354 Journal of Agricultural Research voi. xv. no. e 



MUCOR RACEMOSUS 



In the course of the sweet-potato storage investigations potatoes have 

 been exposed for varying lengths of time to temperature a little above 

 freezing. After a few weeks at a temperature of 2.0° to 4.5° C. they 

 were usually decayed, and in these rotted potatoes two or three fungi 

 always predominated. One of the most common forms was Mucor 

 racemosus Fes. In one experiment four sacks of about X bushel each 

 were stored for two months at 2.0° C, and all the potatoes were decayed 

 with M. racemosus. A large number of potatoes have been inoculated 

 at different times with pure cultures of M. racemosus and stored in the 

 usual way in a sweet-potato storage house, but no infections resulted. 

 However, in other experiments in which the potatoes were inoculated 

 and then stored at a temperature of about 2° C. rot was produced and 

 M. racemosus was isolated in pure culture after two or three months. In 

 one such experiment 100 potatoes were inoculated and M. racemosus 

 was recovered in pure culture from 82 per cent. The remaining 18 per 

 cent were infected with an undetermined species of Mucor and five 

 unknown fungi. That low temperatures are necessary was further 

 proved by a number of experiments in which the potatoes were inoculated 

 with pure cultures of M. racemosus, using the "well" method previously 

 described, and confined in moist chambers. No decay resulted when 

 these potatoes were exposed to the temperatures of the laboratory room 

 or of an ice box (io°-i4° C). If, on the other hand, they were exposed 

 to low temperatures, they were rotted readily, as shown by the following 

 experiments. 



In all these experiments the fungus was grown for 24 hours in sweet- 

 potato decoction, and both the growth and the decoction poured into a 

 "well," after which they were wrapped in filter paper and then in oiled 

 paper. This served both to retain moisture and to prevent contami- 

 nation. Before they were inoculated the potatoes were thoroughly 

 washed and then disinfected with mercuric chlorid. After inoculation 

 the potatoes were divided into several lots and placed in different cham- 

 bers of the Altmann thermostat. In these and subsequent experiments 

 only the average temperature covering the period of the experiment in 

 the Altmann thermostat is given. The maximum and minimum varied 

 I to 2 degrees above and below the average given. In one experiment 

 only two chambers were used — viz, chamber i (average temperature 

 0.43° C.) and chamber 3 (7.2° C). In three weeks 100 per cent of 

 the potatoes in both chambers were from half to completely decayed. 

 The experiment was repeated, using chambers i (0.67° C), 2 (4.37° C), 

 4 (8.1° C), 5 (11.6° C), 7 (14.7° C), 10 (22.8° C). In chambers i, 2, 

 and 4 all the potatoes rotted. The above experiment, in which 10 

 potatoes after inoculation were placed in chambers 2 (5.34° C), 3 (7-81° 

 C), 4 (8.7° C), 5 (ii-o^ C), 7 (15.8° C), 9 (17.5° C), laboratory room 



