Rhizobacteria of maize antagonistic to Fusarium moniliforme, a soil-borne fungal pathogen: Colonization of rhizosphere and roots

Abstract

Rhizobacteria of maize isolated from various soils with a long history of maize cultivation and which exhibited antagonism in vitro to Fusarium moniliforme were screened in a greenhouse for their ability to colonize maize roots. When used as seed coatings, the root “colonizing potential” of the antagonists differed significantly between strains and they also varied depending on the soil type used. For non-sterile soil, larger populations of the inoculant strain developed on maize roots growing in a Darling Downs vertisol than in a silty loam soil from Hawkesbury. However, the reverse was true when plants were grown in the silty loam Hawkesbury soil sterilized by γ-irradiation. The varying response to inoculation measured in both soils was apparently due to the diverse range of competing soil bacteria rather than the total microbial population per se. Of the 16 rhizobacteria screened for their colonizing ability, Pseudomonas cepacia strains 526 and 406 and Enterobacter agglomerans strain 621 formed consistently larger populations on the roots of both seedling and mature maize plants than the other strains. The seed inoculated P. cepacia strains were highly competitive and on 2 week old plants formed 10–80% of the total root-colonizing bacterial populations cultured on nutrient agar, and 1% after 2–3 months of plant growth. In montmorillonite soil, an inoculum density as low as 10 P. cepacia seed −1 resulted in 10 4 bacteria g −1 dry wt root. Nearly 70% of the P. cepacia strains tested were positive for pectinase and lipase activity. It is postulated that production of antifungal substances, rapid growth rates, the capacity to utilize a wide range of carbon sources exuded by maize roots and to a lesser extent the production of extracellular enzymes may facilitate P. cepacia strains to colonize maize roots.