512 



Journal of Agricultural Research 



Vol. XXII, No. 9 



Belmont, and Red Brazil. The parasitism of R. tritici was determined 

 for all the above-named varieties except the last four. 



The mechanical operations involved in carrying out these experiments 

 are identical with those previously employed ' and will not be discussed 

 here. The potatoes inoculated with R. nigricans and R. tritici were 

 incubated at temperatures of from 20° to 22° and 30° C, respectively, 

 these temperatures having been found in previous experiments to be 

 favorable for the growth of these two organisms. Records of the per- 

 centage of infection and of the progress of decay were made at the end 

 of 48 hours after inoculation and every day thereafter to the close of the 

 experiment. The experiments were allowed to run for from four to six 

 days. 



EXPERIMENTAL DATA 



The results of the inoculations of the different varieties of sweet pota- 

 toes with R. nigricans are shown in the following table. The figures in 

 columns i and 2 were obtained by taking an average of the results of the 

 several experiments. The data presented in the second column are based 

 on an estimation of the percentage of the total amount of decay when 

 the experiments were terminated. 



Table I. — Percentage of sweet potatoes infected and the estimated percentage of decay at 



the end of the experimeii t 



Variety, 



Porto Rico 



Big Stem Jersey. .. 



Triumph 



Pierson 



Gold Skin 



Little Stem Jersey 



Georgia 



Early Carolina. .. . 



Variety. 



Nancy Hall 



Florida 



Red Brazil 



Haiti 



Dahomey 



Southern Queen 

 Yellow Belmont 

 Dooley. 



The results show that a large percentage of the potatoes became infected 

 by the method employed. There were cases, especially among the more 

 resistant varieties, where only a very small amount of the tissue about 

 the well decayed. In view of this fact it was frequently difficult to decide 

 whether infection had actually taken place or whether the small amount 

 of decay was due to an enzym in the inoculum at the time the inocula- 

 tions were made. The writers finally decided to regard as infected all 

 potatoes which had been softened for i cm. or more beyond the margin 

 of the well. It is not unlikely that the percentage of infection of some 

 of the varieties, especially those which resisted further decay, was actu- 

 ally placed too high by this method. 



' H.^RTER, L. L., Weimer, J. L,., and Adams, J. M. R. sweet-pot.ito storage-rots. In Jour. Agr. 

 Research, v. 15, no. 6, p. 337-368, pi. 21-27. 1918. Literature cited, p. 366-368. 

 Barter, I,, h., Weimer. L., and Lauritzen ,J. I. op. cit. 



