Family Convolvulaceae 169 



of the sweet potato root. These spots occur singly; 

 but usually several occur in a given area. When 

 very numerous, the spots coalesce, forming a large 

 blotch, which sometimes takes the form of a band, 

 or which may cover the entire root. Soil stain is 

 particularly conspicuous on the white-skinned varie- 

 ties, such as the Southern Queen. Here the color of 

 the spots is that of a deep-black clay loam. On the 

 darker-skinned varieties the color of the spots does 

 not appear so conspicuous. Soil stain is a disease of 

 the underground parts of the plant. The vines and 

 foliage are never attacked so long as they remain free 

 from the soil. But when they are covered, the peti- 

 oles as well as the stems become infected. 



After several months of storage, badly affected 

 roots become a deep brown, contrasting strongly with 

 non-infected potatoes. Occasionally, badly stained 

 roots seem to be subject to more rapid drying and 

 shrinking. This, however, is not often the rule. 

 Usually soil stain is prevalent in over-heated storage 

 houses. It may be, therefore, that the rapid shrink- 

 age is due to the overheating and not to the effect of 

 the disease itself. More data are necessary to deter- 

 mine this point. Soil stain is a disease not only of the 

 epidermis that considerably reduces the market value 

 of the 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 10 per cent, of 

 the crop from the rootlet infection. 



The type of soil seems to be a determining factor 



