SUMMARY. 33 



most interesting to the pathologist and physiologist on account of 

 the profound disturbances set up, as well as from its apparent relation- 

 ship to the mosaic disease. Apart from curly-top the writer has 

 noted stigmonoses of the sugar beet which are identical with those 

 described by Woods, Cobb, and Moulton, as well as with others caused 

 by different species of leaf hoppers. 



SUMMARY. 



The curly-top of beets is related causatively to stigmonose; patho- 

 logically and physiologically to both stigmonose and the mosaic dis- 

 ease. It manifests itself through numerous external and internal 

 symptoms of diverse character. 



The aggregate of these symptoms presents an appearance charac- 

 teristic of this disease, though taken individually all except the vein 

 symptoms have been noted under other conditions. 



Two distinct types of curl are manifested in this disease — the in- 

 ward and the outward, or retracted. 



The symptoms are essentially alike in factory and seed beets. 

 Among seed beets there is a larger proportion of the retracted type. 



The beet leafhopper (Eutettix tenella Baker) is the primary cause 

 of the trouble; no outbreaks are known to have occurred outside of 

 the native habitat of this insect, viz, the Southwestern States, gener- 

 ally in the vicinity of what is known as " sagebrush country." The 

 plants named as the probable host plants of the beet leafhopper are 

 found where outbreaks of curly-top have occurred. The insect 

 probably migrates from those plants to beets. 



One leafhopper is capable of initiating the disturbances in a young 

 beet, and the effect of numerous hoppers on seedlings may be so 

 severe as to kill them in a few days, before the visible symptoms 

 have had time to develop. The younger the beet the more sensitive 

 is it to leafhopper work. 



The nymphs of Eutettix tenella produce the symptoms of curly-top 

 much more rapidly than do the adult insects. 



No other insect and no other species of leafhopper is known to 

 induce the disease herein described. 



Though vigorous beets of four months growth or older will offer 

 considerable resistance, any beet will succumb if attacked by numer- 

 ous beet leaf hoppers. Neither is any variety of beet immune. 



The symptoms appear only on immature leaves where cell division 

 is still active — never on full-grown leaves. They appear first on 

 the inner leaves just as the latter unfold. 



It is considered advisable to plant as early as practicable after 

 danger of late frosts is over, even to take some risks of late frost, 



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