BACTERIAL WILT OF CUCURBITS. 29 



the same virulence relations as the originals. The relative virulence 

 indexes of these four isolations as deduced from many series of 

 cucumber inoculations are, respectively, 0.0384 and 0.0366 for R 230 

 and En 1J. and 0.0218 and 0.0229 for R 235 and En 15. 



The least virulent isolation {En 160 B) of the series which at any 

 time caused complete wilting in cucumbers had a relative virulence 

 index of 0.0149. This was one of four isolations made from slightly 

 wilted cucumber vines collected at Goldsboro, N. C, in June, 1916. 

 One of these cucumber isolations (R 317) made in the field caused wilt- 

 ing of only a few cucumber leaves, never of entire plants. The other 

 three isolations (En 160 A, B, C) were from cucumber material col- 

 lected in this same locality and sent to the laboratory at Washington, 

 D. C, for isolation. These three isolations sometimes caused wilting 

 of entire plants after a comparatively long time and in other cases 

 caused only partial wilting. The bacterial wilt was rare in the 

 vicinity of Goldsboro, and only after careful search were these few 

 cases found. With the exception of this Goldsboro locality all the 

 southern isolations possessed a comparatively high degree of 

 virulence. 



Leaving out the one very exceptional case at Goldsboro, N. C., the 

 average indexes of relative virulence for all cucumber isolations 

 grouped according to geographical source were as follows : Ten isolations 

 from Norfolk,Va., southward, 0.042; 15 from the District of Columbia, 

 0.041 ; 17 from Long Island, 0.040; 7 from middle New York to Canada, 

 0.036; and 9 from Michigan, Wisconsin, and northern Iowa, 0.032. 

 As will be readily noticed, the average virulence is highest in the 

 South and gradually decreases northward. A study of the figures 

 making up these averages shows that the southern isolations as a 

 whole tended to higher virulence, while the northern isolations con- 

 tained examples of both high and low virulence instead of decreasing 

 as a unit. The higher virulence of the southern isolations is shown 

 also by the fact that out of seven isolations from cucumbers obtained 

 in North Carolina, South Carolina, Georgia, and Alabama, six of 

 them (R 318, 320, 321, 323, 324, and 325) were found capable of caus- 

 ing wilt in White Bush Scallop squash. Of the northern isolations 

 from cucumber, the larger number obtained have caused no apparent 

 infection when inoculated into squash. 



The isolations from squash and cantaloupe when grouped according 

 to geographical source have shown this same general tendency, but 

 not in quite so marked a degree, since all isolations from these hosts 

 have fallen within the more highly virulent group. 



Viewing the general results of these relative virulence tests, it 

 seems possible that instead of having only two distinct and well- 

 differentiated biological strains of Bacillus tracheipMlus we may be 

 dealing with a long series of " races' ; or "pure lines" of the organism 



