In-furrow spray as a delivery system for plant growth-promoting rhizobacteria and other rhizosphere-competent bacteria

Abstract

A series of greenhouse and field experiments were conducted to evaluate aqueous in-furrow spray techniques for inoculating crop plants with cell suspensions of rhizosphere-competent root-colonizing bacteria. Maximum root colonization of soybean or canola roots by strains of Serratia, Pseudomonas, and Bradyrhizobium occurred using log 8 colony-forming units (cfu)/mL in the spray in greenhouse conditions. Field experiments evaluating a dose reponse demonstrated that maximum soybean root colonization by strains of Serratia or Pseudomonas was achieved between log 7 and 8 cfu/mL, while root colonization by a Bacillus strain was not related to cell concentration in the spray suspension. Root colonization greater than log 4.5 cfu/g root fresh weight was achieved for most strains by in-furrow spray application of a suspension of log 8 cfu/mL at a rate of 10 mL/m for canola, soybean, and wheat, while root colonization of corn ranged from a maximum of log 3.4 to no recovery. In-furrow spray may be a useful method for inoculating plants with rhizosphere-competent bacteria for experimental purposes, thereby avoiding interactions of formulation. It may also have some value for commercial delivery of bacteria to agricultural crops when it is compatible with accepted agronomic practices. Key words: rhizosphere, plant growth promoting rhizobacteria, formulation, Pseudomonas, root colonization.