18 BULLETIN 45, HAWAII EXPERIMENT STATION. 
Tuber diseases — Continued. 
Tuber rots — Continued. 
Storage rots. Wet and dry rots of various appearances, often 
with inyeeliuni-lined cavities and frequently rather soft 
and watery -brownish. These are the Fusarium rots which 
follow wilt infection of plants and most commonly start 
at the stem end. Slight wounds also furnish entrance to 
the several wound-invading. Fusarium-rotting organisms. 
Fusarium rots (p. 28). 
Surface blemishes. 
Rough, corky scab spots scattered over surface or covering it 
entirely. 
Corky scab (Text figs. 1 and 2 and p. 26). 
Small black raised masses resembling dirt on skin ; not readily 
washed off but easily scraped off with the nail. Badly at- 
tacked tubers roughened and cracked open. A trouble 
occurring simultaneously with rosette and caused by the 
same organism. 
Black scurf or russet scab (PI. Till and p. 24 L 
Numerous small galls and pimples on the surface of the tuber. 
Within the galls are minute, pearly-white, glistening 
bodies. If the tuber be cut a row of these glistening 
bodies may be traced about one-eighth inch under the peel. 
Affected tubers rot quickly. 
Nematode or gallworm (PI. XIV and p. 33). 
PARASITIC DISEASES. 
Fusarium Wilt (Fusarium oxysporum). 
The wilt disease of Irish potatoes caused by the fungus Fusarium 
o.ryspot urn is prevalent in the principal sections of the islands where 
this crop is grown. Potatoes from California, both seed and table 
stock often used here for seed, are frequently found infected with 
this disease. It is the most serious and persistent disease with which 
the growers of that State have to contend, and it is highly probable 
that the disease was introduced into Hawaii on seed from California. 
The disease is characterized by a slight rolling and wilting of the 
leaves and premature drying of the plants. The lower leaves wilt, 
droop, and die first, and the color of the foliage in the early stages 
may be lighter than usual. As a rule the disease is not evident until 
the plants are about a foot or more high. The appearance of the 
plants is the same as that of plants suffering from lack of water, and 
this is really the case, even though there is plenty of water in the 
soiL Upon splitting, the woody portion of the underground stem 
is seen to be discolored, that is, more or less browned (PL I, fig. 2). 
The fungus Fusarium oxysporum enters the plant either from the 
seed or from the soil through the smaller roots and works its way 
up through the water-carrying vessels of the roots and stem, and 
often later through the stolons into the young developing tubers 
(PI. I, fig. 1). The fungus mechanically obstructs the water ducts 
