Apr. 26,1924 Studies on Curly- Top Disease of the Sugar Beet 
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RELATION OF TEMPERATURE TO CURLY-TOP 
The writers’ ideas as to the relation of temperature to the incubation and 
development of curly-top are based principally on observations made under 
natural conditions at different seasons. Facilities for accurately controlling 
temperatures where plants were grown in California have not been available, 
but the evidence obtained indicates that soil temperature as well as that of the 
air affects the duration of the incubation period in beets. For instance, in an 
experiment where all the inoculated plants were exposed to the same range of air 
temperatures but were exposed to two different soil temperatures, symptoms of 
the disease appeared first at the higher temperature. With the plants where the 
maximum soil temperature ranged from 25° to 35° C. symptoms of curly-top 
appeared on the eighth day, while in the case of the plants where the maximum 
temperature varied from 15° to 20.5° C. symptoms first appeared on the 
eleventh day. Results of two series of tests conducted with controlled soil 
temperatures in a greenhouse at Madison, Wis., 6 support the view as to the 
influence of soil temperatures. Plants inoculated in California and sent by mail 
to Madison and there grown at a soil temperature of 33° C. developed the 
symptoms sooner than did those at 15° C. In the case of Stellaria media the 
results of one test indicate that soil temperature is less important than in the 
case of the beet. Where the air temperature was the same for both sets of 
plants the symptoms appeared at the same time in both the high and low tem¬ 
perature soil. If this seeming difference between beets and chickweed holds 
true it may possibly be explained on the basis of the different structure of the 
two plants. In the case of the beet the virus has to pass through the crown of the 
plant, which is usually at or below the soil level, in order to reach the new leaves. 
The main shoots of a Stellaria media plant throw out lateral branches, so that 
the curly-top virus may reach new leaves without necessarily passing through 
the root or crown. It would, therefore, probably be influenced in its rate of 
spread or development chiefly by the air temperature. 
Low temperatures retard the progress of the disease as they lengthen the 
incubation period. This fact probably explains why curly-top is generally less 
injurious, even when it occurs in the cooler coastal districts, than it is in the 
warmer regions. 
AGE OF PLANTS IN RELATION TO SUSCEPTIBILITY 
The question of the relation of the age of a plant to its susceptibility to curly- 
top infection is of great interest from the standpoint of cultural practice as to 
time of planting. Field observations indicate that if beets have attained a 
considerable size before the leaf hoppers appear they are much more likely to 
make a harvestable crop than they will if very small when the leaf hoppers 
come in. This fact seems due, in part at least, to the beets becoming less easily 
infected as they grow older. A limited amount of experimental work on the 
question, which is detailed in Table X, supports this idea. In the experiments 
where plants of different ages were inoculated on the same date both lots were 
growing under similar conditions. The age of the plants used is indicated in the 
table by giving the number of leaves developed. 
* The details of the tests at Madison were handled by Hurley Fellows. 
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