¹ DPI, Qld Horticulture Institute, Indooroopilly, Queensland 4068, Australia; ²DPI, Maroochy Research Station, Nambour, Queensland 4560, Australia; ³ DPI, Redlands Research Station, Cleveland, Queensland 4163, Australia; ⁴DPI, Centre for Wet Tropics Agriculture, South Johnstone, Queensland 4859, Australia

Background and objectives
Micropropagated bananas are increasingly being used by the Australian banana industry as a source of disease and pest-free planting material. Recent work has shown that micropropagated bananas are more susceptible to fusarium wilt (Fusarium oxysporum f. ;sp. cubense) (Foc) [1] and to burrowing (Radopholus sitnilis) and spiral (Helicotylenchus multicinctus) nematodes than conventional planting material. Improvements to micropropagated banana systems are required to reduce susceptibility to soil-borne pathogens and to increase nutrient uptake. This would also result in reduced production costs through reduced fertilizer and pesticide usage, thus leading to fewer chemicals leaching into the enviromnent and more sustainable banana production.

Some soils are suppressive to nematodes and fusarium wilt. Conventional vegetative planting material (i.e. suckers and rhizome pieces) from such soils may be protected by beneficial microorganisms such as mycorrhizal fungi and bacteria which are not available to micropropagated plants raised in an aseptic environment. Research has shown that micropropagated bananas inoculated with mycorrhizal fungi show significant increases in nutrient content and plant growth compared to uninoculated controls [2] and, as a result of this increased vigour, a lower incidence of nematode damage. Rhizobacteria such as fluorescent pseudomonads have been effective in reducing the severity of Fusarium diseases in several crops and significantly reduce infectivity of parasitic nematodes.

This study aims to isolate beneficial microorganisms from plantations growing in wilt- and nematode-suppressive soils and introduce them to micropropagated plants before planting in infested fields, in order to obtain more vigorous plants better able to utilize nutrients and with greater protection against disease.

Materials and methods
Banana roots and soil were randomly selected from fusarium wilt and nematode suppressive soils from North and South Queensland. Rhizobacteria were isolated by dilution and direct plating using selective and nonselective media and cultures,were obtained using streak plating for selection of single colonies.

Results and conclusions
Rhizobacteria were successfully isolated and purified. Isolates were stored on Biolog Universal Growth Medium at l0°C and as a turbid suspension in sterile distilled water.

Bacterial isolates and mycorrhizal fungi are being evaluated in vitro and in glasshouse trials for colonization ability, antagonism towards Foc and nematodes, effect on plant growth and disease control.

References
1. Smith M, Searle C, Whiley A, Langdon P, Schaffer B, Pegg K, 1997. In: Tissue Culture: Towards the Next Century. Arinidale, NSW: University of England, pp. 133-140.
2. Declerck S, Devos B, Delvaux B, Plenchette C, 1994. Fruits 49, 103-109.