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Anna Kisiel
University of Szczecin, Poland
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Abstract
Progressive degradation of the environment caused, among others, by the excessive use of chemical pesticides forces us to look for alternative methods of protecting crops against pathogens. Definitely beneficial for the environment, but also, as confirmed by numerous studies, the use of biological control mechanisms can be an effective solution. The bacteria can inhibit the growth of fungi through the production of enzymes that lyse their cell walls, such as chitinases and glucanases, but also by limiting the availability of microelements important to growth, such as iron, by chelating them to siderophores, and finally producing antibiotics and secondary metabolites, like 2,4-DAPG.

The present study examined 16 strains of bacteria isolated from Medicago sativa rhizosphere for their suitability for the control of fungal pathogens such as Fusarium culmorum and Phoma medicaginis. Among of bacteria were strains belonging to the family Bacillaceae (genus Bacillus, Lysinibacillus and Paenibacillus), the family Pseudomonadaceae (genus Pseudomonas), the family Xantomonadaceae (genus Stenotrophomonas) of the Enterobacteriaceae family (genus Citrobacter, Leclercia and Raoultella) and of the family Rhizobiaceae (genus Sinorhizobium).

In vitro, both on solid and liquid media, all bacterial strains were able to limit the growth of Fusarium culmorum and Phoma medicaginis. The effective inhibitors of Fusarium culmorum Cul-3 were Roultella planticola KK 8a, Pseudomonas corrugata KK 12, and both strains belonging to Stenotrophomonas maltophilia KK 8b and KK 9b. Limiting the development of the second Medicago pathogen, Phoma medicaginis, was the most effective after the use of Pseudomonas corrugata KK 7, Leclercia adecarboxylata KK 6 and Pseudomonas brassicacearum KK 5.

By studying the mechanisms that may be potential for bacteria to inhibit the growth of fungi was tested production of lytic enzymes the cell walls, and siderophores. All bacterial strains showed β-1,3-glucanase activity in the range of 2.3 to 72.5 μmol glucose /mg protein/min. Five strains showed chitinolytic activity, with Stenotrophomonas maltophilia KK 8b being the most active. Thirteen of the 16 analyzed strains of rhizobacteria were able to produce iron chelating compounds, siderophores. In addition, the presence of the phlD gene was confirmed in Pseudomonas brassicacearum KK 5 and P. corrugata KK 12, which may indicate the production of 2,4-DAPG.

The results obtained indicate that the bacteria isolated from the rhizosphere have the potential for biological control of fungal pathogens, which limit the plant cultivation, using various mechanisms. The use of rhizobacteria as biopesticides is an environ­mentally friendly alternative to chemical plant protection products.

Keywords 
fungal pathogens, biological control, rhizobacteria

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