Apr. 26,1924 
Studies on Curly-Top Disease of the Sugar Beet 
307 
of the insect. In view of these facts the question as to whether or not an appre¬ 
ciable interval of time must elapse after a nonviruliferous 5 leafhopper has fed 
on a diseased plant before it is able to effectively transmit the virus may have 
some significance regarding the nature of the virus. Smith and Boncquet (15) 
state that a period of at least 24 hours but not more than 48 hours must elapse. 
Severin (11) worked with batches of 25 to 50 insects instead of single leafhoppers. 
He found that at high temperatures (about 100° F.) the insects were occasionally 
able to produce the disease within four to six hours after they had been placed on 
a diseased plant. For this short period his results showed 8 cases of disease 
from 60 tests. No positive results were obtained from 36 tests for a period of 
one to three hours. Periods of 24 to 48 hours after the insects were placed on the 
diseased plant gave a greater proportion of positve results than shorter ones. 
The experimental tests which the writers have thus far made in regard to this 
question are listed in Table VI. In each of these experiments nonviruliferous 
leafhoppers were placed on a curly-top beet for a given number of hours and then 
transferred individually to young healthy plants for a certain number of hours 
and then transferred again, etc. For example, *in experiment No. 1 the non¬ 
viruliferous insects were caged on a curly-top plant for 24 hours and then caged 
singly on healthy plants for the same length of time. The first period tested was 
from 24 to 48 hours after the insects were placed on the diseased plant. After 
that the insects or those which still survived were transferred to other healthy 
plants for 24 hours. This second period, therefore, was from 48 to 72 hours 
after the insects were placed on the diseased plant. The successive transfers of 
the same insects are indicated in Table VI by attaching the letters a, 6, c, etc., 
to the number of the experiment. 
The results show that some of the insects were able to transmit the virus within 
a period of 21 hours and 45 minutes after they were placed on the diseased beet. 
In another case the period was 22 hours and 20 minutes, and in a third case it 
was 23 hours. In some cases the insects all failed to produce the disease within 
periods of 24 hours or less. A point which may be of some significance is that the 
results seem to indicate that a greater number of the insects become able to trans¬ 
mit the virus after a longer period than 24 hours than are able to do so in the 
shorter time. The facts cited seem to indicate that a multiplication of the causal 
agent takes place within the insect. 
RELATION OF THE NUMBER OF INSECTS TO CURLY-TOP 
Comparative inoculations were made at the same time and on similar plants 
with different numbers of insects. The object was to determine whether or not 
the quantity of virus injected was significant in producing the disease or in 
regard to the period of incubation. Twelve experiments have been performed 
to test this. The procedure in these tests was so varied that the details can not 
be readily tabulated. The results, however, may be summarized. They indicate 
that when 10 insects are used the inoculated plant is more likely to become 
diseased than when the inoculation is made with 1 or 2 insects. It seems probable 
that this fact is due to the variation of individual insects in regard to infection 
rather than to the quantity of virus injected. In the cases where the disease is 
produced there is no difference in the incubation period or severity of the effect 
between inoculations with single insects and those with 10 insects. 
6 It has recently been brought to the writers' attention that the term viruliferous occurs in medical litera¬ 
ture; and as it appears to have the same meaning viruliferous (meaning virus-bearing) as used by the writers 
in previous publications, it is of course desirable to use viruliferous for the sake of preventing a duplication 
of terms. 
