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Rigidoporus microporus is a causal agent of white root rot disease of rubber trees. Chemical control is commonly used against this disease but it is expensive, causes environmental pollution and fungal resistance. Silicon is a beneficial element which enhances plant growth and disease resistance but mostly present in insoluble forms. Silicate solubilizing bacteria (SSB) can solubilize the insoluble forms of silicates. This study aimed to screen and evaluate potential SSB with solubilizing activity, plant growth promoting traits and antagonistic activity against R. microporus. Bacteria were isolated from healthy rubber plantation area. Five out of 26 bacterial isolates which showed the ability to solubilize silicate on magnesium trisilicate media were screened in vitro for growth promoting traits and production of indole-3-acetic acid (IAA), siderophores and hydrolytic enzymes. The investigation of antagonistic activity against R. microporus was conducted using dual culture test. The potential SSB isolates were identified by 16S rRNA gene sequencing. The isolate UPMSSB7 showed the highest solubilization of silicate, phosphate (P) and potassium (K) (solubilizing index 4.67, 2.52 and 2.61, respectively). It also showed highest silicate solubilization at 5 and 10 days (9.76 and 11.55 mg L−1, respectively) in liquid assay. It strongly inhibited the growth of R. microporus with percent inhibition of radial growth of 57.24% and produced highest IAA (19.96 µg mL−1). All the isolates produced siderophores. Two isolates, UPMSSB7 and UPMSSB10, were able to secrete cellulase enzyme, but pectinase was produced by UPMSSB7 only. The isolate UPMSSB7 was identified as Enterobacter sp. This isolate was found to be the most potent antagonistic isolate against R. microporus and simultaneously the most efficient one in solubilizing insoluble silicates, as well as P and K. [ABSTRACT FROM AUTHOR]

• Bioformulation of microbes with Si reduced white root rot disease in rubber seedlings. • Bioformulation was not significantly different than fungicide in disease suppression. • Bioformulation increased plant growth and nutrient contents than fungicide and control. • The mycorrhizae spore density and root colonization were increased by bioformulation. White root rot (WRR) disease, caused by Rigidoporus microporus , is one of the major constraints causing the economic loss to rubber plantation worldwide including in Malaysia. This glasshouse study was conducted to evaluate the efficacy of a peat-based bioformulation of Enterobacter sp. UPMSSB7 and an arbuscular mycorrhizal fungus (AMF, Glomus mosseae) with silicon (Si) for WRR suppression and plant growth promotion of rubber seedlings. Results revealed that the tested bioformulation significantly (P < 0.0001) reduced the disease compared to positive control while, was not significantly different than the propiconazole, a triazole fungicide. The lower values of disease incidence, disease severity of foliar and root rot symptoms, area under disease progress curve, pathogen colonization and higher value of disease reduction were recorded in both bioformulation and the triazole fungicide treated seedlings than positive control treatment 24 weeks after inoculation with R. microporus. Results indicated that pathogen-inoculated seedlings treated with bioformulation had significantly (P < 0.0001) increased stem height, girth size, chlorophyll content, leaf area, shoot and root dry weight, total root length, root volume and root surface area (103.8 cm, 8.08 mm, 56.62 SPAD value, 3534.8 cm2, 37.04 g and 36.76 g, 1617.7 cm, 24.15 cm3 and 402.97 cm2, respectively) as compared to fungicide and control treatments. The tested bioformulation also significantly (P < 0.0001) increased silicon content in root and shoot (18.56 g kg−1 and 27.61 g kg−1, respectively) as well as leaf nutrient (N, P, K and Ca) contents (3.89%, 0.19%, 1.06% and 0.57%, respectively) as compared to the triazole fungicide and control treatments. The population of Enterobacter sp. (1.1 × 108 cfu g−1 soil) recorded in pathogen-inoculated seedlings treated with bioformulation was not significantly different than non-inoculated seedlings treated with bioformulation. AMF spore density and root colonization were recorded higher in bioformulation treated seedlings with or without pathogen inoculation than the triazole fungicide and control treatments. From this study, it was concluded that the peat-based bioformulation of Enterobacter sp. and an AMF (Glomus mosseae) with Si could be a potential approach to suppress WRR and improve growth of rubber seedlings. [ABSTRACT FROM AUTHOR]