AN INVESTIGATION OF THE IMPACT OF BIOCONTROL PSEUDOMONAS-FLUORESCENS F113 ON THE GROWTH OF SUGAR-BEET AND THE PERFORMANCE OF SUBSEQUENT CLOVER-RHIZOBIUM SYMBIOSIS

The biocontrol agent Pseudomonas fluorescens F113 produces the antimic robial metabolite 2,4-diacetylphloroglucinol and controls the extent o f Pythium-mediated damping-off of sugarbeet. A field site without damp ing-off pressure in 1994 was chosen to investigate the effect of a spo ntaneous rifampicin-resistant mutant of F113 (i.e., F113Rif) on growth of sugarbeet and subsequent clover, with the objective of developing a database for future risk assessments of genetically-improved derivat ives of F113. The inoculant established itself at about 6 log CFU per root system of seed-inoculated sugarbeet for 3 months after inoculatio n in 1994. In the absence of damping-off disease, the F113Rif treatmen t did not influence sugarbeet germination and had no effect on root yi eld, sugar yield, and root quality (contents in alpha-amino nitrogen, potassium and sodium, alkalinity of root tissue, purity of juice from extracted roots, sugar extractability). In the second pear, the site w as sown with uninoculated red clover, and F113Rif was found at low pop ulation levels in the rhizosphere (2.0 log CFU per root system at the first harvest). The F113Rif treatment had no effect on nodulation of r ed clover by the resident rhizobia and did not influence foliage dry m atter and total nitrogen at the two harvests carried out in 1995. Over all, inoculation of sugarbeet seeds with the phloroglucinol-producing pseudomonad F113Rif had no apparent ecological impact at the level of the sugarbeet crop and the subsequent clover-Rhizobium symbiosis. (C) 1998 Elsevier Science B.V.