30 BULLETIN 828, TJ. 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 mi nutest 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 tracTieipMlus 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 
