34 THE CURLY-TOP OF BEETS. 



in order to get the beets well established and vigorous before leaf- 

 hoppers are likely to be numerous. 



The disease can not be due to loss of plant juices extracted by leaf- 

 hoppers, but rather to the introduction by those insects of an active 

 agent. 



Few or no beets that develop curly-top symptoms ever recover. 



Beets may fail to show symptoms of curly-top to the end of the 

 season if attacked by leafhoppers after they have attained consider- 

 able size and vigor. Yet the trouble is initiated and transmitted to 

 the root, only to develop with full virulence in the first shoots the 

 following spring if they be planted out for seed production. 



The active agent is therefore persistent and sets up disturbances 

 throughout the whole plant. 



It is risky to select for seed purposes any beets from fields badly 

 infested with beet leafhoppers, even though no large number of 

 cases of curly-top appear. 



It is useless to make selections for seed from fields badly affected 

 with curly-top. 



The inward type of curl is generally produced on young beets. The 

 retracted type appears on seed beets and on sugar beets that had 

 attained considerable size and vigor before the symptoms developed. 



Until recently curly-top symptoms had been noted on no other 

 plants than beets, but the writer noted the symptoms on cabbage in 

 1909. 



The punctures of the beet leafhopper produce w T ine-colored stains 

 on the leaves of dock {Rumex crispus L.) and on stock beets, but no 

 such discoloration has been observed on sugar beets. 



Anything tending to retard or weaken beets renders them less re- 

 sistant to leafhopper attacks. 



The leafhoppers can induce the symptoms of curly-top under ex- 

 tremely varied conditions of moisture, temperature, and atmospheric 

 pressure. 



Weather, soil, etc., may influence the severity of the disease in- 

 directly by modifying the vigor of the beets; possibly, also, the leaf- 

 hoppers are influenced favorably or otherwise by varying weather 

 conditions. 



If seed beets are really healthy when planted — that is, if they have 

 not been subjected to the attacks of leafhoppers during their first 

 season's growth — there is little chance that they will succumb to 

 leafhopper attack during their season as seed beets. An attack in- 

 volving the whole seed beet is perhaps without exception the result 

 of attacks during the preceding season, though no symptoms may then 

 have appeared. Portions of the later shoots and the aftergrowth of 



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