36 
Journal of Agricultural Research 
Vol. XXXI, No. 1 
that found in mosaic cucumber plants. The light-green portions of the 
leaf may later include the greater portion of its surface. They are 
irregular in outline and do not appear to be delimited by the veins 
of tne leaf (pi. 3, A). In the later stages of the disease the plants 
have a typically mosaic appearance, the darker portions of the leaf 
are slightly raised and the foliage has the wrinkled and blistered 
appearance which is associated with many mosaic diseases (pi. 4, 
A, C). The leaves of mosaic pokeweed plants are usually more 
irregular in outline than those of healthy plants. In the older plants 
the leaves are often curled upward at the edges in a manner similar 
to that of mosaic milkweeds. 
Where plants are affected with the disease while young they be¬ 
come mucn stunted and blossom very sparingly. In the field, how¬ 
ever, mosaic plants are commonly found which are 4 to 5 feet in 
height and appear to be making a vigorous growth. In all cases, 
however, the foliage shows the typical symptoms of the disease and 
the leaves are generally somewhat smaller than those of healthy 
plants. 
SUSCEPTIBILITY OP THE POKEWEED TO CUCUMBER MOSAIC 
As a result of the apparent infection of the pokeweed by aphids, 
from mosaic cucumber plants, further cross inoculations were made 
from mosaic cucumber and muskmelon to the pokeweed, both by 
means of aphids and by artificial inoculation with the expressed juices 
of these plants. As shown by Table XIV, these experiments have 
shown that cucurbit mosaic is transmissible to the pokeweed by means 
of aphids from both the cucumber and muskmelon. Up to the 
present, however, no infection has been produced through wound 
inoculations with the expressed juices of these cucurbits. If only 
artificial methods of inoculation had been used the results of our 
earlier work, together with the results of Allard and Clinton in cross 
inoculation with tobacco, would have indicated that the pokeweed 
was not susceptible to other mosaic diseases and that the mosaic 
disease of pokeweed was distinct from that on the cucumber. It 
should be noted, however, that continued efforts at cross inoculation 
with cucurbit mosaic by artificial methods have eventually resulted 
in infection of host plants of other families, although earlier experi¬ 
ments had indicated that these hosts could not be infected except 
by means of aphids. It is possible, therefore, that the pokeweed may 
be infected with cucurbit mosaic by the use of artificial methods of 
inoculation. The use of the insect method is limited to some extent 
by the fact that it is not always possible to find insects that will 
transmit the disease in question and also feed on both the mosaic 
host plant and the one to be inoculated. The wide host range of 
the cucumber aphis, however, has made this insect of especial value 
in such experiments. 
Some difficulty was encountered in the earlier experiments with 
the pokeweed, owing to the fact that during the winter the cucumber 
aphids often refused to feed on the pokeweed unless confined on the 
leaves in small cages. These cages were constructed by covering 
a Van Tieghem cell with fine wire screen and placing the cell over 
the leaf. The aphids were placed in the cage thus formed on the 
leaf and another piece of fine wire was placed on the under side of the 
leaf directly beneath the glass cell. The whole was then held in 
