2 
could be gathered in regard to the loss from this disease in distinction 
from the many other rots, 25 per cent is a low estimate for such sea¬ 
sons as that just passed. The rot in question, which, as is usual with 
fungous diseases, has been ascribed to wet weather, is caused by an inter¬ 
esting species of fungus; but until the authors took the matter into 
consideration little was known as to the real cause of the malady, and 
the fact of its fungous origin, while surmised, was not fully established. 
EXTERNAL APPEARANCE. 
The most conspicuous sign of the disease, and the one which distin¬ 
guishes it from other diseases, occurs upon the potatoes themselves. 
It consists in the presence of dark, somewhat greenish spots, varying 
from a quarter of an inch to 4 inches in diameter, sometimes cover¬ 
ing the greater part of the root and extending some distance into the 
tissue. These spots when once seen can not be mistaken, as they are 
simply sunken areas with distinct margins, like spots burned into the 
potato with a metal dye which has left the skin uninjured. Should 
the slightest doubt as to the identity of the disease remain after a su¬ 
perficial examination, the removal of a small portion of the skin expos¬ 
ing the olive-green tissue below would dispel it. Among the sprouts, 
or young plants grown in hotbeds, the disease manifests itself in dark 
lines upon the lower portion of the shoot and sometimes of the lower 
leaves, giving rise to the name of “ black shank ” among the growers. 
These dark lines or blotches often appear upon etiolated portions of 
the stem and are almost black in color. In very severe cases the tip of 
the sprout wilts and dies. No appearance in the field has so far been 
observed that would distinguish hills diseased with black rot from 
those attacked by some other of the numerous rots; but the dark 
sunken areas on the potato and the black discolorations of the sprouts 
can scarcely be confused with any other sweet-potato disease. 
CHARACTERISTICS OF THE DISEASE. 
A microscopic examination of the discolored tissue in the root re¬ 
veals the fact that the starch-bearing cells have been greatly altered, 
the healthy portions are characterized by thin-walled cells completely 
packed with starch, but these have been replaced in the diseased areas 
by thick-walled, olive-brown ones totally deprived of starch. The thick¬ 
ening of the walls, seen so strikingly upon examination, is due in part 
to granular incrustations which often assume the form of rounded pro¬ 
tuberances. Whether this incrusting matter is anything more than 
the remains of the protoplasmic contents of the cell, was not deter¬ 
mined, but from its irregular appearance this was the natural inference. 
Filling the intercellular spaces and often ramifying through the ad¬ 
joining cells, are the thick-walled, olive-brown hyphrn of the parasite, 
and the dark color of the diseased portions is due in a measure to them. 
