May27, i9i8 PoX of SwCCt PotatO 439 



ECONOMIC IMPORTANCE 

 Pox is undoubtedl}' of great economic importance, but because of 

 inadequate diagnosis estimates of the money losses from the trouble can 

 not be given for all the States where sweet potatoes are grown. In a 

 recent bulletin by Harter (8) there is no mention of this disease. This 

 is certainly surprising when we consider the extent of the geographical 

 distribution and economic importance of the disease. It may be safely 

 stated that in fields where pox has become thoroughly established, the 

 yields may be reduced by about 50 to 80 per cent. The writer has had 

 occasion to make such estimates in many sweet-potato fields in Delaw^are, 

 New Jersey, Maryland, Virginia, and Texas. These observations 

 coincide with those of Halsted (5), who states — 



In some large fields visited this season [1890] the loss was almost total. 



SYMPTOMS 



In the literature pox, or soilrot, is poorly described, the symptoms of 

 the disease not being fully given. The writer's extended field observa- 

 tions on the symptoms of pox may be summarized as follows : In badly 

 affected fields the stand will be somewhat uneven. This, however, may 

 not always be the case. That which attracts the attention most is thin 

 growth, stunted vines, and a pale-green color of the foliage, all of which 

 gives the impression of a very impoverished soil. In fact, growers do not 

 attribute these conditions to the disease, but " to a lack of certain elements 

 in the soil which past sweet-potato crops have removed." Such claims 

 are unfounded, as these soils seem to produce good crops of com, water- 

 melons, etc. In pulling out a sweet-potato hill from a soil of this 

 character one will be surprised to find an almost total lack of 

 secondary feeding rootlets (PL 51, B). This is especially true when 

 the examination is made at the season of maximum growth. Many of 

 the' feeding rootlets will be found totally destroyed, while others will 

 exhibit numerous brownish spots at various intervals. Generally speak- 

 ing, if infection starts at the tip of a growng root, the disease will work 

 its way upward, and destroy that rootlet completely, leaving a discarded 

 stub, which resembles the infected roots of other crops subject to the 

 attacks of species of Thielavia. On the other hand, if infection takes 

 place laterally, the resulting spot mil be limited to about o. i inch. Fre- 

 quently such roots may exhibit from 5 to 10 spots, each separated from 

 the others by a healthy area (PI. 51, A). The color of the spot is a deep 

 chocolate-brown. Such infected rootlets, it is needless to say, become 

 functionless. Besides attacking the feeding rootlets the pox also attacks 

 the small roots which are destined to develop into edible roots (Pi. 51, A). 

 This infection may be as severe and of the same character as in the 

 feeding rootlets. Reduction in yield, lack of the normal green color, and 

 limited vine growth may therefore be directly attributed to the destruc- 

 tion by the disease of the feeding rootlets and young roots. 



