30 BULLETIN" 828, IT. S. DEPARTMENT OF AGRICULTURE. 



exhibiting among themselves closely intergrading degrees of virulence 

 for their various hosts. Wilt goes from cucumber to squash as well as 

 from squash to cucumber. Furthermore, although among cucumber 

 isolations the larger percentage are extremely virulent when inoculat- 

 ed into cucumber, while the larger percentage do not cause any signs 

 of the disease at all in squash, the considerable number of cucumber 

 isolations remaining which do cause appreciable infection in squash 

 exhibit all grades of host reaction — the minutest signs alone, wilt of 

 only a few square millimeters around the inoculation points, wilt of 

 a considerable portion of the inoculated leaf, wilt of one to several 

 leaves, and finally, in the case of the more virulent isolations, wilt of 

 the entire squash plant. 



However, the factors causing resistance or susceptibility to the 

 wilt disease in cucumbers may be more or less distinct from the factors 

 causing resistance or susceptibility in squash. Thus, there may still 

 be a distinct cucumber strain represented by those of our isolations 

 which never caused infection in squash and a squash strain possessing 

 not only the factors necessary for infection in cucumber but additional 

 factors enabling it to attack squash also. If this is the case, in all 

 probability some means will be found of distinguishing the two strains 

 by morphological or cultural characters of the parasite. Thus far, 

 however, no constant morphological or cultural differences have been 

 found between cucumber and squash isolations or between isolations 

 of greater or less relative virulence. The only constant differences 

 as yet noted among the various isolations have been differences in 

 relative virulence to a given host. The fullest tests have been made 

 upon Chicago Pickling cucumber, but smaller tests have also been 

 carried out upon Arlington White Spine cucumber and upon White 

 Bush Scallop and Yellow Crookneck squashes. While in these tests 

 the individual isolations have not preserved exactly the same order of 

 relative virulence when inoculated into the different host species and 

 varieties, the tendency has been in this direction. That is, isolations 

 most or least virulent to Chicago Pickling cucumber tended to be, 

 respectively, most or least virulent to Arlington White Spine cucumber; 

 and many though by no means all isolations highly virulent to 

 cucumbers have been found capable of infecting squashes to a greater 

 or less degree, while isolations of low virulence to cucumbers have 

 invariably given no infection at all on squashes. This last group 

 Dr. Smith called Bacillus tracheipMlus forma cucumis. Apparently 

 the factors in the parasite causing high virulence to cucumbers are 

 necessary to infection in squashes, for none of the isolations weak in 

 virulence to cucumbers has caused any infection at all in squashes. 

 But not all of the isolations highly virulent to cucumbers will cause 

 infection in squashes. Thus additional qualities in the parasite may 

 be necessary for squash infection, and it is entirely possible that 



