12 



CIRCULAR 7 6 5, U. S. DEPARTMENT OF AGRICULTURE 



relatively short exposures. With the wide range of ozone concentra- 

 tions used, considerable variation was obtained in the rates of steriliza- 

 tion. The immediate effectiveness of ozone as a fungicide appeared to 

 be closely related to the concentration used. The high concentrations 

 shown in table 4 and in experiment 1 (table 5) killed spores more rap- 



Table 4. — Effect of intermittent high concentrations of ozone in the 

 air on the spore population of Penicillium expansum as indicated 

 by the colonies developing on nutrient-agar plates exposed for 30 

 minutes 



[Ozonizer operated daily from 8 : 30 a. m. to 4 p. m. ; 1,184 colonies developed per 

 plate exposed to determine concentration of spores before ozonization (last 

 column of table based on this value) ] 



Ozonization period 



Ozone concen- 

 tration at 

 time of 

 exposure 



Relative 



colony 



development 



hour _ 



P. p. m. 





 33.9 





 50.4 

 50.4 







100 



2% hours __ __ _____ 



77 



19 hours l _ _ _ __ 



34 



24 hours _ _ __ 



10 



26 hours _ _ _ _ 



6 



42 hours l 



2 







Plate exposure after ozonizer had been idle 16 hours. 



Table 5. — Effect of continuous high concentrations of ozone in the air 

 on the spore population of Penicillium expansum as indicated by 

 the colonies developing on nutrient-agar plates exposed for 30 

 minutes 



[Ozonizer operated continuously for duration of experiments; 363 colonies in 

 experiment 1 and 498 in experiment 2 developed per plate exposed to determine 

 concentration of spores before ozonization (last column of table based on these 

 values) ] 



Experiment and ozonization period 



Experiment 1 



hour 



5 hours 



7}i hours _ _ 



19 hours 



43 hours *__ 



Experiment 2: 



hour 



48 hours 



72 hours___ 



89 hours 



96 hours 



Ozone con 



- 



centration 



at 



time of 





exposure 





P. p. m. 











28. 







43. 



6 



21. 



9 















20. 



7 



30. 



2 



20. 



1 



20. 



8 



Relative 



colony 



development 



00 

 5. 7 

 3.6 

 7.9 

 7. 6 



100 



37. 



4. 



7.4 

 1. 2 



Plate exposure after ozonizer had been idle 8 hours. 



