Growth promotion of Prunus rootstocks by root treatment with specific bacterial strains

A collection of bacterial strains obtained from a wide-range origin was screened for ability to promote growth in two types of Prunus rootstocks in a commercial nursery. Only few strains promoted growth significantly and consistently, and a strong specificity for the rootstock cultivar was observed. Irrigation of plants with Pseudomonas fluorescens EPS282 and Pantoea agglomerans EPS427 significantly increased plant height and root weight of the plum Marianna 2624 and the peach-almond hybrid GF-677, respectively. Plant height showed a higher rate of growth in early stages of development (2.6-3.5 times the non-treated controls), but the effect decreased with plant age. However, in aged plants growth promotion was more significant on root weight (1.9 times the non-treated controls) than on plant height. The efficacy of growth promotion and the persistence of strains in the root environment were dependent on the bacterial inoculum concentration applied. Increases in root development were maximum at inoculum concentrations of up to 8 log10 CFU ml^–1 (ca 10 log10 CFU L^–1 of potting mix). Population levels at the optimum inoculum concentration were around 7 log10 CFU g f.w.^–1 root material at early stages of development and decreased to 4 log10 CFU g f.w.^–1 after several months of development. The best plant growth-promoting strains were very diverse in secondary metabolite production and antagonistic ability against several plant pathogens.