EFFECTS OF NONPATHOGENIC STRAINS OF FUSARIUM-OXYSPORUM ON CELERY ROOT INFECTION BY FUSARIUM-OXYSPORUM F SP APII AND A NOVEL USE OF THE LINEWEAVER-BURK DOUBLE RECIPROCAL PLOT TECHNIQUE

Soils suppressive to Fusarium yellows of celery are described. Results from a greenhouse experiment in which sterilized or unsterilized suppressive and conducive soils (S and C soils, respectively) were mixed in various proportions and planted to celery, indicated that the suppressive agent was biological and did not multiply during the period of observation. This is in contrast to other S soils in which substantial disease control has been achieved following extensive dilutions and in which bacteria with short generation times have been implicated. Strains of F. oxysporum were isolated from celery roots growing in the S soil that, when incorporated into an experimental soil along with the pathogen, effectively reduced the rate of root infection (colonies per 100 cm of root) by the pathogen. Nonpathogenic strains of F. oxysporum from the C soil were generally ineffective in reducing root infection by the pathogen. Determinations of suppressive ability of nonpathogenic strains of F. oxysporum isolated from symptomless celery roots indicated that there are large differences in this trait among populations from different areas. Reduction in root infection by the pathogen apparently is a function of the ratio of inoculum densities of effective suppressive isolates to those of the pathogen. The relationships between inoculum density of the pathogen vs. rate of root infection as affected by several test isolates were analyzed by the Lineweaver-Burk double reciprocal plot technique. Results of these analyses suggested that isolates that either enhanced or suppressed root infection by the pathogen did so by affecting the susceptibility of a finite number of infection sites rather than by affecting the number of such sites. The most effective isolates reduced disease severity when added to an experimental greenhouse soil and a raw soil collected from a field with severely diseased plants.