Apr. 26, 1924 
Studies on Curly-Top Disease of the Sugar Beet 
315 
those done earlier shows that in its relation to this disease the species is peculiar. 
The climate of Riverside is in general warmer than that of Spreckels, particu¬ 
larly in summer, but opposed to the idea that climatic differences may explain 
the difference in results in the two localities is the fact that some of the positive 
results were secured during the winter. It seems probable that the species is 
resistant to curly-top. 
An interesting observation made in connection with two of the experiments 
seems probably due to this seeming resistance. In two of the tests where nymphs 
from Chenopodium murale , the progeny of viruliferous leaf hoppers, were caged 
on healthy young beets several of the beets soon developed faint clearing 
of the veins of some of the youngest leaves. The first symptom of curly- 
top is a clearing of the veins of the younger leaves. In these two tests, 
however, the vein clearing gradually disappeared in most of the plants and 
normal growth was continued. Only one or two plants developed typical cases 
of curly-top. One theory, which seems a plausible explanation, is that by pas¬ 
sage through the C. murale the curly-top virus was so weakened or modified as to 
be rendered unable, in most cases, to effectively establish itself in the sugar beet. 
The point has not as yet been determined as to whether by means of nonviru- 
liferous leafhoppers the virus may be obtained from those beet plants which 
show the faint vein clearing and subsequently recover. 
As to variation in susceptibility to infection, the fact has been previously 
mentioned (8) that Stellaria media seems much more readily infected than the 
beet or buckwheat. There are some indications that Erodium cicutarium and 
E. moschatum and possibly Calandrinia menziesii are more readily infected than 
the beet. These three species, as well as spinach and tomato, die quickly as a 
result of the disease and in that sense are much more susceptible to the disease 
than several other species, e. g., Malva parviflora f cheese weed, and the sugar 
beet. 
OVERWINTERING OF THE VIRUS 
Few if any of those leafhoppers which leave the beet fields in the fall ever 
return. The observations by which this fact was established were in the main 
made in California. An observation made in Utah in the early summer of 1922 
supports the belief that the same situation exists in that region. Near Elsinore, 
Utah, two sugar beet fields were inspected. In one of these fields a few cases 
of curly-top were found by diligent search, but certainly less than one-half of 
1 per cent of the plants were affected. In the other field 1,200 plants were 
# counted without finding any case of the disease. In both of these fields beet 
leafhoppers were present, and they had been there long enough to allow a con¬ 
siderable number of nymphs, their progeny, to emerge. It was estimated that 
there were 50 to 75 nymphs on each plant. Although the leafhoppers must have 
been on the beets for at least a month, there was practically no curly-top. Evi¬ 
dently most of the insects which flew into these fields in spring were non- 
viruliferous. 
The question therefore arises as to where the new brood of insects which enters 
the fields in spring secures the curly-top virus. The suggestion has been pre¬ 
viously made ( 8 ) that from susceptible annuals of the natural breeding areas 
some of these spring-brood leafhoppers may secure the virus. That this is not 
the only way in which the virus probably passes the winter should be made 
clear. As mentioned elsewhere in this paper, a small proportion of the leaf¬ 
hoppers usually remain in the cultivated area in the fall after the main body of 
the insects has departed. These “ hold-over” insects, or “stragglers,” being 
viruliferous, infect some of the early planted beets. Beets thus infected then 
serve as a source of virus for nonviruliferous insects that may come in later. 
