298 Journal of Agricultural Research voi. vi, no. 9 



infections, by the blackened, more or less sunken spots on the surface 

 of the tuber. Tubers collected in a commercial potato field and infected 

 in this manner are shown in Plate XXXIV, figures i to 6. This black color 

 is lost in part as the infection becomes older, the infected tissues taking 

 on various shades from nearly black to sepia brown. In connection with 

 the stem-end rot, the fungus often proceeds down the vascular tissue, 

 killing and blackening the network of bundles. Figures 5 and 6 in 

 Plate XXXIV show sections of a tuber infected in this manner. Often it is 

 possible to break away the cortical tissues and lay bare the blackened 

 network. Lenticel infection proceeds outward in all directions from the 

 point of infection and may or may not extend down to the main vas- 

 cular system. Very frequently in the case of eye infections the vas- 

 cular strand connecting the eye with the main vascular system is black- 

 ened, but it is seldom that such infection extends far into the main 

 vascular ring. Blackrot is confined principally to potatoes of the Idaho 

 Rural, Pearl, and other round types. 



Closely related to the blackrot of potatoes of the round types is a 

 jelly-end rot attacking principally varieties of the Burbank group. 

 Jelly-end-infected tubers of the Netted Gem variety are shown in Plate 

 XXXV, figures i to 3. The jelly-end rot of the Burbank group differs 

 from the blackrot of round types of potatoes in that it is a softrot, light 

 to dark brown in color, while the blackrot is a comparatively dry rot, 

 black or nearly black in color. Jelly-end rot may be described as a soft, 

 wet rot of the tubers proceeding from the stem end downward through 

 the tuber attacking all tissues but apparently advancing somewhat more 

 rapidly through the vascular bundles. Examination of tubers infected 

 with jelly-end rot, however, often reveals no perceptible discoloration of 

 the vascular tissue below the line of the rot in the other tissues. As the 

 decay becomes older, the stem end becomes somewhat shriveled and 

 dried, often closely resembling the type of decay caused in storage by 

 F. trichothecioides Wollenw.^ Lenticel and eye infections are seldom 

 found in connection with the jelly-end rot of the Burbank group. 



Occasionally a softrot of the seed end is also found. A Netted Gem 

 tuber infected at both the seed end and the stem end is shown in Plate 

 XXXV, figure i. F. radicicola was isolated from both ends of this 

 tuber. There was apparently no infection in the vascular tissues con- 

 necting the tvv^o regions of decay. 



At first it was thought that the jelly-end rot of the Burbank group and 

 the blackrot of round types of potatoes were two distinct diseases, but 

 inoculations made in 191 4 into the stem ends of Netted Gem and Idaho 

 Rural tubers with F. radicicola led to the belief that they might be 

 caused by the same organism. Material collected in the field, whether 

 jelly-end rot or blackrot, when placed in a moist chamber for a few days 



1 Jamieson, Clara O. , and WoUenweber, H. W. An txtemal dry rot of potato tubers caused by Fusarium 

 trichothecioides, Wollenw. In Jour. Washington Acad. Sci., v. 2, no. 6, p. 146-152, illus. 1912. 



