Family Convolvulaceae 153 



Symptoms. This disease is early marked by 

 small, dark, dry spots on the surface of the potato 

 (fig. 25 c). Later the infected portion in most cases 

 cracks, dries,' and falls out, leaving a pit with a newly 

 formed, roughened skin. It is an underground trou- 

 ble and is not detected in the field until late in the 

 season, at a time when healthy hills have formed well 

 developed vines. At this time the lack of an abun- 

 dant vine growth is a characteristic symptom, and the 

 rather meager stem development gives the impression 

 that the soil is very poor and exhausted in plant food. 

 The symptom appears when the sweet potato is begin- 

 ning to form. At this stage the spotted portion 

 ceases to grow, while the healthy portion on each side 

 continues to develop. Frequently roots that are 

 very badly spotted cease to grow altogether, and if 

 they are stored, the spots usually dry and fall out. 

 Unlike black rot, roots affected with soil rot do not 

 lose their food value, as the disease is only skin deep 

 and imparts no bad taste to the potato. The disease 

 also attacks the young rootlets of the plant, and when 

 this is the case there is practically no crop formed. 

 This, however, happens only in badly diseased fields, 

 and especially where lime is used. The latter should 

 never be applied to lands infected with the pox organ- 

 ism. The trouble, unlike many others, seems to be 

 worse in dry weather. 



Besides affecting the sweet potato, pox also causes 

 circular shallow pits on the white potato, and on 

 turnips, the pits on the latter being even more shal- 

 low than on the former. It is also suspected of at- 



