304 Journal of Agricultural Research voi. vi, no. 9 



stolon. The rot in this case was nearly black in color, soft, and resembled 

 the earlier stages of the jelly-end rot often found in commercial fields. 

 Vascular infection also developed in this tuber. The fungus was recov- 

 ered from all infected tissues whenever the attempt was made. None 

 of the checks were infected. Infection resulted in all cases where tubers 

 were punctured with a needle carrying the spores of the fungus. None 

 of the checks were infected. In the case of the checks the punctures 

 could be seen easily but were healed over in each case. The inocula- 

 tions made into the stems of potato plants failed to give very decisive 

 results. In each case a blackening of the tissue adjacent to the puncture 

 was observ^ed. This blackening extended up and down from the point 

 of puncture for from one-eighth to one-half an inch and in most cases 

 also extended into the pith. 



BIvACKROT 



The infections, whether at the stem end, at the lenticels, or at the 

 eyes, produced by the artificial inoculation of Idaho Rural tubers with 

 F. radicicola, could not be distinguished in any way from the infections 

 on decayed tubers collected in the commercial fields. The infections 

 resulting from the inoculation of growing tubers in the station plots 

 when final examination was made were not as deep or as far advanced 

 as many infections occurring naturally in the field, but this can easily 

 be explained by the late date at which the inoculations were made. In 

 fact, at the time the inoculations were made, tubers with well-advanced 

 decay were being found in commercial fields. On the other hand, tubers 

 with decay no farther advanced than that resulting from the inocula- 

 tions have often been found in the field late in the season. In every 

 case where an attempt was made, the fungus was recovered. 



Tubers infected by inoculation in the field, by spraying with the spore 

 suspension, by the puncture of the tuber with an inoculating needle, 

 and by puncture of the tuber stolons, were placed in moist chambers, 

 and in each case, after a few days, tufts of F. radicicola appeared. 

 Blackrot-infected tubers in commercial fields, after being kept in a moist 

 chamber from 3 to 10 days at temperatures ranging from 65° to 75° F., 

 invariably threw out tufts of this fungus (PI. XXXV, fig. 4, 5). Isolations 

 made from the cortical and medullary tissues of blackrot-infected tubers 

 have never yielded any fungus other than F. radicicola, which could be 

 considered as the cause of the disease. Isolations made from stem-end 

 blackrot-infected Idaho Rurals, Pearls, and other round types of potatoes 

 have occasionally yielded F. oxysporum, especially when the culture was 

 made from or near the vascular tissue. The failure to obtain F. oxyspo- 

 rum from lenticel and eye infections of tubers collected in commercial 

 fields leads the writer to conclude that when F. oxysporum is found in 

 stem-end infections it probably entered as a vascular parasite, independ- 



