CAUSE OP THE DISEASE. 13 



the greater part of the root (PL VIII, fig. 1). This is not to be 

 attributed to curly-top, but is probably brought about by the entrance 

 of various organisms, such as Phoma betae, Rhizoctonia, etc. On 

 the other hand, the root may remain sound and crisp, though the top 

 may exhibit every symptom of a severe case of curley-top (PL VIII. 

 fig. 2). 



ONLY ONE SYMPTOM REALLY CHARACTERISTIC. 



It is to be remarked that most of the symptoms of curly-top, taken 

 individually, may appear under other pathological or physiological 

 conditions; so far as observed, the one real exception is the vein 

 symptoms. However, the totality of the symptoms described in this 

 connection has not been observed apart from curly-top. 



CAUSE OF THE DISEASE. 

 THEORIES. 



Until quite recently the cause of curly-top was unknown. Numer- 

 ous theories have been advanced to account for it. Among the more 

 plausible are the following: Bacteria, fungi attacking the leaves, 

 fungi attacking the roots, insects, abnormal moisture supply, abnor- 

 mal soil conditions, poor seed, and a combination of unfavorable con- 

 ditions (26, 27). 



After wide field observations Dr. E. D. Ball, then special agent of 

 the Bureau of Entomology, stated (-1, 5, G) that in his opinion the 

 disease is caused by the beet leafhopper (Eutettix tenella Baker, 

 PL I, fig. 1). Prof. E. G. Titus, formerly collaborator of the Bureau 

 of Entomology, while engaged in entomological experiments observed 

 the appearance of curly-top on beets in breeding cages in which some 

 beet leafhoppers were found (6; also footnote, p. 23). 



PRELIMINARY EXPERIMENTS. 



The writer first turned his attention to the curly-top of beets late in 

 the summer of 1907. Early in June of that year he had noted in the 

 experimental field at Garland, Utah, a few cases of the disease among 

 sugar beets. The earliest of these were among pedigree strains and 

 among garden beets, both of which germinated and grew with much 

 less vigor than the standard varieties of sugar beets. As the season 

 advanced more cases appeared sporadically throughout the field. 



° In this paper the beet leafhopper {Eutettix 'tenella Baker) will be referred to 

 simply as the leafhopper. 

 181 



