198 The Sugar-Beet in America 



swarms fly from their breeding grounds on wild plants for 

 long distances over mountain chains and other barriers. 

 Sometimes there will be only one flight into a partic- 

 ular region ; if so, beets coming up later will not be 

 infested. 



"Leafhoppers taken from wild plants do not transmit 

 the disease until they feed on diseased beets. Three hours 

 on a beet rendered them pathogenic, but they could not 

 transmit until after an incubation period of one or two 

 days. It is probable that some wild plant carries the 

 disease and leafhoppers coming from this plant are able 

 to transmit it to the beets. 



"A large number of leafhoppers, early attack, hot 

 weather, and clean cultivation are favorable to the curly- 

 leaf development. The converse of these factors, together 

 with frequent cultivation, early irrigation, and shade or 

 weeds, are unfavorable. Seed growing is doubly hazard- 

 ous in curly-leaf areas. 



"Loss from curly-leaf may be largely prevented by 

 avoiding dangerous areas, by planting small acreages 

 in a 'blight cycle,' by controlling the time of planting, 

 by not thinning just as the leafhoppers appear, and by 

 knowledge of conditions on breeding grounds. Para- 

 sites doubtless assist somewhat in controlling the leaf- 

 hopper, but to be at all effective should be introduced 

 into the permanent breeding grounds." 



DISEASE INJURY 



The losses due to beet diseases have not been great in 

 America, probably because beets have been growTi here 



