14 BULLETIN 50, HAWAII EXPERIMENT STATION. 
discoloring the skin. A white-skinned variety when attacked 
presents a dark-gray appearance, while a red-skinned variety turns 
black. 
Foot Rot (Plenodomus destruens). 
Foot rot is indicated by rotting of the stem near the surface of the 
soil. The infected plant presents a rather wilted appearance. 
Blight or Wilt (Sclerotium rolfsii). 
Blight or wilt is indicated by decay of the plant at the base and by 
moldy white growth. 
Texas Root Rot {Ozonium omnivorum). 
Texas root rot is indicated by a spotting of the roots and a wilting 
of the vine. 
Pit or Pox (Cytospora batatas). 
Pit or pox, also known as soil rot, is indicated by malformation 
and girdling of the roots of the plants, accompanied by low yield. 
DISEASES ATTACKING THE CROP IN STORAGE. 
Soft Rot and Ring Rot (Rhizopus nigricans). 
Soft rot is indicated by decay of the root at one end, the diseased 
part becoming very soft. 
Ring rot is indicated by a softening of the roots between the ends. 
Dry Rot (Diaporthe batatatis). 
Dry rot is indicated by shriveling of the roots, unaccompanied by 
softness. 
Java Root Rot {Diplodia tubericolu). 
Java root rot is indicated by blackened flesh. 
LEAF DISEASES. 
Three minor diseases which affect the foliage of the plant are leaf- 
blight, giving the plant a withered appearance; leaf -spot, identified 
by minute white specks on the leaves; and white-rust, indicated by 
small white spots. Being only minor diseases, they require no treat- 
ment when occurring on a large area. Infected leaves should be 
removed from small areas, however, and burned. 
CONTROL. 
It is only by the most careful management that sweet-potato 
diseases can be eliminated from the field. The selection of prop- 
agating materials from absolutely sound plants, the careful examina- 
tion of the root crops and vines before moving from one field to 
another, and the practice of crop rotation are the only effective 
measures. 
Cuttings which are intended for propagation should be selected 
with care if they are taken from infected areas. They should be 
treated with a solution of bichlorid of mercury (1 ounce of bichlorid 
of mercury to 8 gallons of water) , being submerged in the solution for 
five minutes. Vines or roots showing signs of disease should be 
removed from the field and burned. If a locality continues to be 
