996 Journal of Agricultural Research voi. v, no. « 
SYMPTOMS OF SOILSTAIN 
Soilstain is characterized at first by small, circular, deep-clay-colored 
spots on the surface of the sweet-potato root. These spots occur singly, 
but usually there are several in a given area. When very numerous, the 
spots coalesce, forming a large blotch which sometimes takes the form 
of a band or may cover the entire root. Soilstain is particularly con¬ 
spicuous on the white-skinned varieties, such as the Southern Queen. 
Here the color of the spots is that of a deep-black day loam. On the 
darker-skinned varieties the color of the spots does not appear so con¬ 
spicuous. Soilstain is a disease of the underground parts of the plant. 
The vine and foliage are never attacked as long as they remain free from 
the soil. However, when these are covered, the petioles as wdl as the 
stems become infected. 
EFFECT OF THE DISEASE ON THE HOST 
After several months of storage, badly affected roots become a deep 
brown, which greatly contrasts with noninfected sweet potatoes. Occa¬ 
sionally, badly stained roots seem to be subject to more rapid drying and 
shrinking. This, however, is not often the rule. Usually soilstain is 
very prevalent in overheated storage houses. It may be, therefore, that 
the rapid shrinkage is due to the overheating and not to the effect of the 
disease itself. More data are necessary to determine these points. Soil¬ 
stain is not only a disease of the epidermis (PI. LXXVII, fig. a) and as 
such considerably reduces the market value of mature roots, but it also 
attacks the very young rootlets, preventing their further development 
and indirectly reducing the yield. In badly affected fields the writer has 
estimated a loss of io per cent of the crop from rootlet infection. 
FACTORS FAVORABLE TO SOILSTAIN DEVELOPMENT 
The type of soil seems to be a determining factor in the development 
of soilstain. Sweet potatoes grown on very light sandy soils, especially 
those which are hilly, are usually free from the disease. The heavier 
lands, or those rich in humus, rarely produce a clean crop. The appli¬ 
cation of manure favors the spread of the fungus and increases the stain. 
In fact, the manure itself is often a carrier of the disease, since diseased 
roots of all sorts find their way ultimately to the manure pile. The trouble 
is also carried directly with the seed stock. These, when planted in the 
seed bed, will produce ioo per cent of diseased sprouts. Experimental 
data, as well as extensive observations in seed beds and in the field, all 
corroborate these statements. Wet weather is favorable to the spread 
and increase of stain. During wet seasons the disease is more plentiful 
than in dry seasons. 
