Utilization of rhizospheric Streptomyces for biological control of Rigidoporus sp. causing white root disease in rubber tree

White root disease causing by Rigidoporus sp. is a severe problem that decreases latex productivity and can even cause mortality of rubber trees. With the aim to control biologically this disease, antifungal actinobacteria were isolated from rhizospheric soils of some medicinal plants cultivated in Thailand. Among all isolated actinobacteria, an isolate TM32 exhibited distinctive antagonistic activity against the fungus. Cell-free culture broth of the isolate showed median effective dose (ED₅₀) of 2.61 ml l⁻¹(equal to 1.19 g l⁻¹of metalaxyl). The isolate was also able to solubilize phosphate and to produce chitinase (enzyme activity = 0.093 ± 0.004 U ml⁻¹), siderophore (average clear zone, 11.75 ± 0.96 mm) and indole-3-acetic acid (54.00 ± 1.00 μg ml⁻¹). Application of biocontrol starters produced by this isolate in nursery stage of rubber trees farming showed greater suppression of the disease than direct use of its biocontrol agents. The biocontrol starters also enhanced growth of the rubber trees by increasing their heights. This might be due to the persistent growth of the isolate by using the organic substrate remaining in soil, which could later antagonize fungal pathogens through colonization at the rhizosphere and immunization of the rubber trees. The isolate revealed phylogenetically related to Streptomyces sioyaensis supported by 99 % similarity of 16S rRNA gene sequences. We concluded that application of the biocontrol starters produced by this Streptomyces isolate would be an alternative approach for sustainable control of soil-borne fungal invasion in long-term rubber tree farming.