Rhizosphere competence of Pseudomonas sp. NBRI9926 and Rhizobium sp. NBRI9513 involved in the suppression of chickpea ( Cicerarietinum L.) pathogenic fungi

Abstract

In order to obtain chickpea ( Cicer arietinum L.) rhizosphere-competent bacteria, 256 bacterial strains, representing different morphological types, were screened for their biocontrol activity against Fusarium oxysporum f.sp. ciceri, Rhizoctonia bataticola and Pythium sp. under in vitro conditions. Pseudomonas sp. NBRI9926 and Rhizobium sp. NBRI9513 were selected for further work because of their unique ability to inhibit all three fungi. Spontaneous rifampicin-resistant (Rif r) derivatives of Pseudomonas sp. NBRI9926 and Rhizobium sp. NBRI9513 showing a growth rate and membrane protein composition comparable to the wild-type were selected to elucidate their rhizosphere competence on chickpea. Both strains showed no difference to their corresponding wild-type strains in terms of chickpea rhizosphere competence. However, when Pseudomonas sp. NBRI9926P3 was mixed in an equal or lower ratio with Rhizobium sp. NBRI9513R7, both in sterile soil and non-sterile soil assays, Rhizobium sp. NBRI9513R7 consistently failed to achieve titres similar to Pseudomonas sp. NBRI9926P3. These results suggest that antibiotic resistance-marked strains should be compared with wild-type parents before being used as monitors of parental strain survival. Pseudomonas sp. NBRI9926P3 was a better chickpea rhizosphere coloniser compared to Rhizobium sp. NBRI9513R7. Greenhouse results demonstrated a superior biocontrol potential of Pseudomonas sp. NBRI9926P3 to control F. oxysporum f.sp. ciceri, R. bataticola and Pythium sp. compared to Rhizobium sp. NBRI9513R7. The screening method should prove useful in identifying rhizosphere bacteria with the greatest potential for controlling diseases caused by phytopathogenic fungi.