Regulation of Fusarium oxysporum populations introduced into soil: the amoebal predation hypothesis

Abstract

Inoculation of fungi into soil has been suggested for biological control of plant diseases. The aim of our work was to test the ability of protozoa to reduce the density of introduced fungal populations. The survival of Fusarium oxysporum in non-sterile soil was studied after introduction at densities of: 1 × 104, 1 × 106 and 5 × 107 cfu/g soil. The dynamics of protozoa were also followed. The fungal populations remained close to the initial inoculation densities and did not induce the growth of indigenous protozoa. A bacterial population (Enterobacter aerogenes) was used to promote and stimulate the predatory activity of amoebae. Then, after simultaneous inoculation with bacteria and fungi, the density of protozoa increased but this had no effect on the fungal population, although some amoebae are able to feed on small fungal propagules such as conidia. The physiological state of Fusarium in soil and intraspecific competition seem to be more important in regulating introduced fungal populations than amoebal predation. We conclude that the regulation of bacterial and fungal populations in soil depend on different mechanisms.