THE VIRUS DISEASES OF PLANTS 421 



diseased plant, the insect frequently fails to transmit the 

 infection. It would appear that either the infective agent is 

 not always in a condition to be taken up by the insect, or it is 

 not always present in the tissues where the insect feeds, or 

 possibly the insect itself is not always willing to feed in the 

 proper way or is otherwise not always liable to take it up. On 

 the other hand, a single leaf-hopper fed for five minutes on a 

 diseased leaf has been recorded to be capable of causing the 

 disease when transferred to a healthy plant. The disease can 

 be transmitted to a number of other plants belonging to various 

 families on which the leaf-hopper can feed, and some of these 

 serve as a means of carrying the disease through the period of 

 the year when there are no beets growing. The insect itself 

 may remain infective for as long as 1 1 1 days after feeding. 



In most cases that have been examined, however, the insect 

 appears to be capable of transmitting the disease immediately 

 after feeding, even though in several of these cases the juice of 

 the diseased plant has not hitherto been found able to cause 

 infection. The evidence in these last cases is not, however, 

 very complete, and it is somewhat surprising that so important 

 a question has been left unanswered in the case of the leaf roll 

 and crinkle of potato, mosaic and leaf curl of raspberries, and 

 the mosaic disease of the sugar beet. In these cases inocula- 

 tions with the juice have failed, and if the insect transmitter 

 does not become infective for some time after feeding, one 

 would be led to suppose that there is a necessary part of the 

 life cycle of the parasite in the insect, and so to understand why 

 a direct transfer by juice is ineffective. Where direct inocula- 

 tion by juice is successful, as in a number of the mosaic diseases 

 (tobacco, potato, tomato, bean, clover, sweet pea, cucurbits, 

 spinach, turnip, sugar-cane, etc.), it is evident that no essential 

 part of the life cycle has to be passed in the insect host. 



In spinach blight, a disease which appears to be of the 

 mosaic type (though, unlike most other mosaic diseases, it is 

 fatal), various interesting points in connection with insect trans- 

 mission have been established.^ It is carried by several insects 

 (at least two species of aphids and the tarnished plant bug 

 Lygus pratensis) and is also transmitted by direct inoculation 

 with the juice of infected plants. The juice of the crushed 

 aphids that have fed on diseased plants is infective. Infection 

 can result if infective aphids are allowed to feed on healthy 

 plants for only five minutes, and non-infected aphids can pick 

 up the virus by feeding on blighted leaves for only ten minutes. 

 Adult aphids are more infectious than those that are immature 

 and the incubation period of the disease in the plant is shorter 



^ McClintock, J. A., and Smith, L. B., True Nature of Spinach- blight and 

 Relation of Insects to its Transmission, Journ. of Agric. Res., xiv, p. i, 1918, 



