66 BULLETIN 879, U. S. DEPARTMENT OF AGRICULTURE. 
the virus was introduced into the roots or flower parts. The first 
symptoms of the disease invariably appear in the youngest leaves or 
fruits and the susceptibility of the plant seems to be closely related 
to its age and vigor of growth. 
The virus spreads through the plant from the point of inoculation 
and is present throughout the leaves and stems 24 to 48 hours before 
any visible symptoms appear. There is some evidence that the vas- 
cular system may be the chief channel of distribution in the plant. 
Transmission under field conditions in the case of the cucumber 
may occur during the thinning, training, or picking operations and 
also by the removal of diseased vines which are intertwined with 
healthy plants. Cucumber insects are among the most important 
agents in the transmission of the disease in both the field and the 
greenhouse. The melon aphis (Aphis gossypii Glover) and the striped 
and 12-spotted cucumber beetles (Diabrotica vittata Fabr. and Dia- 
brotica duodecimpunctata Oliv.) are the insects most concerned. Bees 
have not been proved to be carriers of the disease. 
It has been shown that the disease does not live over winter in the 
soil, and there is no evidence that striped cucumber beetles or other 
insects are a source of primary infection in the spring. Extensive 
field tests of seed from mosaic plants and observations in the field 
and greenhouse indicate that infection through the seed may possi- 
bly occur in rare cases. 
The mosaic diseases of tobacco, tomato, bean, potato, pokeweed, 
and various other hosts do not appear to affect the cucumber. Inocu- 
lations of these and other plants outside the Cucurbitacese (except 
martynia, see p. 6) with the expressed juice of mosaic cucurbits have 
also given negative results. 
The wild cucumber (Micrampelis lobata) is affected with a mosaic 
disease identical with that on the cucumber. Diseased plants of this 
species have been found in Wisconsin and Indiana at least two or 
more weeks before the disease appeared on the cultivated cucumber. 
The striped cucumber beetles feed on the micrampelis from the time 
they appear and later go directly from the wild plants to cultivated 
cucumbers. There is thus a direct means of transmission from the 
wild to the cultivated host during the early part of the season. 
The source of primary infection of micrampelis is not certain, the 
factors which appear to have been eliminated in the case of the cul- 
tivated cucumber probably being excluded from the wild host also. 
The wild host plant, however, offers the most definite source yet dis- 
covered of primary infection for the cucumber. 
The control methods thus far tested have been (1) the removal of 
diseased plants as soon as found; (2) the control of cucumber insects, 
which are largely instrumental in spreading the disease; (3) the dis- 
covery of varieties of cucumbers which are resistant to mosaic; and 
