Dec. io, 1913 
Foot-Rot of the Sweet Potato 
271 
A comparison of these results with similar experiments carried on 
with the pycnidial stage of Diaporthe batatatis 1 shows that Plenodomus 
destruens is limited to a narrower range of temperatures in its growth in 
artificial cultures. The optimum temperature for growth of the dry-rot 
organism was 3 0 C. higher than that of the foot-rot fungus. At the 
lower temperatures the former made as good a growth at an average 
temperature of 7.5 0 C. as the latter did at an average temperature of 
12.6° C. At the higher temperature the foot-rot fungus was killed 
when exposed for 21 days at an average temperature of 37.3 0 C., while 
the dry-rot organism made some growth when exposed for 18 days at an 
average temperature of 37.8° C. 
influence; of light on the growth and production of pycnidia 
It was found that the conidial stage of the dry-rot fungus (Diaporthe 
batatatis) produced pycnidia only sparingly unless exposed to light. 1 
Contrary to these results, the foot-rot organism on rice cultures grew 
equally well in darkness and in the light. Pycnidia were formed in 
about 3 days, and the spores began exuding in small droplets in about 
10 days. 
DISSEMINATION OF THE DISEASE 
From what we already know of the foot-rot disease it is evident that 
there are several ways in which the organism may be carried from one 
place to another. In view of the fact that the pycnospores will live 
through the winter on the dead vines until as late as May 20, the plants 
in near-by hotbeds and even in the fields are liable to infection from this 
source. It has been shown that the organism causes a serious disease 
of the stem of the plant and grows from there to the roots, forming 
pycnidia on the surface. It is evident, therefore, that the use of such 
potatoes for seed might account for a large part of the infections. 
There is no way of determining to what extent insects, the wind, and 
such agencies are responsible for the distribution of the disease, but they, 
do doubt, play an important part. It is believed that this disease, like 
many others, is also distributed from one field to another on farm imple¬ 
ments, the hoofs of animals, or by means of stable manure, etc. It is 
a well-known fact that many farmers are careless about the disposition 
of diseased and decayed sweet potatoes. Without suspecting the risk 
they are taking, they often throw them on the manure pile or feed them 
to stock without cooking. In either case the organism, if present on the 
potatoes, might eventually be carried to the field. The wider distribu¬ 
tion of the disease—i. e., from one locality to another—must largely be 
accounted for by the exchange of seed potatoes and seed plants. 
1 Harter, L- L*» and Field, Ethel C. A dry rot of sweet potatoes caused by Diaporthe batatatis . U. S. 
Dept. Agr., Bur. Plant Indus., Bui. 281, 38 p., 4 pi., 1913- 
