18 

 of Aspergillus ochraceus Wilhelm. The antagonists were selected as 



described in Section I. 



Freshly fumigated soil was divided into five allotments of 20 kg 

 each (Table 2). The pathogen was added to three of the allotments at 

 1000 chlamydo spores per gram of air-dried soil. Of the three allotments 

 infested with the pathogen, one was placed in plastic containers to 

 inhibit recolonization by airborne microorganisms, another was left 

 uncovered in the greenhouse to allow natural recolonization to occur, 

 and the third was left uncovered in the greenhouse and infested with the 

 antagonists at 1000 conidia of each of the five fungi per gram of air- 

 dried soil. The conidia were obtained as described in Section I. The 

 two allotments without the pathogen were left uncovered in the green- 

 house and one was infested with the antagonists as above. The pathogen 

 was also added at the above concentration to a sixth allotment which 

 consisted of nonfumigated soil. The water content of the soils was 

 maintained at approximately 10$ by weight during the experiments. 



The competitive saprophytic and pathogenic abilities of the patho- 

 gen in the different soil communities were quantified by monitoring its 

 population density in the soils and by determining its ability to infect 

 susceptible tomato seedlings, respectively. Every 7 days 1500 g of soil 

 from each allotment were removed. The population of F. oxysporum f . sp. 

 radicis-lycopersici then was determined in those soils previously 

 infested with the pathogen. The pathogen was added to those soil 

 samples which were not previously infested with the pathogen to determine 

 the saprophytic and pathogenic activities of chlamydo spores of the 

 pathogen when introduced to previously recolonized soil. The population 

 of the pathogen in these soils was determined after 2 wk incubation in 



