3o8 Journal of Agricultural Research voi. vi, no. 9 



The vascular infection present in plots 1 and 2 was all of the heavy 

 black type demonstrated to be caused by F. radicicola. Numerous 

 cultures from the vascular systems of tubers from these plots gave 

 the fungus. The percentages of rot include all phases of blackrot, in- 

 cluding stem-end, lenticel, and eye infections. Strangely enough, no 

 tuber-rots developed in plot 3. Of the tubers from plot 3, 16 per cent 

 showed vascular infection, of which 14 per cent were of the type 

 usually ascribed to F. oxysporum and 2 per cent were of the black 

 type caused by F. radicicola. Cultures made from the vascular sys- 

 tems of infected tubers in this plot give F. oxysporum in all cases 

 of light-brown discoloration and F. radicicola in all cases of black 

 vascular discoloration. In the check plots, i per cent of blackrot ap- 

 peared in plot 5. The others were free from all tuber- rots. The vas- 

 cular infection present in the check plots was for the most part of 

 the type ascribed to F. oxysporum. A few tubers showing blackened 

 vascular bundles were found, and F. radicicola was isolated from such 

 tissues whenever the attempt was made. 



The results clearly show that seed infected with blackrot will produce 

 infection in the resulting product. From the fact that no jelly-end rot 

 resulted from planting jelly-end-infected seed, the conclusion should not 

 be drawn that such seed can not cause infection in the resulting product, 

 but rather that it requires conditions for its development different from 

 those required for the development of blackrot. 



CONTROL OF BLACKROT 



Absolute control of blackrot will be difficult. When potatoes are 

 planted on alfalfa or grain lands blackrot is rarely found if the crop has 

 had sufficient water to make good growth conditions possible. Plantings 

 of disease-free seed potatoes on raw desert lands in 191 5 gave as high as 

 II per cent of tubers infected with blackrot in the harvested product, 

 whereas plantings of disease-free tubers on alfalfa or grain lands were 

 usually free from the disease, although as high as 5 per cent of infected 

 potatoes were found in the harvested product of one plot on alfalfa 

 land. Judging from the results of three years' observations in commer- 

 cial fields, it is apparent that losses from blackrot can be reduced to a 

 minimum by planting only on land which has been in cultivation for a 

 number of years and by giving the growing crop the proper amount of 

 water, care, and attention. The crop should be kept in a good growing 

 condition until maturity or frost. Jelly-end rot, on the other hand, has 

 been found in fields where all the conditions of growth were apparently 

 ideal. Some adverse condition, however, is probably responsible for its 

 development. Further research upon jelly-end rot and its cause and 

 occurrence is highly desirable. 



Both jelly-end rot and blackrot-infected tubers may be stored with 

 safety, provided the storage cellar is fairly well ventilated and the tem- 

 perature kept below 50° F, 



