This work aims to isolate and screen the fungicidal endophytic bacterial strains for biocontrol efficacy against Phytophthora palmivora, a soil-borne pathogenic fungus that kills durian trees worldwide. Among more than 100 isolates, 6 strains were screened as potential fungicidal strains with inhibitory efficiency of 67.4-79.8%. Based on 16S rRNA gene sequencing and phylogenetic analysis, these strains were identified as Bacillus amyloliquefaciens EB.CK9, Bacillus methylotrophicus EB.EH34, Bacillus amyloliquefaciens EB.EH18, Bacillus siamensis EB.KN10, Bacillus velezensis EB.KN15 and Paenibacillus polymyxa EB.KN35. In greenhouse tests, the two strains P. polymyxa EB.KN35 and B. velezensis EB.KN15 significantly reduced the damage to diseased roots by P. palmivora (33.3 and 35.6%, respectively), increased the rate of survival of durian trees (only 20.8 and 22.9% plant death, respectively), and showed a positive effect on promoting durian plant growth. Notably, the potential fungicidal effect of last two strains against P. palmivora was recorded for the first time in this work. HPLC analysis showed that these strains can secret several plant growth-promoting compounds, including gibberellic acid (GA3), indole-3-acetic acid (IAA), kinetin, and zeatin. Of these, GA3 and zeatin were produced with a significant amount by both strains. The volatiles bio-synthesized by these isolates were also identified using GC-MS analysis, and some major volatiles were found as fungicidal agents. This study suggested that P. polymyxa EB.KN35 and B. velezensis EB.KN15 may be potential biocontrol candidates for durian P. palmivora and bio-fertilizers for the sustainable production of durian crops.

In order to overcome the damages caused by conventional farming, excessive use of chemical and synthetic fertilizers and for stimulating plant growth, Rhizomicrobiome has been a strategic yet over exploited biological tool. Among rhizospheric microbial communities, use of fluorescent pseudomonads as biocontrol agents remains prominent due to their ability to produce a variety of antimicrobial secondary metabolites. However, Pseudomonas spp. also possess great abilities of nutrient mobilization and can effectively enhance the bioavailability of inorganic zinc and potassium salts to plants. In this study, ten Pseudomonas spp. strains including P. aurantiaca (GS1, GS3, GS4, GS6, GS7, FS2, ARS38, and PBSt2), P. chlororaphis (RP4), and P. fluorescens (RS1) were evaluated for promoting growth of rice under natural climate conditions. Pot-scale experiments with zinc solubilizing P. aurantiaca GS3 and GS7 showed significant increase in dry shoot and root weights of inoculated plants as compared to un-inoculated controls. Pot experiments with potassium-solubilizing pseudomonads including P. aurantiaca PBSt2, ARS38, GS3, GS4, GS7, P. chlororaphis RP4, and P. fluorescens RS1 showed increased biomass and weight of tillers, as compared to control plants. Maximum zinc concentration was recorded for the shoots and tillers of P. aurantiaca GS3-inoculated plants. However, maximum potassium concentration in shoots, roots, and tillers of rice plants was observed for P. aurantiaca ARS38-inoculated plants. A significant increase in plant biomass and weight of tillers was attributed to increased nutrient mobilization by biofertilizing Pseudomonas spp. Successful nutrient uptake and increased grain yield of rice suggest the use of biofertilizing pseudomonads to lessen the environmental burden of chemical pesticides and enhance plant productivity. (c) 2024 SAAB. Published by Elsevier B.V. All rights reserved.