23 

 The maximum number of fungal propagules (approximately 2 X 10-5 



propagules per gram of soil) in amended and nonamended soils was 

 similar when naturally occurring recolonization was allowed. The 

 population of F. oxysporum f. sp. radicis-lycopersici in fumigated soils 

 which had been infested with the pathogen and placed in plastic contain- 

 ers to inhibit naturally occurring recolonizers was higher than the 

 total fungal populations in any of the other treatments (Fig. 4). Under 

 these conditions the pathogen population increased to 10 propagules per 

 gram of soil 25 days after fumigation. By direct observation it was 

 determined that the pathogen population consisted predominantly of 

 microconidia. 



When the pathogen was added 4 days after fumigation (Table 2, 

 Treatment 3>4), its population density increased and then decreased with 

 time in nonamended soils (Fig. 4,5). The proportion of infected plants 

 was not related to the population density of the pathogen in nonamended 

 soils. When natural recolonization was inhibited, the pathogen popula- 

 tion increased and then decreased as the incidence of infection remained 

 relatively constant at 100% (Fig. 4). When natural recolonization of 

 fumigated soil was not inhibited, infection decreased with time and the 

 population of the pathogen increased and then decreased (Fig. 6). 



When the pathogen was introduced into soils that were either non- 

 fumigated or fumigated and amended with the antagonists (Table 2, Treat- 

 ment 5»6), the pathogen populations remained relatively stable (Fig. 6, 

 ?). In fumigated, amended soils the infection incidence increased to 

 44% 11 days after fumigation and then decreased to approximately 23% by 

 39 days after fumigation. In nonfumigated soils, the infection incidence 

 remained relatively stable, and varied from 40 to 50% (Fig. 7). 



