June i8 r 19x7 
Potato Disease Produced by Rhizoctonia 
423 
starch grains all remain in position, forming a dry '‘plug'/ and suggesting 
the name “dry core.” 
The second phase of this type of injury is most often found in the 
older stages in the progress of the disease or where the infected area 
reaches a diameter greater than 3 mm. This stage is most likely to be 
found when the potato tubers have reached maturity and the disease 
has run its course, or after the tubers have been stored. Owing to a 
drying out and shrinkage of tissues, a pit or canal is formed in the center 
of the affected area. Doubtless in harvesting and storing the tubers 
this pitting is furthered by parts of this granular material being shaken 
out or loosened. There is no doubt that the greater part of this pitting, 
particularly in cases where the disease has penetrated deeply into the 
flesh, has been attributed in Maine to the work of the wireworm. How¬ 
ever, close observation even by the layman will readily show a great 
difference between these two types of injury. 
The diseased areas are approximately circular in outline and at the 
surface vary in size from that of a lenticel to 6 or 7 mm. in diameter. 
They usually extend into the flesh of the tuber to a depth equal to or 
somewhat greater than the diameter. The dry core or pit thus formed 
gradually tapers off, forming a somewhat rounded end, very seldom 
becoming pointed. The majority of these pits are proportioned and 
shaped quite like a thimble, but some of them have a tendency to become 
longer and more slender. In such cases a dry, roughly cylindrical core 
may penetrate the flesh of the tuber for some distance. A casual 
observer might readily attribute this form of injury to insect attacks. In 
a few cases the pit has taken a more or less horizontal direction in refer¬ 
ence to the surface of the tuber, and, as shown in Plate 27, F, two of 
these pits have joined together some little distance below the surface of 
the tuber. 
Infection takes place in lenticels. Even from the very earliest stages 
the infected areas become slightly darker and sunken from the surround¬ 
ing tissues. The mycelium of the fungus seems to travel equally in all 
directions; thus, as time passes, the infected area becomes a larger and 
larger circle dark brown in color. Surrounding the mature pit there 
is a very definite line of demarcation separating the diseased tissue from 
the healthy. In fact, this line is so definitely laid down that one has 
little or no difficulty in inserting the point of a knife and lifting out the 
whole core, leaving a clear-cut cavity in the healthy flesh of the tuber. 
By boiling tubers affected in this manner, the majority of the cores will 
come out clean with the peeling when it is removed. Plate 28, E-I, 
shows a group of these cores that came out with the peeling after the 
tuber has been boiled for 20 minutes. It will be seen (PI. 29, A) that 
this division line is formed by three or four layers of compact suberized 
cells that have been laid down by the potato to prevent further penetra¬ 
tion of its tissues by the fungal hyphae. This leather-like lining of the 
