Dec. io, 1913 
Foot-Rot of the Sweet Potato 
253 
CAUSE OF THE FOOT-ROT 
The organism causing the foot-rot of the sweet potato has been de¬ 
scribed as Plenodomus destruens . 1 2 It has also been pointed out that the 
fungus does not fit well into this genus or into any of the present-known 
genera. At the time, however, it was thought better to describe it as a 
new species of the genus Plenodomus rather than to create a new genus 
in a group where there are already a great many genera. It is probable 
that this organism is the conidial stage of an ascomycete which will 
eventually be discovered, and in view of that fact its generic position 
can only be temporary. It falls naturally in the order Sphaeropsidales 
and is more closely related to Phoma, Phomopsis, and Phyllostycta than 
to any of the other genera in the order. 
The diagnosis of the genus Plenodomus as found in Saccardo’s Sylloge 
Fungorum is somewhat brief. In 1911 Diedicke 3 * * worked over this genus, 
describing it more fully and pointing out the characteristics which distin¬ 
guish it from Phomopsis, the genus with which it is most likely to be con¬ 
fused. 
Since it is quite evident that the foot-rot fungus is not a Phoma, 
differing from that genus (1) in having more irregularly shaped pycnidia 
(PI. XXV, B) and (2) in having a well-defined beak (PI. XXV, A) y atten¬ 
tion will be given only to the characteristics which distinguish the foot-rot 
organism from Phomopsis, the conidial stage of the sweet-potato dry-rot. 
According to Diedicke, Plenodomus is characterized by having only 
two walls composing the pycnidium—a dark outer wall and a hyaline 
one within. The outer wall completely surrounds the pycnidium and is 
of uniform thickness at the top and base. The inner hyaline layer is 
composed of several layers of cells and is somewhat thicker than the 
outer wall. The conidiophores are short, fragile, and inconspicuous. 
The spores are rounded at both ends. 8 
On the other hand, the pycnidium of Phomopsis, according to the 
same author, is composed of four walls. The upper portion of the 
pycnidium, especially about the beak, is composed of thick black cells. 
The dark color of this layer of cells becomes less conspicuous in the 
lower portion and practically disappears at the base of the pycnidium. 
Phomopsis is further characterized by the development of a stroma and 
chambering of the pycnidium. The conidiophores are long, conspicu¬ 
ous, and awl-shaped, and the spores are spindle-shaped. Because of 
the variation in the shape of the spores this latter character is of less 
importance than some of the others in separating the genus from Pleno¬ 
domus. Stylospores are found in some species of Phomopsis. 
1 Harter, L. I*. Foot rot, a new disease of the sweet potato. Phytopathology, v. 3, no. 4, p. 243-245, 
2 fig., 1913. 
2 Diedicke, H. Die Gattung Plenodomus Preuss. Ann. Mycol., Jahrg. 9, No. 2, p. 137-141, pi. 8, 1911. 
3 This last character is perhaps of the least importance, since it is well known that the spores vary 
greatly within the genus and even in the same species. In fact, the spores of some species of Phomopsis 
have rounded ends. 
