WILT OP 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- Fig. 79.f 



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 



*Further studies should be made to determine whether infection may not also take place through the root- 

 system. 



fFio. 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. 



