Apr. 26, 1924 
Studies on Curly- Top Disease of the Sugar Beet 
309 
The question has been raised as to whether the infectivity of the insect or 
ability to produce the disease runs in cycles, such as might occur if the virus 
were an organism with a definite life cycle. The results as listed in Table VII 
seem to give no evidence of definite cycles of infectivity. The evidence seems 
to justify the conclusion that, in general, when an insect has produced or failed 
to produce the disease in a given period it is more likely to give a similar result 
in the subsequent one or more periods before changing to the other result. In 
other words, the positive or negative results are more likely to be grouped together 
in consecutive periods than they are to alternate. This idea is based on the results 
shown in the table, as well as on the results of other tests. As to what is the 
significance of this seeming tendency of the results, the evidence thus far justifies 
only speculation. It is of interest to note the preponderance in the number 
of cases in which the disease was produced over the number of negative results. 
Table VII .—Variation of individual insects in regard to causing infection 
[The plus sign (+) indicates the production of the disease, and the minus sign (—) indicates that no disease 
resulted] 
The numbers at the top of the columns were used to designate the individual 
insects. The figures in the left-hand column show the number of hours the insects 
were on the plants. The plus sign (+ ) indicates that the disease was produced as a 
result of the inoculation in that period. Likewise the minus sign (—) indicates that 
