634 



Journal of Agricultural Research 



Vol. VI, No. 17 



subcultures from five separate isolations of the fungus were used, and 14 

 potatoes were inoculated from subcultures from each isolation, 2 for each 

 of the constant-temperature chambers. They were maintained at con- 

 stant temperatures for one week and were then removed and examined. 

 The results of these experiments are shown in Table II. 



Table II. — Results of experiments in which inoculated potatoes were maintained at 

 constant temperatures, 10 Bur bank potatoes in each chamber, maintained at constant 

 temperature for one week. All sound potatoes were then placed in the 30 chamber for 

 three days 



Temperature. 



°c 



5 



10 



15 



20 



25 



3° 



35 



Number of 



tubers showing 



infection in 



one week. 



Number of tubers 

 which did not show 



evidences of in- 

 fection in chambers 

 originally used but 

 which were rotted 

 three days after 

 removal to 30 

 chamber. 



Total 

 number of 



rotted 

 potatoes. 



7 

 5 

 8 

 8 



9 

 10 

 10 



The results given in Table II show that a higher percentage of inocu- 

 lated potatoes are rotted at temperatures near the optimum for growth 

 of the fungus in artificial culture media than at the lower temperatures. 

 It is evident, however, that temperatures near this optimum are not 

 necessary for infection. As was to be expected, no rot was produced 

 while the inoculated potatoes were maintained at 5 C, but when these 

 potatoes were moved from this chamber to the incubator maintained at 

 30 , 70 per cent of them were rotted in three days. The growth of the 

 fungus is apparently inhibited at the low temperature, but begins as soon 

 as the temperature is raised. The lowest total amount of rot was in the 

 potatoes maintained at io° for the week. In this case 50 per cent of the 

 inoculated potatoes rotted. The growth of the fungus in the potato is 

 slower at the lower temperatures, io° and 15 , than at the higher tem- 

 peratures, as was found to be the case -with this fungus on artificial- 

 culture media. 



It is evident from these experiments in which P. debar yanum was iso- 

 lated from 49 diseased tubers, inoculations made from 30 of these isola- 

 tions into healthy tubers, the disease produced, and the fungus subse- 

 quently reisolated that this fungus is frequently present in potatoes 

 affected with leak and that when inoculated into the tubers, it causes 

 this rot. 



