174 Diseases of Truck Crops 



indicate that infection takes place from a bruise on 

 the epidermis and from there the fungus works slowly 

 inward. It is not uncommon, therefore, in slight 

 cases of infection, to find, upon making a cross section 

 of the root, a jet black ring ranging from one third 

 to one half an inch in diameter immediately under the 

 epidermis, the color grading off from dark to a light ash 

 as it nears the center of the root. At this stage the 

 infected sweet potato is watersoaked but quite solid. 

 A liquid, brownish in color, may be squeezed from 

 such roots. Charcoal rot is almost unknown in well 

 ventilated houses. There seems little doubt that 

 the fungus 5. bataticola is a common field saprophyte, 

 which may be brought into the storage house with 

 the sand or soil which clings to the roots. Diseased 

 roots kept dry for one year will readily yield a pure 

 culture of the fungus, thus showing that these roots 

 carry the fungus from year to year. 



The Organism. The sclerotia are jet black, very 

 minute, smooth, and made up exteriorly of anasto- 

 mosed black hyphas. The interior of the sclerotia is 

 light to dark brown, composed of thick walled cortical 

 hyphal cells (fig. 28 m). The sclerotia appear singly, 

 and oftentimes in long chains, and abound through- 

 out the entire affected root. 



COTTONY ROT 



Caused by Sclerotium Roljsii Sacc. 



Cottony rot is mostly a disease of the seed bed. 

 Infected sprouts suddenly wilt and topple over, giv- 



