Abstract
To avoid the use of agrochemicals in agriculture, alternative methods are emerging to control plant pathogens. Some plant growth-promoting rhizobacteria (PGPR) can be used as biocontrol agents since they can induce protection against pathogens. The aim of this study was to evaluate the protective effect of several PGPR strains against the main Fusarium species involved in asparagus decline syndrome (ADS): F. proliferatum, F. oxysporum f. sp. asparagi and F. redolens. In vitro antagonism assays showed that all the bacteria inhibited the mycelium growth of the three Fusarium species. The most effective strains (Streptomyces fradiae Hvs6, Bacillus paralicheniformis Hvs2 and Bacillus velezensis FC37) were tested to evaluate their protective effect on asparagus plants inoculated with pathogenic Fusarium isolates. Strains FC37 and Hvs2 were the most effective in controlling pathogenic F. proliferatum and F. oxysporum f. sp. asparagi, but neither could protect against F. redolens isolates. The production of hydrolytic enzymes such as β-glucosidase, amylase and protease by these bacterial strains could be involved in the structural degradation of the fungal cell wall. In addition, the production of toxic volatile compounds, such as hydrogen cyanide, may inhibit the fungal growth, and the production of phosphate solubilizers could be related to the plant growth promotion. These results suggest that strains FC37 and Hvs2 could be used as potential biocontrol agents as a sustainable and environmentally friendly control strategy for ADS-affected fields.
Highlights
Asparagus (Asparagus officinalis L.) is a highly important crop worldwide
The objective of this work was to evaluate the ability of five rhizospheric bacterial strains of the species Bacillus amyloliquefaciens, B. paralicheniformis, B. velezensis, Brevibacterium frigoritolerans and Streptomyces fradiae in the control of the main Fusarium species associated with Asparagus decline syndrome (ADS) (F. proliferatum, F. oxysporum f. sp. asparagi and F. redolens), through both in vitro and in planta assays, and to propose possible mechanisms of action of the bacterial strains by characterizing the antifungal metabolites and growth promoter compounds produced by the biocontrol strains
The strain Hvs6 significantly showed the highest percentage of fungal growth inhibition (81–84%) for all tested isolates of F. proliferatum, F. oxysporum f. sp. asparagi and F. redolens
Summary
Asparagus (Asparagus officinalis L.) is a highly important crop worldwide. Asparagus decline syndrome (ADS) is one of the most important diseases of asparagus in most producing areas [3]. Fields crops with previous ADS symptoms display replanting problems, which imply premature uprooting of plants and prevents affected fields from being planted with asparagus. Several factors have been suggested to be involved in ADS, including abiotic factors, such as the accumulation in the soil of allelopathic compounds produced by the asparagus plant [4,5], the accumulation of herbicide residues, nutrient starving or the alteration of the soil structure. ADS has been closely associated with the presence of pathogenic species of Fusarium in the soil. Asparagi and F. redolens were the main Fusarium species associated with ADS in Spain [6,7] F. oxysporum f. sp. asparagi and F. redolens were the main Fusarium species associated with ADS in Spain [6,7]
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