12 

 nonfumigated, or fumigated and amended with antagonists. In all soils 



the slope was approximately 1.0. 



The attenuation of isolates of Fusarium spp. grown on artificial 

 media must he considered in quantifying the relationship of inoculum 

 density to disease incidence and severity. As an isolate becomes less 

 virulent, more inoculum will be required to cause disease regardless of 

 the particular treatment. The axenic production of chlamydo spores of a 

 pathogen reduces the chances of attenuation because the isolate does not 

 reproduce vegetatively between experiments; thus the possibility of 

 genetic variation is minimized. Changes in virulence of F. oxysporum f . 

 sp. radicis-lycopersici were not observed in any of the experiments. 

 The germination rate of chlamydo spores of the fungus on potato dextrose 

 agar was not significantly different from 100$! (p = 0.05) for up to one 

 year after their formation. French (8) reported that chlamydo spores 

 formed from macroconidia of F. oxysporum f . sp. batatas remained viru- 

 lent for 7 yr when stored in water. 



The application of a broad spectrum biocide to soil creates a 

 biological vacuum which disrupts the stability of the soil community. 

 The early recolonization pattern of treated soil involves a shift to an 

 early successional pioneer stage that consists of a few species which 

 occur in large numbers (^2). Thus the community is of low diversity and 

 may be readily invaded by new species if the environmental conditions 

 are not severe. In this study the pathogen population remained stable, 

 as determined by dilution plating, in the more advanced serai stages of 

 nonfumigated soils. In the pioneer successional stage of fumigated soils, 

 however, the pathogen was able to compete and increase in population 



