Onion rhizobacteria as potential plant health promotants

Sclerotium cepivorum Berk. (Berkeley, 1841), the causal agent of Allium White Root Rot (AWRR) disease poses a major threat to the sustainability of the Tasmanian onion industry, an export commodity which is worth $A40 million to the state's economy. Tasmanian research into alternatives to fungicides for the management of this disease has been primarily focussed on use of Trichoderma spp. as fungal antagonists and novel bioactive substances such as diallyl disulphides. In this study, antifungal bacteria from the onion rhizosphere were examined for their potential as plant growth promotion agents with the view to provide an additional component in the present integrated control strategy. Bacteria isolated from root samples of healthy onions growing in an AVVRR pathogen infested paddock were found to possess antifungal properties when co-cultured in vitro with the pathogen at 10°C (typical of cropping soil temperatures for onions in Tasmania). The five bacterial strains most active in S. cepivorum inhibition were identified as members of the Enterobacteriaceae. Molecular taxonomy studies indicated these were putatively novel species. These isolates were screened in glasshouse trials with one isolate, M1RB2, inducing production of significantly greater onion shoot biomass in dry warm conditions. All bacteria trialled were suitable for formulation as a broadcast powder, and exhibited high tolerance to Benlate, Thiram and Folicur, the three fungicides most commonly used on Tasmanian onion crops. Studies on the impact of these fungicides on native microbial populations showed that bacterial numbers may be elevated by the application of a benlate-thiram seed dressing. These two traits are of particular significance if a commercial product was to be developed. ELISA surveys of the rhizosphere showed an increase in the proportion of M6SA1-like bacteria in pots to which the bacteria had been introduced but no significant change in total bacterial numbers.