The study of antagonistic bacterial strains isolated from the soil around durian tree roots demonstrated their ability to inhibit the growth of Phytophthora palmivora. The pathogens were screened from 30 samples collected around durian trees (leaves, soil around the roots, and debris under the tree) showing symptoms of root and stem rot disease. A total of 17 pathogen strains were isolated and grouped into 3 groups, TNP05, MNP13, and KNP21, originating from Chanthaburi province, Thailand. When P. palmivora isolates were tested for pathogenicity on leaves and durian trees, it was found that the strain MNP13 had the highest capacity to cause root and stem rot disease. A total of 196 beneficial bacteria isolates were collected from several samples around durian trees. The samples included leaves, soil surrounding the roots, and organic debris beneath the trees. Based on their colony characteristics on nutrient glucose agar (NGA), these isolates were divided into 8 groups. The efficacy of the beneficial bacteria against root and stem rot disease was tested using the Dual culture method and arranged in a Completely Randomized Design (CRD) with 5 replications. The experiment showed that bacterial isolates NJTU05, NJTU10, and NJTU13 effectively inhibited the growth of P. palmivora isolate MNP13, with inhibition rates of 76.66, 67.59, and 69.07%, respectively, compared to chemical control using metalaxyl 80% WP. Among the tested strains, NJTU05 was identified as the most effective bacterial strain for controlling major durian diseases. Biochemical identification and 16S rRNA sequencing revealed that bacterial strain NJTU05 was closely related to Brevibacillus formosus with a 99.70% identity.

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.