Rhizobacteria of Sugar Beets: Effects of Seed Application and Root Colonization on Yield

Abstract

Selected strains of fluorescent Pseudomonas spp. isolated from the rhizosphere-rhizoplane of field-grown sugar beets caused statistically significant yield increases of sugar beets in replicated greenhouse studies and field trials when applied as a seed-coating formulation. Significant increases in dry and fresh weight of seeding roots and/or shoots of plants grown in the greenhouse ranged 20-85%. The effect did not occur in sterile University of California mix or in peat soils. Significant growth increases in seedling weight, mature root weight, and total sucrose yield were attained in field trials in California and Idaho [USA] over a 3-yr period. Increases in root weight and total sucrose, averaging 13% above untreated controls, were as great as 4.6 t[tons]/ha and 26.8 cwt/ha, respectively. Protocols were developed for the application and preservation of populations of up to 1012 colony-forming units (cfu) of plant growth-promoting rhizobacteria (PGPR) on seed, using cellulose methyl ether or gum xanthan in combination with neutralized peat or talc coatings. The seed application techniques developed were compatible with commercial planting procedure used for field trials. Colonization of roots by PGPR resistant to rifampicin and nalidixic acid persisted throughout the growing season, reaching populations as great as 5.2 .times. 104 cfu/cm of root. Populations of PGPR reached 103-104 cfu/cm of treated roots compared with natural populations of total fluorescent pseudomonads, which ranged 90-500 cfu/cm of root. In vitro antibiosis by PGPR strains was observed against Erwinia carotovora spp. carotovora and E. carotovora ssp. atroseptica; Pseudomonas marginalis pv. marginalis; P. syringae pv. syringae, P. syringae pv. phaseolicola, and p. syringae pv. tomato; and fungal pathogens Rhizoctonia solani, Pythium ultimum, P. aphanidermatum and P. debaryanum. The antagonism was biostatic rather than biocidal. The establishment of high populations of antagonistic PGPR on roots appears related to plant growth promotion effects.