WILT OF CUCURBITS. 285 
It has been observed or reported in the following varieties of muskmelons: Early 
Hackensack, Shumway’s Giant, Dudaim, Rocky Ford. 
It has been observed in the following varieties of squash: Hubbard, yellow Crookneck, 
Long Island White Bush, Early Yellow Bush Scallop, White Summer Crookneck, Boston 
Marrow. 
July 14, 1909, the disease was observed near Washington on Venetian squashes, grown 
from imported seed. 
MORBID ANATOMY. 
There are no hyperplasias in connection with this disease. It is principally a disease 
of the spiral and ring-vessels, and their entourage in the stem of the host. These vessels 
are arranged in a group toward the 
inner part of each bundle. They 
are embedded in a mass of thin- 
walled living parenchyma, which is 
separated from the inner phloem 
by a thin band of tissue somewhat 
resembling cambium in structure, 
and sometimes called pseudocam- 
bium. By its outer face, the tissue 
containing the spirals and ring-ves- 
sels joins on to the lignified tissue 
or xylem proper which contains the 
large pitted vessels embedded in 
pitted, lignified connective tissue. 
The spiral-vessels and ring-vessels 
are always the first part of the stem 
to be occupied by this bacillus. 
The reason for this is not far to 
seek. It lies in the fact that they 
are the only part of the xylem- 
portion of the bundle which passes 
out from the stem into the leaves 
to form the xylem-part of the veins 
of the leaf. Since the infections 
are entirely through the leaf-sur- 
face (so far as we yet know*) and 
since the organism passes downward into the stem exclusively by way of the spiral-vessels 
of the petiole, it is at once apparent why the spiral-vessels of the stem are the first part of 
that organ to be invaded. In the stems of the cucurbits subject to this disease there are 
nine or ten separate vascular bundles and, consequently, on cross-section there are, or may 
- be, nine or ten distinct bacterial foci corresponding to as many groups of spiral-vessels 
and ring-vessels (figs. 67, 77, 78, 80). The organism always appears in these spiral-vessels 
in enormous numbers, soon filling them completely (fig. 79). The next stage in the progress 
of the disease is the destruction of the walls of these vessels. This appears to take place by 
solution, but perhaps also by rupture, these walls consisting of a very thin non-lignified 
membrane the only lignified parts being the spiral-thickenings or ring-thickenings. The 
bacteria now invade in great numbers the thin-walled living parenchyma surrounding these 
" *Burther studies should be made to determine whether infection may not also take place through the root- 
system. . 
tFic. 79.—Cross-section of a cucumber stem from field, showing a small spiral vessel filled with Bacillus trachei- 
philus, contents of non-parasitized vessel-parenchyma-cells omitted. Drawn from a photomicrograph of a thick, 
unstained glycerin mount made in 1893 (the year I discovered the disease). x 1000. 
