32 



Discussion 



Decreases in the saprophytic activity of F. oxysporum f . sp. 

 radicis-lycopersici due to competition from naturally recolonized and 

 artificially amended soils similar to those observed in this study have 

 been reported by other investigators (13.27); however, the compositions 

 of the communities have not been examined. In this study, the fungal 

 recolonization of soils was monitored by soil dilution plating, and the 

 theories of successional mechanisms were invoked to explain why the 

 pathogen population decreased as recolonization continued. 



For comparison purposes, the pathogenic and saprophytic activities 

 of the pathogen also were monitored in nontreated soils. Saprophytic 

 proliferation of the pathogen did not occur in nontreated soils. Nash 

 et al. (19) reported similar results when they added chlamydospores of 

 F. solani f. phaseoli to nontreated soils. The lack of chlamydo spore 

 germination in natural soils has been attributed to insufficient nutri- 

 ents and to the presence of inhibitory substances in the soil (M). 



The ability of the pathogen to infect the host, as determined by 

 the ratio of inoculum density to infection incidence, was higher in 

 nonfumigated soils than in fumigated soils that had been allowed to 

 undergo recolonization for 25 days. This phenomenon was termed induced 

 antagonism by Welvaert (^2). The practical aspect of induced antagonism 

 is that it prolongs the effect of a soil treatment after the treatment 

 is no longer active. The decreases in infection incidence of tomato and 

 saprophytic proliferation of the pathogen in fumigated soils were corre- 

 lated with the microbial population that formed during recolonization. 



The relationship of the inoculum density of F^ oxysporum f . sp. 

 radicis-lycopersici to the infection incidence of tomato was influenced 



