514 



Journal of Agricultural Research 



Vol. XXII, No. 9 



cubated at 30° C, a temperature favorable for R. tritici, and the other 

 lot at 20° to 22°, a temperature favorable for the growth of R. nigricans. 

 None of the potatoes inoculated with R. nigricans and incubated at 30"^ 

 became infected, showing that this temperature is unfavorable to in- 

 fection by this fungus. On the other hand, all those inoculated with 

 R. tritici became infected and were completely decayed in three or four 

 days. A direct comparison of these two organisms at 30° was therefore 

 impossible. The percentage of infection as well as the total amount of 

 decay of all three of these varieties by both organisms when incubated 

 at a temperature of 20° to 22° for seven days is shown in Table II. 



Table II. — Percenlaije of infection and estimated percentage of decay of three -varieties of 

 sweet potatoes by R. nigricans and R. tritici after seven days at 20° to 22° C. 



An examination of this table shows that at this temperature the Little 

 Stem Jersey variety was decayed much more readily than either Nancy 

 Hall or Southern Queen, by both organisms. However, decay was much 

 more rapid by R. tritici than by R. nigricans, being completed by the 

 former in three to four days and not quite completed by the latter in 

 seven days. The potatoes of the Nancy Hall and Southern Queen varie- 

 ties were 100 and 40 per cent and 85 and 15 per cent decayed by R. tritici 

 and R. nigricans, respectively, at the end of seven days. A comparison of 

 the three varieties shows that the estimated amount of decay caused by 

 R. nigricans at the end of the experiment was about 15, 40, and 80 per 

 cent for Southern Queen, Nancy Hall, and Little Stem Jersey, respec- 

 tively. The total amount of decay caused by R. tritici, on the other hand, 

 was 85, 100, and 100 per cent, respectively. The results therefore show 

 that the Little Stem Jersey is quite susceptible to decay by both species, 

 while Nancy Hall and Southern Queen are very resistant to decay by 

 R. nigricans only. The percentage of infection of the Nancy Hall potatoes 

 was 95 and 80 by R. tritici and R. nigricans, respectively, while the 

 Southern Queen potatoes were 100 and 70 per cent infected by the same 

 organisms. On the other hand, Little vStem Jersey potatoes were 100 per 

 cent infected by both species. 



These results indicate what occurs at temperatures of 20° and 30° C. 

 under the conditions existing in these particular experiments. A tem- 

 perature of 30° proved to be too high for R. nigricans and 20° to 22°, while 

 not optimum for either organism, is probably more favorable for decay 

 by R. tritici than by R. nigricans. On the other hand, if a much lower 

 temperature had been employed it would have been more favorable to 

 R. nigricans but less favorable to R. tritici. The results, however, do 

 show a considerable difference in the three varieties with respect to their 

 relative susceptibility to decay by the two species of Rhizopus at the 

 temperatures tried. 



