THE VIRUS DISEASES OF PLANTS 423 



Rubbing the leaves wdth the fingers acts in the same way. With 

 needle inoculations, single pricks often fail, the best results being 

 got by multiple inoculations. 



The virus does not seem to be equally active or equally 

 concentrated in all parts of the plant ; thus inoculations from 

 the roots give a lower percentage of success than those from the 

 leaves. " Carriers " are known, e.g. Physalis may be inoculated 

 and transmit the virus by subsequent inoculation from it to 

 other plants, but itself may show no sign of the disease. So 

 also the grafting of an infected plant on an immune variety of 

 Nicotiana makes the juice of the latter infective, though no 

 symptoms are produced. Direct inoculation of the virus into 

 an immune species may set up a destructive tissue rot, though 

 no other symptoms appear and though rotting is not a symptom 

 of mosaic in susceptible plants. Possibly immunity here is 

 related to those cases known in the plant rusts in which varie- 

 ties whose cells are too readily attacked and killed are immune 

 merely because the parasite is unable to feed and grow in the 

 presence of dead cells, and soon dies out. Mixing the virus 

 with the sap of immune species of Nicotiana does not reduce its 

 virulence. The virus travels slowly in the plant. It has been 

 found to take three days to traverse a leaf and its stalk. It 

 can be transmitted to several other plants of the same famil}'- 

 (Solanaceae), including tomato, petunia, Datura, Hyoscyamus, 

 and Capsicum, but the divergent results obtained by different 

 investigators in Europe and America, together with other 

 circumstances, indicate that there are two or more distinct 

 mosaic diseases of tobacco, just as it is clearly proved that there 

 are several of potato. Nicotiana viscosum, is known to have a 

 distinct mosaic disease not communicable to tobacco, though 

 both it and the tobacco mosaic can be transmitted to Datura 

 stramonium. Transmission by insects (aphids) is common, but 

 not all insects that feed on tobacco can carry the virus. The 

 large green aphis of lettuce {Macrosiphon lactucae) will feed and 

 multiply readily on tobacco but does not transmit mosaic, nor 

 does the white fly or red spider. Though the fresh juice is 

 infectious, the best inoculum of all tried is the dried leaf tissue 

 macerated in water. 



The allied potato mosaic is not so readily transmitted as the 

 last ; for instance, rubbing the leaves is not sufficient. Bean 

 mosaic is also more refractory, as it is often difficult to get 

 success by inoculating the juice into the tissues from a syringe 

 or by rubbing the leaves ; the best results are got in this case by 

 using the crushed tissue of the leaf. In the cucumber mosaic/ 



^ Doolittle, S. P., The Mosaic Disease of Cucurbits, U.S. Dept. of Agric. 

 Bull., 879, 1920, and The Relation of Wild Host Plants to the Over-wintering 

 of Cucurbit Mosaic, Phytopathology, xi, p. 47, 1921. 



