Dec. io, 1913 
Foot-Rot of the Sweet Potato 
255 
On corn meal the dry-rot organism forms a black stroma composed of 
several pycnidia with long exserted beaks. The stroma is % to 1 or more 
mm. in diameter and is preceded by a profuse growth of mycelia. The 
foot-rot organism, on the other hand, forms no stroma on com meal. 
The pycnidia stand separately and are very numerous, while the mycelial 
growth is slight and inconspicuous. The pycnidia follow closely after the 
growth of hyphae, the pycnidial zone increasing with the increase in 
diameter of the mycelial growth. Spores are exuded in great quantities, 
forming a yellowish transparent liquid over the surface of the medium. 
ISOLATION OF THE FUNGUS 
Pure cultures of the foot-rot organism were particularly easy to secure 
by the poured-plate method. Stems on which the pycnidia were present 
were thoroughly washed in hydrant water or, preferably, disinfected with 
mercuric chlorid for about 40 seconds and then rinsed in sterile water. A 
few of the pycnidia were then macerated in a watch glass in sterile water 
and one or two loopfuls transferred to tubes of synthetic agar and plates 
poured. The fungus grows very slowly on agars, particularly on syn¬ 
thetic agar. The colonies are not visible in the plates for three days and 
often not until five or six days after they are made. Because of the char¬ 
acteristic growth on synthetic agar the organism can easily be picked out 
from other fungi when the appearance of the colony is once known. 
DESCRIPTION OF THE FUNGUS 
Mycelium. —The appearance of the mycelium varies so markedly on 
different culture media and according to the age of the culture that it 
would be difficult to give a simple, characteristic, general description. In 
young cultures and for the most part in old cultures it is nearly always 
hyaline, although occasionally browned hyphae may be found. Oil glob¬ 
ules are found in the mycelia at all ages (PI. XXV, C). Hyaline, spherical 
and oval, thick-walled bodies 8 to 13// in diameter, generally filled with oil 
globules, intercalated or, rarely, terminally, in chains or singly (PI. XXV, 
D ), occur in most media and at nearly all ages. Browned bodies morpho¬ 
logically similar to the hyaline ones but occurring mostly at the end of the 
hyphae (PI. XXV, E ) are frequently found in older cultures. In 7-months- 
old corn-meal cultures which were quite well dried out the brown bodies 
were abundant, especially where the media came in contact with the glass. 
In these cultures the hyaline forms were few. In 4-months-old cultures 
of string beans brown and hyaline bodies and brown hyphae were present. 
The brown hyphae were filled with numerous beadlike swellings. On the 
other hand, in a rice culture of the same age only hyaline hyphae and 
hyaline spherical or oval bodies were found. 
Pycnidia. —The pycnidia are at first buried, but later break through 
the epidermis, appearing as black dots scattered over the surface. They 
stand close together on the stem and roots, but they are not confluent 
