454 



aOVTHERN FIELD CROPS 



and, if necessary, to spray the vines with this or with Paris 



green. Cutworms may be poisoned before setting the 



slips, as suggested in paragraph 379. 



432. Fungous diseases. — The enlarged root of the 



sweet-potato is subject to various forms of decay, each 

 one due to a different germ or 

 disease-producing organism. The 

 most serious of these is the follow- 

 ing : — 



Black-rot (Sphceronema fimbria- 

 tum). — The presence of this fungus 

 within the potato root causes black 

 spots on the surface (Fig. 198). 

 These spots are shghtly depressed, 

 and the dark color extends deep 

 into the enlarged root, which com- 

 pletely decays in the field or during 

 storage. If diseased potatoes are 

 bedded, the slips are also diseased. 

 The remedies consist in (1) bedding 

 no tubers thus diseased; (2) de- 

 stroying any slips on the white 

 stems of which are found any dark 

 spots; and (3) rotation of crops, 



avoiding the planting of sweet-potatoes for two years in 



succession on the same land and avoiding any land where 



this disease has occurred in recent years. 



Fig. 198. — Plack-bot on 

 Root and Slip of Sweet- 

 potato. 



In addition to these measures, J. L. Winslow soaks the roots 

 for five minutes just before bedding, in a weak solution of for- 

 malin, using 1 ounce of this liquid to 8 gallons of water. He also 

 dips the slips into a slightly stronger solution of formalin. It is 



