CAUSE OF THE DISEASE. ' 17 



September 21. (6) Three plants show distinct curly-top symptoms on one or 

 more leaves. 



September 28. (&) Six plants affected with curly-top. 

 October 15. (&) All plants plainly affected. 



None of the control plants became affected with curly-top, nor 

 did any of those in compartments a and c. Most of the seedlings in 

 this experiment were only about ten days above ground and possessed 

 but two pairs of true leaves when the first hoppers were introduced. 

 The first unmistakable signs of curly-top were noted within thirteen 

 days after the introduction of leafhoppers. 



In another frame beet seed had been sown on June 10 ; this frame 

 was immediately covered with netting to exclude hoppers. Some 

 preliminary experiments, already mentioned, were tried in this frame, 

 but, as stated, with negative results so far as curly-top is concerned. 

 The plants were simply checked in growth. On August 12 this 

 frame was divided in the same way as cage 1. Compartment a was 

 used for controls and b for the confinement of leafhoppers, and the 

 plants in c were individually covered with inverted 2-quart Mason 

 jars as follows: Nos. 1, 2. 3, and 10 with thrips; Nos. 6, 7, and 8 as con- 

 trols; No. 4 with red spiders and thrips; Nos. 11 and 12 with red 

 spiders; No. :> with four leafhoppers; and No. with two leafhoppers. 

 At the beginning of this experiment these plants were two months 

 old. The outer leaves were stripped off so that those remaining 

 should not be too crowded under the jars. All the leaves left on the 

 plants were immature, and in them active cell division was in opera- 

 tion. The record for this cage is as follows: 



September 1. No. 5, inner leaves curing and vein symptoms showing (PI. IX, 

 fig. 1) : all controls unaffected, except from confinement under glass. 



September 4. No. 9, inner leaves curling and vein symptoms appearing; 

 controls still normal. 



In eighteen days curly-top symptoms appeared on the growing 

 leaves of plants two months old at the start, with only four and two 

 adult leafhoppers, respectively. Several days after the insects were 

 imprisoned some of them became inoperative because they stuck to 

 the wet walls of the jars. The plants were constantly bathed in 

 moisture during this experiment. 



On August 26 a cage was placed over a 2^-month beet. Four leaf- 

 hoppers were placed in this cage. On August 30, four more hoppers 

 were placed in this cage. On September 6, the inner leaves were 

 curling and vein symptoms appeared. (See PI. IX, fig. 1.) 



Early in September some living beet leafhoppers were sent in mail- 

 ing tubes to Dr. C. O. Townsend, at Washington, D. C. These were 

 plaeed on beets in isolation cages in the greenhouse, as follows: Sep- 

 tember 9, two hoppers; September 10, four more hoppers intro- 



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