38 
BULLETIN 45, HAWAII EXPERIMENT STATION. 
spots are as a rule smaller than those of the corky or common scab, 
are more regularly rounded, and are characteristically bordered by 
the uplifted and torn skin. A canker stage also occurs, and it is 
this stage that causes the greatest loss. This scab may open the 
way to various storage rots. 
Control. — Since this disease occurs in the coast States, importa- 
tions of potatoes from these should be carefully inspected for signs of 
this disease, and all mainland po- 
tatoes to be used for seed should be 
disinfected with corrosive subli- 
mate solution as a precautionary 
measure. This solution is known 
to be only partially effective in 
destroying powdery scab spores 
carried by the seed, but it is better 
than formalin. Lots of potatoes 
containing suspicious tubers should 
not be planted, and specimens 
should be forwarded to the station 
for examination. 
Blackleg (Bacillus phytophthorus) . 
Blackleg is a serious bacterial 
disease of the potato occurring in 
the New England States, also in 
Washington and Oregon, and pos- 
sibly in California, but not as yet 
observed in the Hawaiian Islands. 
This is a disease of the stem and 
tuber, and, as its name suggests, the 
lower part of the stem of affected 
plants becomes blackened. The 
tubers themselves are destroyed by 
a soft rot. In the advanced stages 
the stem shows a black color which 
extends from the seed piece to 
some distance above the ground 
(fig. 7). In the tubers a rot often extends from the stolon, or else the 
tubers may simply remain undeveloped. The use of infected seed 
results in a poor stand, and the plants become diseased. 
In Europe the losses are reported at from 5 to 94 per cent, and in 
Maine the most common losses range from 1 to 5 per cent with occa- 
sional losses of 50 per cent. The damage is heaviest on low, wet, 
and poorly drained soils. 
Fig. 7. — A potato plant affected with 
blackleg in the summer stage, the result 
of planting diseased tubers. The lower 
part of the stem is shriveled and black, 
the leaves are yellow and rolled up- 
ward. (From L\ S. Dept. Agr., Farm- 
ers' Bui. 544.) 
