312 
Journal of Agricultural Research 
Vol. XXVIII, No. 4 
leaves. In most of the trials, however, the insects secured the virus from the 
normal-appearing leaves as well as from the younger, evidently diseased leaves. 
The peculiar fact should be noted, however, that the insects which were placed 
on the leaves showing the symptoms were able to transmit the virus sooner than 
those insects which were placed on the older, normal-appearing leaves. 
The fact that the virus is present in the roots of diseased plants has been repeat¬ 
edly determined by the use of nonviruliferous insects. 
IS A BACTERIAL ORGANISM ASSOCIATED WITH CURLY-TOP ? 
The discovery of a bacterial organism associated with beets affected by curly- 
top was announced by Smith and Boncquet (15) and earlier workers. The 
work of the present writers on this point may be divided into two phases. In the 
first place, attempts have been made to grow beet plants under aseptic conditions 
by the use of steamed soil and disinfected seed. The plants were covered with 
bell jars to protect them from contamination by air-borne organisms. After 
curly-top had been produced in some of the plants, pieces of affected leaves of 
these were transferred, without disinfection, to tubes or plates of a culture medium. 
Bacteria grew from the leaf pieces in some'instances, but the important phase of 
the results is that in several of the cases the medium into which the curly-top 
tissue was introduced remained sterile or showed only fungus contamination. 
The other phase of the study of the question has been the plating of pieces of 
tissue of curly-top plants. Pieces of the interior of roots and of petioles have 
been cut out with aseptic precautions and then plated without disinfection. 
Pieces of diseased leaves have been plated after disinfection with mercuric chlorid 
for 5 to 15 minutes. In the majority of the cases both the untreated and the 
disinfected tissue remained sterile. In some instances bacteria grew from the 
tissue pieces, but several different types were represented. None of these were 
tested by inoculation. 
The results of these two lines of work indicate at least that no easily culturable 
specific bacterial organism is constantly associated with curly-top. That bac¬ 
teria may get into curly-top tissues through the open wounds caused by the dying 
back of lateral roots and the ruptures on the leaves made by the liquid exudate 
seems probable. 
THE INCUBATION PERIOD OF THE DISEASE 
The length of time that elapses from the inoculation of the plant until the 
first symptoms appear is considered as the incubation period of the disease. 
Temperature affects the length of the period, as is noted elsewhere in this paper. 
The incubation period is shortest when, with high temperatures, conditions are 
still favorable for growth of the plant. With beet plants in cheesecloth cages 
out-of-doors in the summer at Riverside, Calif., as short an incubation period 
as four days has been occasionally noted. With Stellaria media an incubation 
period of four days has several times been observed. In one case with this 
plant symptoms were visible within 48 hours after inoculation. In general the 
incubation period in sugar beets varies from 7 to 14 days. With low tempera¬ 
tures or conditions otherwise unfavorable for growth the period may be pro¬ 
longed, sometimes for a month or more. 
The discussion of the incubation period of the disease as given here should 
not be confused with the discussion by Severin (11). He speaks of the time 
elapsing between inoculations and when “ noninfective” insects were able to secure 
the causative agent from the plant as the incubation period of the causative 
agent. 
