The aim of this study is to find bacterial strains with antagonistic effects against Fusarium oxysporum f. sp. lycopersici ( Fol) and Ascochyta sp, which are phytopathogens responsible for fusarium wilt of tomato and ascochyta blight of peas, respectively. One hundred thirty- six bacteria isolated from the rhizosphere of tomatoes were screened. Five strains with the largest inhibition zones were selected. These strains were identified by the phenotypic method, later confirmed by sequencing of 16S rDNA. All strains belonged to the genus Bacillus spp. Their inhibition capacity was evaluated by the direct method by doing a dual culture, the inhibition rates ranged from 44.32 ± 0.8 to 61.36 ± 0.2 against Fol and 62.04 ± 0.8 to 74.1 ± 0.2% against Ascochyta sp. They were then evaluated by the indirect method by evaluating, on one hand, the antifungal effect of the volatile compounds produced by the strains and on the other hand, the antifungal effect of the filtrates. The results showed that volatile compounds inhibited plant pathogens’ growth with average inhibition rates of 55% against Fol and 17% against Ascochyta. For filtrates, the average inhibition rates were 33.01% against Fol and 33.74% against Ascochyta sp. Finally, the plant growth promoting rhizobacteria (PGPR) effect of B. halotolerans RFP57 was evaluated. This involved assessing their ability to stimulate the germination of tomato seeds and the growth of their vegetative organs. The results showed significant improvement in treated seedlings compared to controls. All these results show that the strains selected for this study have the potential for use as a biocontrol agent. However, it is clear that further in-depth studies are needed to confirm their true potentiality.

The purpose of the studies conducted in the years 1996-1998 was to determine the quantitative and qualitative composition of bacterial and fungal communities in rhizosphere of soybean cultivated in monoculture and non-rhizosphere soil. Besides, the proportion of bacteria and fungi, which were distinguished by their antagonistic effect towards soil-borne pathogens was established. A microbiological analysis of lg of dry weight of soil from rhizosphere of soybean resulted in 3.21 x I ()6 to 8.70 x 106 bacterial colonies and from 70.51 x I 03 to 123.74 x 103 fungal colonies. In the case of non-rhizosphere soil, 3.50 x 106 to 4.75 x 10" bacterial colonies and 16.16 x 10' to 51.38 x 10' fungal colonies were obtained. Besides, soybean cultivation in monoculture had a negative effect on the number of antagonistic isolates of bacteria (Bacillus spp., Pseudomonas spp.) and fungi (Gliocladium spp., Penicillium spp., Trichoderma spp.). Smaller numbers of antagonistic bacteria and fungi in rhizosphere soil of soybean cultivated in monoculture as compared to non-rhizosphere soil, can prove little biological activity, which results in a worse phytosanitaty condition of the soil.

Plants can recognize molecules derived from pathogens and trigger systemic acquired resistance (SAR). In phytopathogenic bacteria, elicitors are constituent components of cellular structures, such as flagellin. We sought to select structural components of Xanthomonas spp. incompatible with tomato, aiming to control bacterial spot ( Xanthomonas perforans). Initially, cell suspensions from 11 Xanthomonas spp. isolates were infiltrated into the leaves to assess their ability to cause a hypersensitivity response (HR) and the incompatible ones had their flagellin purified. The flagellin of the isolates were first applied at different concentrations, via infiltration and spraying. The pathogen, X. perforans, was inoculated after 24 h, to assess whether there would be any harmful reaction. No harmful reaction was observed in any treatment. Then, a second experiment was conducted to assess the severity of all isolates, at a concentration of 8.35 μg · ml–1, via spraying, infiltration, and soil. The greatest reduction in Area Under the Disease Progress Curve (AUDPC) was observed in the treatment with XapRR, applied via spraying. Thus, prospecting for elicitors is the first step in developing a product for agricultural use. The flagellin elicitor of XapRR is promising and capable of producing these molecules on a large scale.

Several species of Solanum produce secondary metabolites with antimicrobial activity. In

the present study, the inhibitory activity of Solanum chrysotrichum, S. erianthum, S. torvum

and S. rostratum against phytopathogenic Curvularia lunata was determined. Methanol extracts

from roots, stems, leaves and fruits were evaluated by the method of mycelial inhibition

on agar and the minimum inhibitory concentration (MIC) was determined on a liquid

medium. To increase the antimicrobial activity, the combined activity of the most active

extracts for each phytopathogen was also determined (a combination of intra and interspecies

extracts). The results showed that 12 of the 16 methanolic extracts of Solanum species

had antifungal effects against C. lunata. The extracts of S. rostratum and S. erianthum

developed the highest activity (~80% inhibition and 28.4 MIC μg . ml–1), even, equal to or

greater than, the reference fungicide. The mixture of the active extracts of S. chrysotrichum

and S. torvum increased their activity. Various extracts affected the macro and microscopic

morphology and most of them reduced the number of conidia of the fungus. This resulted

in the capacity to control the vegetative growth and reproduction of C. lunata, the causal

fungus of corn leaf spot disease.

The purpose of the presented studies was to determine the species composition of the fungi occurring on the underground parts of potato at an thesis and to establish the qualitative and quantitative composition ofmicroorganisms living in the soil environment of this plant. Besides, the studies aimed at finding antagonistic microorganisms inhibiting potato infection by soil-borne phytopathogens. The mycological analysis showed that the symptoms of necrosis on the roots and the stem base of potato were caused by Alternaria alternata, Fusarium spp., Colletotrichum coccodes and Rhizoctonia so/ani. The microbiological analysis of the potato rhizosphere gave twice as many bacteria and fungi as from the non-rhizosphere soil. The dominating pathogenic fungi in the examined soil samples were Fusarium spp., A. alternata and R. so/ani. Three times as many antagonistic bacteria Pseudomonas spp. and Bacillus spp. and more than twice as many antagonistic fungi (Penicillium spp., Trichoderma spp., Gliocladium spp.) were obtained from the potato rhizosphere as compared to the non-rhizosphere soil.