Effects of Rhizosphere Colonization by Plant Growth-Promoting Rhizobacteria on Potato Plant Development and Yield.

Two strains of fluorescent Pseudomonas spp. isolated from potato periderms and from celery roots significantly increased growth of potato plants up to 500% greater than controls in greenhouse assays. Mutants of these plant growth-promoting rhizobacteria (PGPR) resistant to rifampicin and nalidixic acid rapidly colonized rhizospheres of roots emerging from treated seed pieces in field tests. The bacteria colonized the entire rhizosphere of treated plants including developing daughter tubers and the apical roots of adjacent nontreated plants. PGPR rhizosphere populations were as great as 9.6 × 10⁵ colony-forming units per centimeter (cfu/cm) of root 2 wk after plant emergence and averaged 10³ cfu/cm throughout the season, but declined to approximately 10² cfu/cm at harvest time. PGPR populations were relatively constant throughout the root system. Field tests were done in various soil types with pH ranges from 6.8 to 7.8. PGPR rhizosphere colonization resulted in significant increases in stolon length 2 wk after plant emergence in all six sampled fields. The early season plant growth promotion caused by PGPR was followed by significant yield increases up to 17% compared to untreated controls in four of five harvested fields.