12 BULLETIN" 1038, IT. S. DEPARTMENT OE 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 
(Macrosiphum solanifolii Ashmead) and the spinach aphid (Rho- 
palosiphum 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. 
