440 Journal of Agricultural Research voi.xiii. no. 9 



Infection on the older growing roots may result in a constriction. 

 Growth may seem to cease at this point, although it is uninterrupted 

 on either side (Pi. 52). Infection on the older roots, besides mis- 

 shaping them, does not, however, result in a total loss. Such roots 

 usually attain a fair marketable size, and do not suffer in the least in 

 edible quality. Here, however, the disease is manifested differently, 

 from that on the young rootlets. On the older roots infection may be of 

 two types. 



The normal and typical one is characterized by small, dry, darkish, 

 circular, more or less superficial spots the size of a dime or less. Later, 

 the tissue of the spot in most cases dries up cracks (PI. 52, A), and falls 

 out, leaving a pox, or pit, whence the name of the disease. As a rule, a 

 new skin is formed immediately below the area of the fallen spot. The 

 depth of the spot seems to vary with the w^eather conditions; in dry 

 weather the pox spots seem to enter more deeply into the tissue than during 

 wet spells. In light cases of infection, there may be but one to three spots 

 on the potato. In severe cases, however, the spots may be so numerous as 

 to coalesce. These, on dropping out, leave a large, ragged, irregular pit. 

 (PI. 52, B). The tissue of the pox spot is dry and leathery, but is readily 

 pulverized when rubbed between the fingers. 



The second form of infection on the older potatoes is what ElHott (4) 

 terms "blister" infection. It was observed by the writer but once; and 

 in that case the infection took place at a feeding rootlet, then worked 

 down to the main root, and was later apparent as a blister-like elevation 

 on the epidermis. 



CAUSE AND PATHOGENICITY 



It has already been pointed out by the writer (/j) that Halsted's fun- 

 gus Acrocystis hatatis, is not the cause of pox, although species of Rhi- 

 zoctonia are frequently isolated from pox spots. The fungus, after 

 repeated inoculation, failed to produce the typical disease. It is to the 

 credit of Elliott (4) to have proved that pox is caused by a myxomycete 

 Cystospora batata Elliott. 



The pathogenicity of the organism may be readily proved in the fol- 

 lowing manner: Mature roots with unbroken pox spots may be easily 

 secured. The roots are then carefully washed in running tap water to 

 remove all trace of soil particles. They are then placed for tv/o minutes 

 in a mercuric-chlorid-alcohol solution,^ and rinsed several times in steri- 

 lized water to remove all trace of the bichlorid. The roots are then placed 

 in the dark in a sterilized moist chamber, heavily lined wdth clean, wet 

 filter paper. In about 48 hours or more, a dark slimy mass will emerge 

 from the now split pox spot. Upon transferring this slime to healthy 

 sweet potatoes, which have been carefully disinfected and kept in a moist 

 chamber, the typical disease may be reproduced in about three to five 

 days. 



' Made up of equa( parts of mercuric chlorid Cii.ooo) and 50 per cent alcohol. 



