Mixtures of plant disease suppressive bacteria enhance biological control of multiple tomato pathogens

Abstract.

Plant growth-promoting rhizobacteria (PGPR) strains CHA0 (Pseudomonas fluorescens), IE-6 S⁺ (Pseudomonas aeruginosa) and 569Sm^r (Bradyrhizobium japonicum) were tested singly and in combinations for biological control against multiple tomato pathogens (root-infecting fungi and root-knot nematodes). Strains CHA0 and IE-6S⁺ inhibited in vitro growth of 569Sm^r while IE-6S⁺ suppressed CHA0. The bacterial species not only inhibited the radial growth of three root-infecting fungi, Macrophomina phaseolina, Fusarium solani and Rhizoctonia solani (AG 8), but also caused substantial mortality of Meloidogyne javanica juveniles. Used as a soil drench the three bacteria not only suppressed root-infecting fungi and root-knot nematodes but also enhanced growth of tomato plants both under glasshouse and field conditions. The suppressive effect was generally more pronounced when the bacteria were employed together. Strain IE-6S⁺ exhibited better rhizosphere colonization than CHA0 and 569Sm^r. Populations of CHA0 in the rhizosphere declined when the bacterium was used with either IE-6S⁺ and/or 569Sm^r, while populations of IE-6S⁺ in the rhizosphere were enhanced when used in combination with CHA0 and/or 569Sm^r. IE-6S⁺ was the only bacterium that colonized inner root tissues of tomato plants. When using an iron chelator to create iron deficiency in the soil, the biocontrol efficacy of the bacteria against F. solani and R. solani was enhanced while against M. phaseolina and M. javanica this activity remained unchanged. Only strain 569Sm^r gave significant suppression of M. phaseolina in both iron-deficient and iron-sufficient soils.