Antagonistic Activity and Preliminary Toxicity Assessment of a Bacillus subtilis TU089 Bioproduct Against Phytophthora palmivora Causing Durian Root and Stem Rot

Durian is an economically important fruit crop in Thailand, with growing export demand, particularly from China. However, production is severely affected by root and stem rot caused by Phytophthora palmivora. This study aimed to screen Bacillus strains for antagonistic activity against P. palmivora strain MNP13, develop the most effective strain into a bioproduct, and evaluate its safety. Five Bacillus strains were tested on PDA using the dual-culture method. Bacillus subtilis TU089 exhibited the highest inhibitory activity, with 76.56 ± 0.23% inhibition. The selected strain was subsequently developed into an encapsulated bioproduct and propagated in F3 medium, yielding a bacterial population of approximately 108 CFU/mL. The developed bioproduct retained strong antagonistic activity against P. palmivora, with efficacy comparable to that of fresh B. subtilis TU089 cells. In vitro safety assessment using the MTT assay showed IC10 values of 1090 and 309.82 µg/mL in MRC-5 and HaCaT cells, respectively. Acute oral toxicity evaluation according to OECD Guideline 423 revealed no significant adverse effects under the experimental conditions. These findings highlight the promising in vitro antagonistic activity of the B. subtilis TU089 bioproduct against P. palmivora and support further greenhouse and field evaluation for its potential application in durian disease management.