442 Journal of Agricultural Research voi. xiu. No. 9 



and would seem to indicate that the active plasmodium is necessary for 

 infection. This had already been shown by Elliott (4), who, however, 

 also had claimed that infection of the growing tips of the tender rootlets 

 takes place by means of amebae. 



METHODS OF SPREAD 



It has already been stated that in dry weather the pox spot seems to 

 be deeper. It is the general opinion of Halsted (5), Townsend (14), 

 and Duggar (2), that pox is worse during dry weather. This in reality 

 is true only in so far as the pox spots are deeper, and cause much more 

 visible damage by distortion and disfigurement of the marketable roots. 

 In wet weather pox is just as severe, and the causal organism perhaps 

 more active, but the spots are more shallow and less noticeable. The roots 

 are not so disfigured, and, hence, are more saleable. According to Halsted 

 (5) and others, pox is spread about from field to field by wind-blown spores 

 of the causal organism. The work of the writer does not seem to bear 

 this out. Extended observations and studies have shown that pox does 

 not spread readily from field to field nor even to adjoining neighboring 

 fields by means of wdnd-blown spores. If it did, the disease would spread 

 very rapidly over large areas. This, however, is not the case. Pox will 

 not become very noticeable until 8 or 10 years after its introduction in 

 a field, and then only when the crop is continually grown on the same 

 land. The disease does not seem to spread rapidly from an infected 

 field to the neighboring fields, but advances slowly, unnoticed and 

 unsuspected. Definite evidence is also lacking as to whether pox is 

 carried over on the small potatoes (seeds) in storage. As a rule, sweet- 

 potato growers never hesitate to plant infected stored seed. According 

 to the writer's observation, and numerous communications from growers, 

 sprouts from such seed, when pulled for transplanting, have not shown 

 evidences of pox on any of the underground portions of the plant. This 

 evidence is further strengthened by Duggar (2), who states that — 



Soil rot has not been observed to spread by way of the hot-bed, but only through 

 contamination of the soil of the field. 



In order to verify this, the writer has often planted infected seed which 

 wintered over in storage in a sterilized soil. These seeds were surface- 

 sterilized in the usual way. At no time, however, did the resulting 

 sprouts show any marks of pox. The mode of spread of pox needs further 

 careful investigation. It seems reasonable to suppose, however, that the 

 disease is probably disseminated with lumps of soil which have been 

 carried on farm implements on wet days. At that time the sandy soil 

 is more likely to stick together. It is also likely to spread with diseased 

 sprouts which had been grown in a seed bed the soil of which has been 

 taken from a previously infected field. Washing by rain is also likely 

 to carry the disease in the field. In lands with a natural slope pox 



