12 BULLETIN 1038, U. S. DEPAETMEISTT OF AGRICULTURE. 



The infectious chlorosis of the sugar beet known as curly-top is not 

 transmitted by expressed plant juices nor by way of the seed, but is 

 disseminated by the sugar-beet leafhopper {Eutettix tenella Baker). 

 This insect is capable of producing infection only after a definite 

 incubation period subsequent to feeding upon diseased plants. Ap- 

 parently there is no other agent of transmission for this disease 

 (10, 68). 



Some extremely interesting insect relations of spinach blight have 

 recently been worked out by McClintock and Loren B. Smith (49). 

 Not only were healthy plants successfully inoculated by needle 

 pricks with the contagium from diseased plants and with the crushed 

 juice of aphids fed upon diseased plants but the potato aphid 

 {Macrosiphwn solanifolii Ashmead) and the spinach aphid {Rho- 

 jyalosiphuTti persicae Sulzer) , free from infection at first, were demon- 

 strated to transfer the blight to healthy spinach after feeding upon 

 diseased plants. Control plants invariably remained healthy. Later, 

 these two species were obtained from four different States where 

 spinach blight did not occur, and they failed to induce the disease 

 on healthy plants until after they had fed on blighted spinach. The 

 same two species collected locally and tested at the same time pro- 

 duced the disease. These investigators demonstrated that the con- 

 tagium may be carried from spring to fall by a direct line of aphids. 

 Transmission tests with several other species of insects gave nega- 

 tive results, thus also tending to show that the insect relation to 

 spinach blight is not that of a purely mechanical disseminator. 



Sugar-cane mosaic has been shown by Brandes (19-21) to be 

 transmitted by cuttings, by expressed juices from diseased plants, 

 and by certain insects {Aphis maidis Fitch) fed upon infected plants. 

 No evidence of seed transmission was found. Insect transmission 

 of corn mosaic has also been demonstrated by Brandes. 



That cucurbit mosaic is transmitted by the expressed plant juices 

 and by insects has been definitely proved by Doolittle (28), by 

 Jagger (43, 44), and by Doolittle and Gilbert (31) ; and the latter 

 investigators have apparently shown that at least in some cases the 

 disease is carried over by the seed (30). In this disease both foliage 

 and fruit become yellow mottled and distorted, and growth of the 

 entire plant is seriously checked. The dark-green portions of dis- 

 eased leaves are slightly thicker than normal, thus accounting for 

 their blistered and distorted appearance. The yellow areas, though 

 thinner than contiguous dark-green parts, are of about the same 

 diameter as in the normal leaf. The palisade cells of the green areas 

 are crowded closely together and are somewhat longer and narrower 

 than in the normal leaf. In the yellow parts these cells are more 

 nearly isodiametric and less in number than normal per unit of area. 



