20 
Journal of Agricultural Research 
Vol. XIV, No. i 
remaining plants were healthy on March 2. A duplication of this experi¬ 
ment was made with Rhopalosiphum persicae which had been bom and 
reared on supposedly healthy spinach in the greenhouse. The parents 
of these aphids were collected from a field of apparently healthy spinach 
which at the time gave no evidence of the presence of the blight. The 
aphids were placed on spinach in the greenhouse and allowed to remain 
on the seedlings until there was a sufficient number of adults to use in 
this experiment. Four days after the transfers had been made to the 
outdoor cages, or 29 days after the original aphids had been brought 
to the greenhouse, six of the seedlings on which they had been placed 
became diseased. In the outdoor series three plants were infected, the 
symptoms developing in an average time of 27 days. 
About 10 plants in each of the field cages remained untreated and 
were used as controls. There were 84 plants used in this manner, and 
all remained healthy until the experiment closed on March 2. 
The results obtained through the transfers of live aphids from diseased 
to healthy plants proved the ability of the aphids to transmit the disease 
from plant to plant in the field. The ability of Macrosiphum solanifolii 
and Rhopalosiphum persicae to transmit the disease was about the 
same, although the incubation period for plants inoculated by M. solani¬ 
folii was 0.8 day longer than the average incubation period of plants 
inoculated by R. persicae . The juice obtained from crushed aphids from 
blighted plants proved to be infectious; that obtained from M. solani¬ 
folii gave 50 per cent and that from R. persicae 25 per cent of positive 
infections. It is noticeable that in this case the average incubation 
period was one to seven days longer than where the inoculations were 
made through either the feeding punctures of the aphids or by pricking 
the expressed virus into the plant tissues. The peculiar results which 
were obtained by the transfers of lettuce-fed aphids to healthy plants are 
not easily explained. It was known that the M. solanifolii had not come 
in contact with diseased spinach from the time of their birth until they 
were transferred to the healthy spinach plants, and yet they produced 
positive cases of blight in 2 cases out of 10 inoculations. R . persicae from 
supposedly healthy spinach plants produced 3 cases of blight out of the 10 
plants inoculated. It will be noticed that the average incubation period 
of the disease in all five of these positive cases was 8 to 10 days longer 
than the average incubation period of the disease when caused by the 
direct transfers of aphids from diseased plants. During the experiment 
it was supposed that in the series of R. persicae there may have been a 
latent case of blight in the healthy spinach from which the aphids had 
previously been taken in the field and that the virus had been carried 
from it to the experimental plants. The possibilities of seed or soil 
infection were eliminated, as thousands of plants were growing at the 
time under insect-free conditions though in soil known to have grown 
diseased spinach. No cases of blight developed on any of these plants. 
