upon tobacco plants and the subsequent 

 occurrence of the mosaic disease, it does not appear that he looked upon 

 them as actual carriers of the infective principle of the disease. 



SUMMARY 



(i) The virus of the mosaic disease of tobacco is present in the trichomes 

 of the leaves, as well as in the tissues of the lamina. The disease may be 

 communicated to healthy plants by inoculating the virus into the 

 trichomes alone. 



(2) The infective principle of the disease does not readily invade un- 

 injured trichomes or leaf tissues when merely sprayed upon the plants. 

 Infection readily follows when the virus is sprayed upon the leaves and 

 subsequently rubbed in. 



(3) Infection is more likely to follow if the virus is inoculated into the 

 leaves at more than one point. 



(4) Cutting across the midrib at the base of the leaf, or severing all 

 the larger veins on one or both sides of the midrib, does not prevent the 

 final dissemination of the virus from distant points of inoculation in the 

 leaf to other leaves on the plant. 



(5) Thorough washing with soap and water serves to remove the 

 virus from the hands for all practical purposes. 



(6) Thorough steam sterilization of the soil of the seed bed completely 

 destroys any virus of the mosaic disease of tobacco which may be present 

 in the soil. 



(7) The mosaic disease of tobacco does not necessarily follow when 

 large quantities of mosaic-diseased material are introduced into the soil 

 so that the roots of healthy tobacco plants must ultimately reach it. 

 Infection from such material appears to depend upon injury to some 

 portion of the root system which allows the virus to enter at these points. 

 It is possible that root parasites may sometimes produce this injury to 

 the roots and root hairs. 



(8) The species Nicotiana glauca is susceptible to the mosaic disease of 

 tobacco, although visible symptoms of the disease may be very slight. 

 The sap of such plants, although apparently but little affected by the 

 disease, so far as visible symptoms are concerned, is highly infectious to 

 healthy plants of A^. iabacum. 



(9) Some species of aphids may become active carriers of the infective 

 principle of the mosaic disease. Experiments have shown that the green 

 peach aphis (Myzus persicae) may become an active carrier of infection 

 in the greenhouse, after it hks been feeding upon mosaic-diseased plants. 

 Not all species of aphids appear to transfer the disease with the same 

 readiness. Negative results were always obtained with the large lettuce 

 aphis, Macrosiphum lactucae. Under field conditions the large plant 

 louse Macrosiphum tabaci which sometimes becomes very common on 



