13 

 density. The limited ability to compete as a saprophyte is charac- 

 teristic of other pathogenic Fusarium spp. (21,2^,35). 



The ability of other plant pathogens to compete as saprophytes in 

 treated soil also may be reduced with the reestablishment of the micro- 

 bial community (^,23,30,39). A decrease in the percentage of isolations 

 of Verticillium albo-atrum from seedlings of Senecio vulgaris was attrib- 

 uted to the recolonization of autoclaved soil by airborne propagules of 

 other microorganisms (30 ) • This decrease is similar to that observed by 

 Rowe and Farley (27) with Fusarium crown rot of tomato. 



The observed decrease in the population of the pathogen from an 

 initial inoculum density of 500 chlamydo spores per gram of fumigated 

 soil amended with antagonists to 50 propagules per gram of soil may have 

 been caused by the production of toxins or by parasitism of the pathogen 

 by the antagonists. The increase in the ratio of inoculum density to 

 infection in the same soil may have been due to the adverse effects of 

 the anxagonists on the growth of the pathogen or the successful competi- 

 tion of the antagonists at potential infection sites. The reduction in 

 the ability of the pathogen to infect roots is evident by the compar- 

 ison of the TD^qS of 300 and 6500 chlamydo spores per gram of soil in 

 fumigated soil and fumigated soil with antagonists added, respectively. 

 The reduced number of infected seedlings was correlated with the 

 decreased mean lesion lengths when antagonists were added as compared to 

 when the antagonists were not added in the greenhouse experiments. The 

 correlation of increased numbers of infections and the increase in 

 disease severity has been reported in other diseases caused by Fusarium 

 spp. (20,31,32). 



