20 



growth chambers. The populations of the pathogen in the soils were 



quantified by dilution plating on Komada's (l6) medium, which is select- 

 ive for F. oxysporum . Pathogenic isolates were identified by the tech- 

 nique of Sanchez et al. (29) » as described in Section I. The ability 

 of the pathogen to infect host plants grown in soil from the different 

 treatments was determined by placing two germinated 'Bonnie Best' tomato 

 seeds in a 100-ml polypropylene beaker which contained 60 g of soil 

 layered over 50 g of autoclaved sand. The beakers then were placed in 

 growth chambers at 20 G and watered every 48 hrs. After 2 wk, the soil 

 was washed from the roots; the roots and lower stem were soaked in .6% 

 sodium hypochlorite for 1 min, rinsed in autoclaved deionized water, and 

 plated on Komada's (l6) medium. The plates were examined after 10 days 

 at 25 G. The plants were considered to be infected if F. oxysporum f. 

 sp. radicis-lycopersici grew from the crown area of the seedling. 



The soil fungal communities were monitored by dilution plating of 

 soil samples on potato dextrose agar which contained 1 ml of Tergitol 

 NPX (Sigma Chemical Co., St. Louis, MO 63178) and 50 mg of chlortetracy- 

 cline hydrochloride (Sigma Chemical Co., St. Louis, MO 63178) per liter 

 of medium. The plates were incubated for 7 days at 25 C and 2000 lux of 

 fluorescent light. Benomyl (Benlate 50% WP, E. I. du Pont de Nemours 

 and Co., Wilmington DE 19898) was added to a replicate set of plates at 

 2.5 ppm when soil dilutions in water of 1:25 or lower were used to 

 inhibit the fast growing colonies of Trichoderma spp. Dilution series 

 ranged from 1:10 to 1:10° (wt:vol). The particular series of dilutions 

 used was dependent upon the expected populations of fungi. 



The experiments were repeated at least twice; 10 petri plates were 

 used for each soil dilution and 48 tomato plants were used to determine 

 the incidence of infection for each experiment. 



