Abstract
Bacillus subtilis strain NCD-2 and Pseudomonas protegens strain FD6 produce a range of antimicrobial metabolites known to be effective against soil-borne plant pathogens. Quantitative qPCR assays were designed from sequence characterized amplified fragment length polymorphisms (AFLP) diagnostic for each inoculant genotype. The qPCR minimum detection thresholds (MDT) for NCD-2 and FD6 ranged between 5.2 × 103 (log 3.71) and 1.5 × 106 (log 6.18) genome (cell) equivalents g−1 of soil or wheat root tissue and were independent of soil type. Wheat rhizosphere populations of strains FD6 and NCD-2 significantly declined during the first 14 days postinoculation and then remained stable for the next 14 days. Colonization of wheat roots, rhizosphere soil and bulk (non-rhizosphere) soil by FD6 was significantly greater than that of NCD-2. Strains FD6 and NCD-2 significantly decreased in vitro growth of wheat root pathogenic Rhizoctonia solani AG-8, Pythium irregulare and Gaeumannomyces graminis var. tritici, fungal antibiosis by FD6 greater than NCD-2. Wheat rhizosphere colonization by NCD-2 and FD6 significantly decreased abundance of R. solani AG-8 in the rhizosphere by 3- and 13-fold respectively, pathogen suppression increasing (21%–29%) foliar biomass of wheat seedlings to levels not significantly different to the non-diseased control. The qPCR assays developed in this study were used to monitor soil and rhizosphere populations of P. protegens FD6 and B. subtilis NCD-2 and their suppressive efficacies against R. solani AG-8 in wheat. These assays can be applied to monitor the dynamics of these inoculants in suppressing Rhizoctonia root rot and other crop diseases.
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