## 4 Discussion  
Soil-borne fungal diseases cause worldwide economic losses in agriculture. Alternative control methods are desirable to avoid using chemical fungicides, which can potentially be harmful to the environment and human health. Numerous studies have been published about the ability of PGPR to control soil-borne fungal diseases in different crops [34; 35; 36], however there are few studies based on PGPR to control _Fusarium_ species involved in ADS, as far as we know. In this work, we have evaluated the potential of five PGP bacterial strains as antagonists and biological control agents of ADS-involved _Fusarium_ species. Bacteria belonging to genera _Bacillus_ (Adb14, Hvs2 and FC37), _Brevibacterium_ (Hvs8) and _Streptomyces_ (Hvs6) were selected because of their demonstrated biological control activity against different crop diseases tested earlier [37; 38; 39; 40]. _In vitro_ tests, through dual confrontation experiments, clearly showed that _S. fradiae_ Hvs6, _B. velezensis_ FC37 and _B. paralicheniformis_ Hvs2 were the most effective strains to inhibit the fungal radial growth of the pathogenic _Fusarium_ isolates, reaching from 73 to 84% inhibition. Interestingly, Hvs2 and FC37 were the bacterial strains that produced \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Bacterial Strain** & **Siderophore** & **Phosphate Solubilization** & **Auxin** \\ \hline Adb14 & + & - & - \\ FC37 & - & + & - \\ Hvs2 & - & - & - \\ Hvs6 & +++ & - & - \\ Hvs8 & - & - & + \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Plant growth promoting compounds produced by bacterial strains Adb14, FC37, Hvs2, Hvs6 and Hvs8. Figure 3: Percentage of biocontrol enzymatic activities displayed by strains Adb14, FC37, Hvs2, Hvs6, and Hvs8. the greatest diversity of cell wall degrading enzymes among the five strains tested. The production of cell wall degrading enzymes has been described as a mechanism used by biocontrol agents to control soil-borne pathogens [21; 41]. Enzymes such as \(\beta\)-glucosidase, chitinase, amylase and protease secreted by biocontrol strains of PGPR provoke a direct inhibitory effect on the mycelium growth of fungal pathogens by degrading their cell wall [42]. _In planta_ challenge assays suggested protection against pathogenic isolates of _F. proliferation_ and _F. oxysporum_ f. sp. _asparagi_ by the three bacterial strains. FC37 displayed the best protection effect, which could be related to the higher protease and amylase activities exhibited by this strain. In addition, plants inoculated with the FC37 presented higher root and shoot weights than non-inoculated plants or inoculated with the other two bacteria, suggesting that this strain could also stimulate the asparagus growth. _B. velexensis_ FC37 is the only tested strain able to solubilize phosphorus, suggesting that FC37 could protect the plant by both, limiting the growth of fungi by producing cell wall degrading-enzymes, and enhancing plant growth by converting unavailable forms of phosphorus into forms available for subsequent uptake by the plant. Furthermore, it was found that in addition to some fungal cell wall degrading enzymes [43], _B. velexensis_ strains can produce volatile compounds which can affect the growth of plant pathogenic fungi including _Fusarium_ spp. [44]. It could be possible that strain FC37 produces a volatile compound that it has not been tested in this work. In the same way, the mechanism of action of strain Hvs2 could be related to the production of hydrogen cyanide (HCN), which is a toxic volatile compound that affects the respiratory system of pathogenic fungi resulting in their growth inhibition [45]. However, whether volatile compounds play a role in biocontrol _in planta_ has not yet been confirmed. On the other hand, Hvs2 could effectively control the _F. proliferation_ isolates and the _F. oxysporum_ Fo14 isolate but not the _F. oxysporum_ Fo125 isolate. The ability of controlling pathogenic _Fusarium_ depends not only on the fungal species (e.g., _F.