Groundnut (Arachis hypogaea) is an economically important legume crop in Vietnam and many other countries worldwide. Stem and pod rot, caused by the soilborne fungus Sclerotium rolfsii, is a major yield-limiting factor in groundnut cultivation. To develop sustainable measures to control this disease, fundamental knowledge of the epidemiology and diversity of S. rolfsii populations is essential. In this study, disease incidence was monitored in eight groundnut areas in central Vietnam with a total of 240 observational field plots. The results showed that 5 to 25% of the field-grown groundnut plants were infected by S. rolfsii. Based on internal transcribed spacer (ITS) ribosomal DNA sequence analyses, three distinct groups were identified among a total of 103 randomly selected S. rolfsii field isolates, with the majority of the isolates (n = 90) in one ITS group. S. rolfsii isolates originating from groundnut, tomato, and taro were all pathogenic on groundnut and relatively sensitive to the fungicide tebuconazole but displayed substantial diversity of various genetic and phenotypic traits, including mycelial compatibility, growth rate, and sclerotial characteristics.

Aims:  To determine the role of phenazines (PHZ) and lipopeptide surfactants (LPs) produced by Pseudomonas in suppression of stem rot disease of groundnut, caused by the fungal pathogen Sclerotium rolfsii. Methods and Results:  In vitro assays showed that PHZ‐producing Pseudomonas chlororaphis strain Phz24 significantly inhibited hyphal growth of S. rolfsii and suppressed stem rot disease of groundnut under field conditions. Biosynthesis and regulatory mutants of Phz24 deficient in PHZ production were less effective in pathogen suppression. Pseudomonas strains SS101, SBW25 and 267, producing viscosin or putisolvin‐like LPs, only marginally inhibited hyphal growth of S. rolfsii and did not suppress stem rot disease. In contrast, Pseudomonas strain SH‐C52, producing the chlorinated LP thanamycin, inhibited hyphal growth of S. rolfsii and significantly reduced stem rot disease of groundnut in nethouse and field experiments, whereas its thanamycin‐deficient mutant was less effective. Conclusions:  Phenazines and specific lipopeptides play an important role in suppression of stem rot disease of groundnut by root‐colonizing Pseudomonas strains. Significance and Impact of the Study:  Pseudomonas strains Phz24 and SH‐C52 showed significant control of stem rot disease. Treatment of seeds or soil with these strains provides a promising supplementary strategy to control stem rot disease of groundnut.

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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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