Stem rot of groundnut caused by Sclerotium rolfsii, seriously damages groundnut production in central Vietnam. Biological control is a promising strategy for sustainable groundnut cultivation. In this study, indigenous bacteria were isolated from the rhizosphere of groundnut and tested for fungal inhibition against S. rolfsii in vitro and disease control under net house condition. Genetic diversity of isolated bacterial population was evaluated by BOX-PCR and 16S rDNA sequences. Bacterial strains that showed high disease control in net house were evaluated under natural conditions in farmer fields. The antifungal mechanism of the best bacterial strain was identified. Results of the study showed that the antagonistic bacterial population in groundnut rhizosphere is separated in three bacterial genera including Bacillus, Pseudomonas and Burkholderia. One bacterial strain which produces 2,4-DAPG reduced stem rot of groundnut caused by S. rolfsii and increased yield from 20.3 to 26.3% compared to the control.

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Summary This study aimed to evaluate the nematicidal activity of the Tithonia diversifolia leaf extract (TDE) against Meloidogyne incognita under in vitro and pot conditions. In addition, the antioxidant activity of TDE was evaluated using the 2,2-Diphenyl-2-picrylhydrazyl hydrate (DPPH) and 2,2′-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) assays. The results revealed that TDE possesses antioxidant potential. Nematicidal activity bioassays revealed that the TDE treatment resulted in 3.0, 16.3, 36.6, 52.4 and 64.2% second-stage juvenile (J2) hatch inhibition at MeOH control, 0.125, 0.25, 0.5 and 1.0 mg ml−1, respectively, for a 5-day treatment. The corresponding values for a 10-day treatment were 3.3, 16.0, 30.7, 56.0 and 69.3%, respectively. The TDE treatment resulted in 64.6, 86.4, 95.3 and 100% J2 movement inhibition at 0.125, 0.25, 0.5 and 1.0 mg ml−1, respectively, for a 48 h treatment. In pot experiments, at 60 days after treatment (DAT), the J2 populations in non-treated, 0.25, 0.5, 1.0 mg TDE (g soil)−1, and Tervigo 020SC 1.0 mg (g soil)−1 were 477.1, 277.3, 142.7, 23.9 and 19.7 per 5 g of root, respectively. The corresponding values in 100 g of soil were 33.2, 5.5, 3.6, 1.1 and 4.0, respectively. The relative galling formation on the Robusta coffee plants was 100, 55.3, 39.1, 33.5 and 80.8% in the non-treated, 0.25, 0.5, 1.0 mg TDE (g soil)−1, and Tervigo-treated plants, respectively. These results may provide crucial information for exploring the nematicidal metabolites of TDE against M. incognita. Therefore, the purified active compounds of TDE should be identified in future studies before the commercialisation of TDE for the biocontrol of M. incognita.