632 Journal of Agricultural Research vol. vi, no. n 



peratures below 5 , though growth was inhibited. The cultures from this 

 chamber grew readily when placed in the incubator maintained at 30 . 

 The experiments show that the range of temperature for growth is wide, 

 about 30 , and that the optimum is high. Johnson (9) found the 

 optimum temperature for growth of P. debaryanum to be 33 . 



INOCULATION EXPERIMENTS 



Inoculations were made into healthy California-grown Burbank pota- 

 toes from 30 of the 49 isolations of P. debaryanum obtained from diseased 

 potatoes. Tubers were rotted and the fungus reisolated in all cases. 

 Inoculations were also made with the bacterium which was sometimes 

 obtained from the rotten tubers with no apparent effect. It seemed to 

 be present as a saprophyte. 



In the earlier inoculation experiments with P. debaryanum, the sterile 

 tubers were inoculated with the fungus in wounds made with a flamed 

 knife as in some of the experiments with Rhizopus nigricans. The inocu- 

 lated tubers were then placed in moist chambers. Inasmuch as moist 

 chambers, because of their limited volume of oxygen and their high 

 humidity furnish rather abnormal conditions for the storing of potatoes, 

 another method was developed in which the potatoes were maintained 

 after inoculation under conditions which more nearly approached those 

 found in storage. According to this method, the potatoes were disin- 

 fected as before and a small hole made in one side with a sterile knife. 

 A ring, usually the ring of a Van Tieghem cell, was placed over the open- 

 ing and cemented to the potato with petrolatum. A small quantity of 

 sterile water was poured into the hole in the tuber and the inoculation 

 made by placing some of the mycelium of the fungus in the water. A 

 cover glass was then sealed on top of the cell with petrolatum. Various 

 modifications of this method were tried to determine the size and depth 

 of the wound necessary to insure a high percentage of successful inocula- 

 tions. It was found that if the skin was removed from a small area of 

 the potato which came within the ring when it was cemented in position 

 and the inoculation made in a drop of sterile water on this wounded area, 

 the results were as good as when deep wounds were made. Further 

 experiments showed that it was sufficient to make a rather deep incision 

 in the tuber with a sterile knife and introduce some mycelium to inocu- 

 late the potato successfully. The rots produced by such inoculations, 

 however, became contaminated more frequently with bacteria than when 

 the raw surface of the tuber was inclosed with a ring and cover glass. 

 Numerous controls were prepared by cementing the ring to the unbroken 

 surface of the tuber and placing therein some bits of mycelium in sterile 

 water ; also by pouring sterile water into wounds in the potatoes and seal- 

 ing them as in the inoculation experiments. In none of these controls 

 was there any infection. 



