July 1,1925 
Overwintering and Dissemination of Cucurbit Mosaic 
11 
The wild cucumber seedlings in the vicinity of Madison, Wis., 
usually appear in the field between May 1 and May 15, and prac¬ 
tically all the observations were made within two weeks after the 
development of the first leaf. All the plants which developed mosaic 
showed definite symptoms of the disease in the first leaves, and in 
many cases the presence of the disease was confirmed by inoculations 
from suspected plants to healthy cucumbers in the greenhouse. The 
possibility of infection from other sources was slight, as cultivated 
cucurbits are rarely planted at the time these observations were 
made, and near-by groups of healthy Micrampelis plants remained 
mosaic-free each season. The experiments previously mentioned, in 
which mosaic Micrampelis plants appeared in spots which had been 
caged early in the spring, also seem to reduce the possibility of the 
infection coming from any source except the seed. As will be shown 
later in this paper, the milkweed, Asclepias syriaca , is also an agency 
in overwintering cucurbit mosaic, but as the mosaic milkweed shoots 
do not appear until some time after the disease is found on the wild 
cucumber seedlings, the infection on Micrampelis could hardly come 
from this source. It is also of interest to note that the percentage of 
mosaic plants found in the open each year is approximately the same 
as that obtained in trials with seed from mosaic wild cucumber plants 
in the greenhouse. All the evidence, therefore, seems to warrant the 
conclusion that cucurbit mosaic is carried in the seed of mosaic plants 
of the wdld cucumber. 
It is a peculiar fact that cucurbit mosaic should be carried in the 
seed of the wild cucumber but not in that of the cultivated cucurbits. 
This anomaly is as yet unexplained, but Butler (5), in a review of 
the literature on mosaic diseases, makes an interesting suggestion. 
He points out that the embryo is not connected with the vascular 
system, as the bundles do not penetrate the nucellus, and states 
that— 
In the Cucurbitaceae the nucellus is provided on the outside of its epidermis 
with a well-marked cuticle, and this epidermis persists in the ripe seed, though 
the rest of the nucellus is absorbed. The stalk end of the ovule (the chalaza) 
is suberified. Thus the whole of the central part of the ovule is cut off from the 
seed coats and stalk by cutin or suberin. The embryo also, in the spherical 
stage, is provided with a complete investment of cuticle over its free parts. All 
the evidence available in regard to infection by the virus diseases indicates 
strongly that they are unable to pass through cutin and probably unable to 
pass through suberin (e. g., uninjured branches or a potato tuber). Hence 
one would expect that the virus of cucumber mosaic would fail to be carried 
over to the next generation in the seed. But this is one of the diseases which 
is sometimes transmitted by seed. The explanation of this anomaly is perhaps 
to be found in the work of Longo {15), who has shown that since the cucurbit 
embryo is so completely isolated* by impermeable layers, an adaptation of a 
curious nature has been developed, in which the pollen tube opens a communica¬ 
tion between the neck of the nucellus and the embryo-sac. Possibly the same 
channel could serve to transmit the virus to the embryo. 
INSECT TRANSMISSION OF MOSAIC FROM THE WILD TO THE CULTIVATED CUCUMBER 
Mosaic wild cucumber plants often occur at considerable distances 
from fields of cultivated cucurbits, and these and other wild host 
plants would be of relatively minor importance were it not for the 
fact that insects serve as carriers of the disease. Experiments have 
demonstrated, however, that the insects which transmit the disease 
in the case of the cucumber ( 8) also transmit mosaic from Micram¬ 
pelis to the cultivated cucurbits. 
