Effect of integration of two bacterial bioagents and a plant residue extract for biocontrolling root-knot nematode, Meloidogyne incognita infesting potatoes

Background Recently, there has been an increasing attempt to explore nature-friendly compounds that could be substitutes for chemically synthesized products. It was found that some plant residues and certain microorganisms, including antagonistic bacterial species such as Bacillus spp. associated with plants, can act as biocontrol agents, achieving various degrees of control against Meloidogyne incognita, as well as increasing the plant growth and yield parameters. Objectives This research was designed to study the effect of Bacillus subtilis (Bs) and B. pumilus (Bp) alone or in combination with pomegranate peel aqueous extract (PP) on root-knot nematode, M. incognita, infesting potato cv. Spunta, as well as to examine the biochemical changes and total microbial counts under field conditions. Materials and methods Overall,100 g of crushed pomegranate fruit peel (PP) water extract and two bacterial biocontrol agents were applied in a field experiment for controlling root-knot nematode, M. incognita. These bacteria, B. subtilis (Bs) and B. pumilus (Bp), were isolated from rhizosphere soil and identified according to standard microbiological characteristics. In a field naturally infested with M. incognita, potato cv. Spunta tubers were planted during winter growing season. After planting, each of the bacteria was added in the soil at the tested rate (107–109 CFU/ml). Moreover, some tubers that were planted in the soil served as untreated control. The treatments included (a) B. subtilis (Bs)+pomegranate peel (PP) residue extract, (b) B. pumilus (Bp)+PP residue extract, (c) PP residue extract+medium (M), (d) Bs, (e) Bp, (f) PP residue extract, (g) medium (M), and (h) untreated control. Results and conclusions Based on the percentages of juvenile reduction in soil at the harvest time, Bs+PP recorded 84.0% juvenile reduction followed by Bp+PP (82.3%), revealing combined treatments to be more effective than single treatments (78.4% by Bs and 72.8% by Bp). The examination of co-toxicity of the two applied combined treatments at the harvest stage showed synergistic effects. All treatments significantly (P≤0.05) increased plant growth and yield criteria, especially individual treatments. Biochemical compounds and the total bacterial and fungal counts in potato rhizosphere varied with different treatments. It could be concluded from the present study that the combined treatments of B. subtilis or B. pumilus +PP inhibited M. incognita proliferation in potatoes more than single treatments. However, single treatments improved plant growth and yield more than the combined cases. Biochemical changes and microbial counts of potatoes were influenced by different treatments.