40 
Journal of Agricultural Research 
Vol. XXXI, No. 1 
fields near mosaic pokeweed plants contained a number of pokeweed 
seedlings, all of which were free from mosaic. Practically all of the 
old pokeweeds in the vicinity, however, were severely affected with 
mosaic. 
INSECTS AS AGENCIES IN THE DISSEMINATION OF POKEWEED MOSAIC FROM THE 
POKEWEED TO THE CUCURBITS 
Aphids 
It was first thought that the cucumber aphids would feed on the 
pokeweed only when forced to do so by a lack of other host plants. 
This belief was at first confirmed by an inspection of mosaic poke- 
weeds in southern Illinois during the summer of 1922. No aphids 
or other insects were found on the pokeweed plants examined at that 
time, although they showed signs of insect injury at some earlier 
date. No aphids were found in the surrounding cucumber fields, 
however, so the evidence in this case was not conclusive. There was 
some question as to whether there was any common insect carrier 
of the disease from the pokeweed to the cucumber, and unless such 
a carrier existed it was not certain that the pokeweed was an impor¬ 
tant source of infection to the cucurbits. Later in the summer, how¬ 
ever, it was found that the cucumber aphids were present in consid¬ 
erable numbers on pokeweeds in the experimental plats at Madison, 
and that their occurrence was not due to artificial conditions. The 
aphids had appeared naturally on the cucumbers and later had 
apparently migrated to the near-by pokeweed plants. The natural 
transference of the aphids from one host to the other was also demon¬ 
strated by experiments in which five mosaic pokeweed plants were 
placed in separate cages which also contained a number of healthy 
cucumber plants. The experiments were conducted in the field, and 
care was taken to see that the pokeweed and cucumber plants were 
not in contact. In each of these cages the mosaic pokeweeds soon 
became covered with aphids from the cucumbers, and in every case 
the cucumber plants in Xhe cage became mosaic diseased within 12 
days after the pokeweeds were introduced. In the control cages in 
which healthy pokeweeds had been placed there was no mosaic 
infection on the cucumbers in any case. 
The migration of aphids from cucumbers to the pokeweed was also 
demonstrated by experiments in which healthy pokeweed plants were 
placed in three cages containing mosaic cucumber plants which 
were infested with aphids. In these experiments all of the poke- 
weeds became covered with the aphids, although they were not in 
contact with the cucumbers, and later developed symptoms of 
mosaic. These observations and experiments indicate that the 
cucumber aphis is a carrier of mosaic from the pokeweed to the 
cucurbits and that the aphids may also be responsible for a portion 
of the mosaic infection found on the pokeweed. The situation in 
the case of pokeweed seems much like that of the milkweed, although 
the evidence of the transmission of mosaic from the pokeweed to the 
cucurbits in the field is not as complete as that which exists with 
regard to the milkweed, owing to the fact that it has been necessary 
to conduct most of the work in Wisconsin, where the pokeweed is 
of rare occurrence in the field. 
