2 



pathogen. In Florida, an environment conducive to the development of 



Pusarium crown rot is established when plastic mulch is applied during 

 fumigation and maintained during the entire growing season. If antago- 

 nists of F. oxysporum f . sp. radicis-lycopersici could be introduced 

 under the plastic before the soil is recolonized by the pathogen, it 

 should be possible to reduce disease. Thus, the nature of the pathogen 

 and tomato production methods provide an excellent system for a quanti- 

 tative field study of biological control. 



Before field studies are undertaken, however, the importance of 

 fumigation and recolonization of soil by antagonists can be evaluated 

 critically by quantitatively determining the relationships of the 

 pathogen and antagonists to incidences of infection and disease in 

 fumigated or nonfumigated soil under growth- chamber and greenhouse 

 conditions. The relationships of inoculum density to disease severity 

 in root rots caused by Fusarium spp. are well documented ( 1,7,11,31). 

 Baker (3) proposed that biological effects of antagonists on disease 

 could be quantified by the analyses of curves derived by plotting dis- 

 ease severity to inoculum density of the pathogen. Significantly 

 greater inoculum densities of the pathogen should be required to cause 

 a proportionate amount of disease when antagonists are present. 



The quantification of inoculum required the development of a pro- 

 cedure in which defined levels of chlamydospores of the pathogen could 

 be established in freshly fumigated soil. The present procedure used 

 in soil density studies with Fusarium spp. involves placing plants in 

 soil with populations of the pathogen established by assaying artifi- 

 cially infested, aged soil with selective media and diluting the as- 

 sayed infested soil with noninfested soil (ll). This procedure is not 



