POTATO WILT, LEAF-ROLL, AND RELATED DISEASES. 1 
In contrast with the slow-developing cases described, one finds 
many hills where there is actual wilting and rapid death of the plants, 
due to the water supply having been cut off by the fungous mycelium 
in the vascular bundles. Weekly examinations of fields during 
August and September show that the plants are dying prematurely 
and in increasing numbers as the season advances. 
It will be brought out later in describing leaf-roll that the latter 
does not cause such a rapid and early death as the wilt, but that 
plants showing distinct symptoms of leaf-roll in June may live till 
harvest time. 
In the root. — The fungus appears to enter through the smaller 
roots, and there are some indications that its injuries to the feeding 
roots are the cause of the dwarfed and checked development of the 
plant during the early stages of the disease. As a result of partial 
destruction of the roots, the plants are easily pulled up, and the roots 
are partly dead and brittle. As Smith and Swingle (1904) write: 
All the smaller roots are so friable that they can be broken with almost no effort, and 
some can even be rubbed to pieces between the thumb and finger. The main root 
also is much more tender and brittle than that of healthy plants, and this condition 
extends nearly to the line marked by the surface of the ground. Such diseased roota 
are usually covered with a white, pink, or even reddish growth of mycelium, which is 
distributed very unevenly and is much more conspicuous in some places than in others. 
Microscopical examination shows that this mycelium invades all parts of the root, 
though the bark is most affected. 
It is possible, and from some recent observations it seems quite 
likely, that some of this root injury is due to secondary invasion of 
other species of fungi. 
The underground stems on which the tubers are borne are nearly always attacked, 
but they do not as a rule become so soft and brittle as roots of the same size. The 
mycelium passes through the whole extent of these underground stems into the base 
of the tubers. 
In the tuber. — The infection of the tubers by the Fusarium is in 
well-marked cases almost universal. This is evidenced by the dis- 
tinct browning of the vascular ring shown when tubers are cut across 
at the stem end (PI. II, fig. 1). From these browned vessels Fusar- 
ium oxysporum can readily be isolated. To quote again from Smith 
and' Swingle (1904): 
Numerous cultures made from the extreme ends of the discolored portions of the 
bundles very seldom failed to develop the fungus. These cultures were made by 
carefully paring especially favorable pieces of diseased tubers with a hot scalpel, 
heating it nearly to redness before each stroke and cutting out pieces a few millimeters 
in diameter, containing a length of about two millimeters of the extreme end of the 
discolored part of the bundle. These pieces were cut from the main part of the speci- 
men with the hot scalpel and allowed to drop directly into a tube of sterile culture 
media. Potato cylinders were used principally for media. Slant tubes of beef agar 
(+15 on Fuller's scale) also were sometimes used. One hundred and twenty-two 
