The effect of an extract on the target pathogen population over time also is an issue when determining whether repeated soil treatment is necessary throughout the growing season. For example, soil populations of F. oxysporum f. sp. chrysanthemi were lowest after 3 to 7 days of incubation when the soil was treated with 5 and 10% aqueous emulsions of the formulated pepper/mustard extract. Population numbers then increased over time. In greenhouse experiments, the pepper/mustard extract controlled disease initially, but disease incidence increased later in the experiments, especially with the 5% aqueous emulsion. Fusarium populations may have increased, as in the laboratory studies, to sufficient levels in the greenhouse over time to cause symptom development. Observation of the lack of background microflora on dilution plates suggests that the pepper/mustard extract may act as a general biocide. At this concentration, the extract may affect a wide range of soil microorganisms and thus may create a biological vacuum. Fusarium has been shown to rapidly colonize fumigated soil in the absence of competition (17,19). We hypothesize that the extract may break down rapidly in soil, and a small surviving population of Fusarium may have flourished in the soil environment with the natural microflora at low population levels once the active ingredient in the extract was no longer present. Treatment of soil with a 5% aqueous emulsion may not have reduced Fusarium populations enough, thus allowing rapid recolonization with reduced microbial competition, to effectively control disease over a long time frame. Populations of Fusarium also increased numerically over time when the pepper/mustard extract was added as a 10% aqueous emulsion. However, disease control was nearly 100% during the 6 weeks of the experiment in the greenhouse. Treatment of the soil with a 10% aqueous emulsion of the pepper/mustard extract may have reduced Fusarium populations sufficiently to suppress disease over a
longer period of time. A very low Fusarium population may not compete as well over time with other soil populations during recolonization (26). Thus, any threshold needed for observable symptom development may be delayed. It is within this time frame that the addition of biological control microorganisms may be most effective (19). Retreatment of soil during the growing period may not be feasible with the pepper/mustard extract because of potential phytotoxicity (J. H. Bowers and J. C. Locke, unpublished). Season-long disease control in the field needs to be evaluated.