###### 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. proliferation_, _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 (_Streetomyces fradiae_ Hv86, _Bacillus paralicheniformis_ Hv82 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. proliferation_ and _F. oxysporum_ f. sp. _asparagi_, but neither could protect against _F. redolens_ isolates. The production of hydrolytic enzymes such as \(\beta\)-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. Fusarium proliferation; F. oxysporum; enzymatic activity; Bacillus parallelism; Bacillus vallezensis; biocontrol; PGPR; hydrogen cyanide; phosphate solubilization +
Footnote †: journal: Applied Sciences  
residues, nutrient starving or the alteration of the soil structure. Among biotic factors, ADS has been closely associated with the presence of pathogenic species of _Fusarium_ in the soil. _Fusarium_ _proliferation_, _F. oxysporum_ f. sp. _asparagi_ and _F. redolens_ were the main _Fusarium_ species associated with ADS in Spain [6; 7]. The control of ADS is challenging because the causal agents can survive for long time in the soil and plant debris and can be disseminated by nursery plant material and irrigation water [8; 9]. In addition, the commercial varieties have demonstrated high susceptibility to _Fusarium_ infection, and tolerant or resistant varieties are not currently available. Chemical soil disinfestation prior to planting is one of the most used control strategies for ADS. However, the drastic decrease in the use of chemical fungicides applied to agriculture, in accordance to the European strategy \"Farm to fork\", aimed to prevent fungicide resistance and to avoid damages to human health and the environment, which has prompted the search for more sustainable alternatives. Biological control has emerged as one of the most sustainable control strategies. Fungal-based biological control agents such as non-pathogenic strains of _F. oxysporum_[10; 11], arbuscular mycorrhizala [12], _Trichoderma hurzianum_[13], and endophytic [14] and rhizospheric [15] fungi have been used for achieving tolerance to _Fusarium_ root rot of asparagus. Regarding bacteria, _Streptomyces griseus_ could control _Fusarium_ diseases of asparagus _in vitro_, via antibiotic production [16], and species of _Pseudomonas_ and _Serratia_ could suppress _Fusarium_ crown and root rot of asparagus [17]. Another asparagus disease, such as _Phytophthora_ rot of asparagus, could also be controlled by the use of rhizosphere bacteria [18]. In this sense, the use of bacterial strains isolated from the soil or the rhizosphere of healthy plants as biological control agents has allowed reducing the incidence and severity of multiple fungal plant diseases.