POTATO WILT, LEAF-ROLL, AND RELATED DISEASES. 7 



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 roots 

 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 

 mycelitun 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 readUy 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 cultvires 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 cultm-e 

 media. Potato cylinders were used principally for media. Slant tubes of beef agar 

 (-f 15 on Fuller's scale) also were sometimes used. One hundred and twenty-two 



