July i, 1925 Overwintering and Dissemination of Cucurbit Mosaic 
23 
CROSS-INOCULATIONS FROM MOSAIC MILKWEED TO THE CUCUMBER 
Inoculation with Aphids 
Aphids from mosaic milkweeds found in the fields at Rockland 
were brought to Madison and transferred to healthy cucumber 
plants under cages in the greenhouse. Definite cases of mosaic 
occurred on all these plants, and the controls on which aphids from 
healthy milkweeds had been placed remained free from mosaic. 
These results were followed by a series of experiments in which 
aphids from healthy cucumbers were transferred to mosaic milkweed 
plants which were free from insects. These plants were covered with 
cheesecloth cages, and after the aphids had been allowed to feed 
on them for some time a number of the insects were transferred to 
healthy cucumber plants under cages in the greenhouse. This 
method eliminated the possibility that mosaic infection produced by 
aphids from mosaic milkweeds in the field might be due to aphids 
which had recently migrated from mosaic cucumber plants to the 
milkweed. The results given in Table IX show that nearly all cucum¬ 
ber plants inoculated in this manner developed typical cases of mosaic. 
Table IX. — Results of cross-inoculation from mosaic milkweeds (Asclepias 
syriaca) to healthy cucumber plants by means of Aphis gossypii 
Date, 1920 
Source of 
inoculum 
Method of inoculation 
Number 
of plants 
inocu¬ 
lated 
Number 
of mosaic 
plants 
Date ob¬ 
served, 
1920 
Ang. 5 
Rockland, Wis__ 
Aphids from mosaic milkweed_ 
9 
9 
Aug. 12 
Do_ 
_do... 
Aphids from healthy milkweed (control) 
9 
0 
Aug. 18 
Allg. 15 
Madison, Wis_ 
Aphids from mosaic milkweed ®_ 
7 
6 
Aug. 27 
Do_ 
.do__. 
Aphids from healthy milkweed (control) 
7 
0 
Sept. 1 
Aug. 16_ 
_do_ 
Aphids from mosaic milkweed °_ 
6 
5 
Aug. 27 
Do_ 
_do. 
Aphids from healthy milkweed (control) 
6 
0 
Sept. 1 
Sept. 1_ 
_do_ 
Aphids from mosaic milkweed ®_ 
8 
8 
Sept. 19 
Do_ 
.do_ 
Aphids from healthy milkweed (control),. 
8 
0 
Do. 
Sept. 4.. 
_do_ 
Aphids from mosaic milkweed «.. 
4 
3 
Sept. 13 
Do. 
.do.. 
Aphids from healthy milkweed (control)-. 
4 
0 
Do. 
Sept. 5 
do 
Aphids from mosaic milkweed ®_ 
9 
9 
Sept. 20 
Do. 
_do.. 
Aphids from healthy milkweed (control).. 
5 
0 
Sept. 23 
« Aphids from healthy cucumber plants transferred to mosaic milkweed plants under cages in the field* 
Artificial Inoculation 
In addition to the cross-inoculation by means of Aphis gossypii , 
the susceptibility of the cucumber to the mosaic disease on the milk¬ 
weed was confirmed by artificial cross inoculations. The crushed 
leaf tissues of mosaic milkweeds were used as inocula and a small 
fragment of the crushed leaf was inserted in a slight longitudinal cut 
in the lower portion of the stem of healthy cucumber plants. The 
oldest leaf of the inoculated plant was cut off close to the stem and 
the incision made in this wounded surface. The controls were treated 
in the same manner, except that the crushed leaf tissue of healthy 
milkweed plants was inserted in the stem. Inoculations were made 
both in the field and in the greenhouse and all plants inoculated were 
kept under cheesecloth cages to avoid outside infection. The results 
of these experiments, shown in Table X, have been uniformly success¬ 
ful, and the fact of the transmission of the mosaic disease from the 
milkweed to the cucumber is thus established. 
