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Rubber growers in Malaysia depend on soil drenching with propiconazole fungicide to control white root disease (WRD) caused by Rigidoporus microporus. The fungal infection affected the environmental ecosystem, giving rise to fungicide resistance. Recently, silicon (Si) has become an alternative to reduce and delay pathogenic fungal invasion. Therefore, the present study investigates the antifungal property of soluble silicon against R. microporus in rubber trees (Hevea brasiliensis). In vitro dose-response towards soluble silicon types, i.e., silicic acid, sodium meta-silicate, sodium silicate, and calcium silicate with different concentrations (10, 100, 500, 1000, 1500, 3000, 5000, and 8000 ppm) were determined on the Ayer Molek strain of R. microporus using the Poisoned Food Technique. Results showed that sodium meta-silicate inhibited mycelial growth (100%) at 5000 and 8000 ppm concentrations compared to other types of soluble silicon. However, silicic acid inhibited more than 50% R. microporus at a minimal concentration of 500 ppm, which could be considered the most effective antifungal from the soluble silicon group. Moreover, the higher pH values did not solely affect the inhibition rate of R. microporus. Microscopic observation showed the changes of R. microporus hyphae width grown on soluble silicon medium agar compared to the control (without Si). The Dipped Stick Inhibition Assay revealed that a higher concentration and more frequent soluble silicon application effectively inhibited R. microporus growth. Thus, this study proved that soluble silicon, especially silicic acid and sodium meta-silicate, showed promising results as antifungal agents and fungicidal in controlling white root disease