Soil-borne Plant Pathogens Research Articles

Long-term grazing effects on soil-borne pathogens are driven by temperature

Soils support a highly diverse community of plant pathogens, which are highly responsive to global change. Climate and livestock grazing are the main global changes in grasslands, yet, how long-term grazing alone, and in interaction with climate, influence the distribution of soil-borne plant pathogens remain virtually unknown. Here, we present the first long-term regional-scale experimental investigation on the impacts of livestock grazing on soil-borne fungal plant pathogens and their association with plant community across 10 experimental sites spanning a climate gradient in the steppe in Northern China. Our results showed that long-term grazing effects on the diversity and proportion of soil-borne fungal plant pathogens are strongly controlled by temperature, with grazing increasing pathogen richness and proportions largely in cooler grasslands. We further show that long-term grazing supported stronger connections between soil-borne fungal pathogens and plant communities. Our work demonstrates that climate controls the effects of grazing on plant pathogens, which is critical to understand and manage grasslands in a changing world.

Genome sequences for twenty-four Phytophthora and one Nothophytophthora species for the study of the evolution of pathogenicity in Peronosporaceae

Oomycetes classified in the genera Phytophthora and Nothophytophthora encompass a wide range of lifestyles, from saprotrophic and water-borne to destructive soil-borne or wind-dispersed plant pathogens. To clarify the evolutionary and genomic basis of these transitions, genome sequences for a wide range of species are necessary. However, the amount of genomes currently available is not representative of the full range of these lifestyles. To address this gap, we present draft genomes of 24 Phytophthora and one Nothophytophthora species, of which 14 have 100% BUSCO completeness for the Straminipila dataset. We hope that this resource will stimulate further research into the evolution of pathogenicity in Peronosporaceae.

Cultural and morphological characters of Trichoderma harzianum isolates and its antagonistic activity against soil borne plant pathogens

Cultural and morphological characters of Trichoderma harzianum isolates and its antagonistic activity against soil borne plant pathogens

Profiling the biocontrol agents, nitrogen-fixing bacteria, and indole acetic acid-producing bacteria in anthill soil

Due to the adverse ecological impacts of prolonged and excessive use of agrochemicals, many researchers have called for the adoption of eco-friendly materials in farming. Consequently, the aim of this study is to profile the biocontrol agents, nitrogen-fixing, and indole-acetic-acid-producing bacteria present in anthill soil. Anthill soils and adjacent soils were collected, and their physicochemical properties were analysed using standard analytical methods. Viable bacteria were isolated and screened for plant growth-promoting (PGP) activity using standard biochemical, morphological, and bacteriological methods. The PGP capacity of the isolates was evaluated using standard protocol for nitrogen fixation and indole-acetic-acid production, while antagonistic effect against plant pathogens was evaluated using the disk diffusion assay. Results revealed that Pseudomonas aeruginosa, Klebsiella pneumoniae, and Bacillus sp. isolated from the anthill soil demonstrated PGP characteristics including IAA production and nitrogen fixation. Bacillus sp. exhibited zones of clearance with values of 20.00 ± 1.50 mm, 20.00 ± 1.75 mm, 17.00 ± 1.33 mm, and 17.00 ± 1.33 mm against Aspergillus niger, Trichoderma sp., Penicillium sp., and Fusarium sp., respectively. This study demonstrated that anthill soils contain beneficial microbes with the potential to stimulate plant growth and suppress soil-borne plant pathogens.

Pyricularia oryzae enhances Streptomyces griseus growth via non-volatile alkaline metabolites.

Chemical compounds that affect microbial interactions have attracted wide interest. In this study, Streptomyces griseus showed enhanced growth when cocultured with the rice blast fungus Pyricularia oryzae on potato dextrose agar (PDA) medium. An improvement in S. griseus growth was observed before contact with P. oryzae, and no growth-promoting effect was observed when the growth medium between the two microorganisms was separated. These results suggested that the chemicals produced by P. oryzae diffused through the medium and were not volatile. A PDA plate supplemented with phenol red showed that the pH of the area surrounding P. oryzae increased. The area with increased pH promoted S. griseus growth, suggesting that the alkaline compounds produced by P. oryzae were involved in this growth stimulation. In contrast, coculture with the soilborne plant pathogen Fusarium oxysporum and entomopathogenic fungus Cordyceps tenuipes did not promote S. griseus growth. Furthermore, DL-α-Difluoromethylornithine, a polyamine biosynthesis inhibitor, prevented the increase in pH and growth promotion of S. griseus by P. oryzae. These results indicated that P. oryzae increased pH by producing a polyamine.

Isolation, identification and multi-locus sequence typing of Phytophthora capsici from capsicum fields and its cross-infectivity in different crop species

Phytophthora capsici is a major soil-borne oomycete plant pathogen, which severely affects the productivity of peppers including Capsicum annuum L., by causing fruit rot, root rot, and foliar blight diseases. It has a broad host range beyond Capsicum species and evolves rapidly to find new potential hosts for survival under different climatic conditions. In the present study, P. capsici was isolated from soil using castor seed baiting, and characterized through growth patterns and spore morphology. The species identification and molecular characterization were achieved through multi-locus sequence and phylogenetic analysis of three genomic loci (ITS, tef1 and cox), and also attributed given P. capsici isolate Pc_UHF into India-specific ITS-based evolutionary lineage Pc-X. Interestingly, dN/dS estimates revealed reasonably high positive selection in the tef1 gene, which could have possible implications on the pathogen's adaptation and/or virulence. Pathogenicity of this isolate was tested in three commercially growing bell pepper varieties and also in other crop species using detached leaf and/or in planta disease screening. Differential disease symptoms were induced by Pc_UHF in three bell pepper varieties, such as both collar rot and foliar blight symptoms observed in "Yolo Wonder" and "California Wonder", whereas only foliar blight symptoms were recorded in "Solan Bharpur". Furthermore, cross-pathogenicity was also recorded in tomato, citrus, cucumber and eggplant. This is the first report of molecular characterization and cross-pathogenically assessment of P. capsici isolate from farm soils in the North Western Himalayan region of India, which has a dynamic virulence profile and distinct host range.

Effect of integration of eco-friendly components for the management of stem rot of berseem caused by Sclerotinia spp under in vivo conditions

Stem rot of berseem caused by Sclerotinia spp. is the most devastating and cosmopolitan plant disease, infecting more than 400 species of plants worldwide. The disease is reported to cause by a soil-borne plant pathogen and is responsible for significant amount of green fodder and seed yield losses in the forage crop. Integrated disease management is the practice of using a range of measures to prevent and manage diseases in crops. Total seven different combinations of eco-friendly components were integrated for their effect on the management of stem rot of berseem caused by Sclerotinia spp. under in vivo conditions. The experiment was conducted in a polyhouse using sick soil pots. Out of them, treatment T7 (TRN-10 + Eupatorium adenophorum + panchgavya + ghanjeevamrit) was found the most promising integration for the management of the stem rot disease, with terminal disease severity (13.17%) & AUDPC value (177.33) and maximum disease control (75.00%) after 35 days of inoculation. Thus it was inferred that the integration of various eco-friendly management components proved to be most effective in reducing the disease severity of stem rot of berseem.. KEYWORDS :Berseem, fodder, stem rot, Sclerotinia spp. , yield loss

Genus-wide analysis of Trichoderma antagonism toward Pythium and Globisporangium plant pathogens and the contribution of cellulases to the antagonism.

Parasitism is an important lifestyle in the Trichoderma genus but has not been studied in a genus-wide way toward Pythium and Globisporangium hosts. Our approach screened a genus-wide set of 30 Trichoderma species in dual culture assays with two soil-borne Pythium and three Globisporangium plant-parasitic species and used exo-proteomic analyses, with the aim to correlate Trichoderma antagonism with potential strategies for attacking Pythium and Globisporangium. The Trichoderma spp. showed a wide range of antagonism from strong to weak, but the same Trichoderma strain showed similar levels toward all the Pythium and Globisporangium species. The Trichoderma enzymes from strong (Trichoderma asperellum, Trichoderma atroviride, and Trichoderma virens), moderate (Trichoderma cf. guizhouense and Trichoderma reesei), and weak (Trichoderma parepimyces) antagonists were induced by the autoclaved mycelia of one of the screened Pythium species, Pythium myriotylum. The variable proportions of putative cellulases, proteases, and redox enzymes suggested diverse as well as shared strategies amongst the antagonists. There was a partial positive correlation between antagonism from microscopy and the cellulase activity induced by autoclaved P. myriotylum mycelia in different Trichoderma species. The deletion of the cellulase transcriptional activator XYR1 in T. reesei led to lower antagonism toward Pythium and Globisporangium. The antagonism of Pythium and Globisporangium appears to be a generic property of Trichoderma as most of the Trichoderma species were at least moderately antagonistic. While a role for cellulases in the antagonism was uncovered, cellulases did not appear to make a major contribution to T. reesei antagonism, and other factors are also likely contributing.IMPORTANCETrichoderma is an important genus widely distributed in nature with broad ecological impacts and applications in the biocontrol of plant diseases. The Pythium and Globisporangium genera of fungus-like water molds include many important soil-borne plant pathogens that cause various diseases. Most of the Trichoderma species showed at least a moderate ability to compete with or antagonize the Pythium and Globisporangium hosts, and microscopy showed examples of parasitism (a slow type of killing) and predation (a fast type of killing). Hydrolytic enzymes such as cellulases and proteases produced by Trichoderma likely contribute to the antagonism. A mutant deficient in cellulase activity had reduced antagonism. Interestingly, Pythium and Globisporangium species contain cellulose in their cell walls (unlike true fungi such as Trichoderma), and the cellulolytic ability of Trichoderma appears beneficial for antagonism of water molds.

Evaluation and Characterization of Rhizobacteria and Endophytes for Antagonistic and Growth Promotional Traits Against Collar Rot (Sclerotium rolfsii Sacc.) of Soybean

Background: Sclerotium rolfsii Sacc. is one of the destructive soil-borne plant pathogens due to its significant loss in terms of yield and quality in several crop plants including soybean. Use of biocontrol agents in the management of disease is an ecofriendly strategy. Therefore, the study aimed to assess in vitro antagonisticactivity of rhizobacteria and endophytes against Sclerotium rolfsii following dual culture technique. Methods: A total of 75 rhizobacterial isolates, 3 endophytes along with reference cultures were evaluated for the antagonistic activity against Sclerotium rolfsii following dual culture technique. The effective rhizobacterial isolates were tested for the various plat growth promoting rhizobacteria traits viz., indole acetic acid production, siderophore production, Phosphate solubilization, production of ammonia, hydrogen cyanide production and chitinase test following the standard procedure. Result: The two promising rhizobacterial strains i.e., AUUB 209 (Streptomyces enissocaesilis) and Streptomyces racemochromogenes (AUDT 626) were found effective with 76.79 and 64.75 percent mycelial inhibition of Sclerotium rolfsii under in vitro. The antagonistic activity of these two rhizobacteria is mainly attributed to the positive confirmation of various PGPR activities viz., production of Indole acetic acid, siderophore, phosphate solubilization and chitinase production.

GC-MS analysis and antifungal potential of flower extract of Acacia nilotica subsp. Indica against Macrophomina phaseolina

Macrophomina phaseolina is a wide host ranged soil-borne fungal plant pathogen. It infects more than 500 host plant species belonging to 100 families. Many important oil-seed and leguminous crops are known to be attacked by this devastating plant pathogen. In the present study, antifungal potential of flowers of a leguminous tree Acacia nilotica subsp. indica, was assessed against this pathogen through bioassays guided fractionation. Initially, methanolic extracts of 1 %–5 % of leaf, flower, root-bark and stem-bark of the plant species under consideration were evaluated for their antifungal potential against the target pathogen. Among these, the best antifungal activity was shown by flower extract. The reduction in growth of the test fungal strain was 27–49 %, 4–40 % and 2–27 % due to flower, root-bark and leaf extracts, respectivey, over control. Flower extract was partitioned using n-hexane, chloroform, ethyl acetate and n-butanol as the solvents. Bioassays guided study of these fractions of methanolic extract of flower revealed that high antifungal potential was shown by n-hexane and chloroform fractions against M. phaseolina causing 26–53 % and 28–50 % decline in fungal biomass, respectively, as compared to that of control. GC-MS analysis of chloroform fraction revealed the presence of 27 compounds in this fraction. Among these cyclopentanol,-1-methyl (10.93 %) was the predominant compound followed by methyl, 4,4-dimethyl butanoate (7.04 %), 1-pentanol (6.80 %), 2-propanol, 1-cyclopropyl (6.11 %), 1H,imidazole-4-5-dihydro-2-methyl (5.93 %), trichloroethane (5.91 %), carbonic acid-ethyl hexyl ester (4.59 %), 1,4-butandiol,2,3-bis(methylene)- (4.54 %) and [S]-3,4-dimethyl pentanol (4.48 %).

Towards sustainable biocontrol: inhibition of soil borne fungi by microalgae from harsh environments.

Using microorganisms as biocontrol agents against soilborne plant pathogens is a promising alternative to chemical pesticides. However, only some biocontrol agents have proven effective under field conditions. This study explores the potential of highly resilient microalgae isolated from harsh environments, such as Biological Soil Crusts and agricultural fields in semi-arid regions, as a novel and sustainable approach to biocontrol. Fifty-nine microalgal strains, including thirteen cyanobacteria and forty-six green algae, were isolated and identified. Dual-culture plate assays and toxicity tests of microalgal growth media were conducted to evaluate the antifungal activity of the isolates against eight representative soilborne pathogens. The results showed that many microalgae strains exhibited significant inhibitory effects on the growth of specific fungal pathogens, although the activity varied among different microalgal strains and pathogen species. Some strains even promoted the growth of certain fungi. The lack of a clear pattern in the antifungal activity highlights the complexity and specificity of the interactions between microalgae and soilborne pathogens. An "Inhibition Effectiveness" metric was developed to quantify biocontrol potential based on fungal growth inhibition. The green algal genus Desmodesmus, particularly Desmodesmus subspicatus isolates, showed higher antifungal efficacy than other genera. While the inhibitory mechanisms remain unclear, the results demonstrate the promising biocontrol capabilities of microalgae from extreme environments like BSCs. Further research could unlock novel opportunities for sustainable disease management by harnessing specific microalgal strains or synergistic strain combinations targeting soilborne pathogens.

Insights for Soil Improvements: Unraveling Distinct Mechanisms of Microbial Residue Carbon Accumulation under Chemical and Anaerobic Soil Disinfestation

Soil disinfestation has been widely used as an effective strategy to improve soil health and crop yield by suppression of soil-borne plant pathogens, but its effect on soil organic carbon (SOC), a crucial factor linked to climate change, remains unknown. A microcosm trial was conducted to evaluate microbial residue carbon (MRC) and its contribution to SOC under chemical soil disinfestation (CSD) with quicklime (QL) and chloropicrin (CP), as well as anaerobic soil disinfestation (ASD) with maize straw (MASD) and soybean straw (SASD). The SOC concentrations were increased by both CSD and ASD. Also, total SOC-normalized MRC concentration was enhanced, with a considerable increase in soil bacterial and fungal MRC, particularly evident under CP and SASD treatment. Due to broad-spectrum biocidal activities, decreased SOC-normalized microbial biomass carbon (MBC) was consistent with the reductions in bacterial and fungal phospholipid fatty acids (PLFAs), consequently increasing MRC accumulation under CSD. Similarly, ASD decreased fungal PLFAs while shifting bacterial PLFAs from aerobic to anaerobic taxa or from gram-negative to -positive taxa, both of which contributed to both MBC and MRC buildup. Collectively, the findings demonstrate that ASD can efficiently increase SOC concentration, with distinct mechanisms underlying MRC generation when compared to traditional CSD.

Clostridium omnivorum sp. nov., isolated from anoxic soil under the treatment of reductive soil disinfestation.

Reductive soil disinfestation (RSD), also known as biological soil disinfestation, is a bioremediation method used to suppress soil-borne plant pathogens by stimulating the activity of indigenous anaerobic bacteria in the soil. An anaerobic bacterial strain (E14T) was isolated from an anoxic soil sample subjected to RSD treatment and then comprehensively characterized. Cells of the strain were Gram-stain-positive, curved to sigmoid, and spore-forming rods. Cells were motile with a polar flagellum. Strain E14T grew in peptone-yeast extract broth, indicating that it utilized proteinous compounds. Strain E14T was also saccharolytic and produced acetate, isobutyrate, butyrate, isovalerate and gases (H2 and CO2) as fermentation products. The strain did not decompose any of examined polysaccharides except for starch. The major cellular fatty acids of strain E14T were iso-C15:0 and iso-C15:0 DMA. The closest relative to strain E14T, based on 16S rRNA gene sequences, was Clostridium thermarum SYSU GA15002T (96.2 %) in the Clostridiaceae. Whole-genome analysis of strain E14T showed that its genome was 4.66 Mb long with a genomic DNA G+C content of 32.5 mol%. The average nucleotide identity (ANIb) between strain E14T and C. thermarum SYSU GA15002T was 69.0 %. The presence of the genes encoding glycolysis and butyrate production via the acetyl-CoA pathway was confirmed through genome analysis. Based on the obtained phylogenetic, genomic and phenotypic data, we propose that strain E14T should be assigned to the genus Clostridium in the family Clostridiaceae as Clostridium omnivorum sp. nov. The type strain is E14T (=NBRC 115133T=DSM 114974T).

Identifying walnut rootstocks with resistance to multiple soil-borne plant pathogens

Identifying walnut rootstocks with resistance to multiple soil-borne plant pathogens

Soil and Biochar: Attributes and Actions of Biochar for Reclamation of Soil and Mitigation of Soil Borne Plant Pathogens

Soil and Biochar: Attributes and Actions of Biochar for Reclamation of Soil and Mitigation of Soil Borne Plant Pathogens

Genetic diversity and antagonistic properties of Trichoderma strains from the crop rhizospheres in southern Rajasthan, India

There are fewer studies on Trichoderma diversity in agricultural fields. The rhizosphere of 16 crops was analyzed for Trichoderma species in 7 districts of Rajasthan state of India. Based on DNA sequence of translation elongation factor 1α (tef-1α), and morphological characteristics, 60 isolates were identified as 11 species: Trichoderma brevicompactum, species in Harzianum clade identified as T. afroharzianum, T. inhamatum, T. lentiforme, T. camerunense, T. asperellum, T. asperelloides, T. erinaceum, T. atroviride, T. ghanense, and T. longibrachiatum. T. brevicompactum is the most commonly occurring strain followed by T. afroharzianum. No new species were described in this study. T. lentiforme, showed its first occurrence outside the South American continent. The morphological and cultural characteristics of the major species were observed, described, and illustrated in detail. The isolates were tested for their antagonistic effect against three soilborne plant pathogens fungi: Sclerotium rolfsii, Rhizoctonia solani, and Fusarium verticillioides in plate culture assays. One of the most potent strains was T. afroharzianum BThr29 having a maximum in vitro inhibition of S. rolfsii (76.6%), R. solani (84.8%), and F. verticillioides (85.7%). The potential strain T. afroharzianum BThr29 was also found to be efficient antagonists against soil borne pathogens in in vivo experiment. Such information on crop selectivity, antagonistic properties, and geographic distribution of Trichoderma species will be beneficial for developing efficient Trichoderma-based biocontrol agents.

Potential of different species of actinobacteria in the management of Meloidogyne javanica.

Root-knot nematodes (RKN) are one of the most harmful soil-borne plant pathogens in the world. Actinobacteria are known phytopathogen control agents. The aim of this study was to select soil actinobacteria with control potential against the RKN (Meloidogyne javanica) in tomato plants and to determine mechanisms of action. Ten isolates were tested and a significant reduction was observed in the number of M. javanica eggs, and galls 46days after infestation with the nematode. The results could be explained by the combination of different mechanisms including parasitism and induction of plant defense response. The M. javanica eggs were parasited by all isolates tested. Some isolates reduced the penetration of juveniles into the roots. Other isolates using the split-root method were able to induce systemic defenses in tomato plants. The 4L isolate was selected for analysis of the expression of the plant defense genes TomLoxA, ACCO, PR1, and RBOH1. In plants treated with 4L isolate and M. javanica, there was a significant increase in the number of TomLoxA and ACCO gene transcripts. In plants treated only with M. javanica, only the expression of the RBOH1 and PR1 genes was induced in the first hours after infection. The isolates were identified using 16S rRNA gene sequencing as Streptomyces sp. (1A, 3F, 4L, 6O, 8S, 9T, and 10U), Kribbella sp. (5N), Kitasatospora sp. (2AE), and Lentzea sp. (7P). The efficacy of isolates from the Kitasatospora, Kribbella, and Lentzea genera was reported for the first time, and the efficacy of Streptomyces genus isolates for controlling M. javanica was confirmed. All the isolates tested in this study were efficient against RKN. This study provides the opportunity to investigate bacterial genera that have not yet been explored in the control of M. javanica in tomatoes and other crops.

Farmed Insect Frass as a Future Organic Fertilizer

The aim of this incubation experiment was to evaluate the effect of Tenebrio molitor L. frass on selected chemical and microbiological properties of acid peat. The optimal rate of mealworm frass in the substrate for growing ornamental trees and shrubs was determined. Acid peat was fertilized with frass or urea at five nitrogen (N) rates: 0, 50, 100, 200, and 400 mg dm−3. Mineral N content and electrical conductivity increased, and calcium content decreased in peat with increasing N rates. Similarly to urea, frass increased the ammonification rate at the beginning of incubation and the nitrification rate from the second week of incubation. Higher frass rates increased the abundance of plant-available nutrients (N, P, Mg, K, and Na) in acid peat. Frass undesirably decreased the counts of bacteria with antagonistic activity against soil-borne plant pathogens. Regarding the abundance of functional genes, the optimal N rate was 100 mg dm−3, which promoted the growth of N-fixing and chitinolytic bacteria. Higher N rates promoted the development of aerobic spore-forming bacteria, which produce antibiotics that can be used as biocontrol agents. Moderate fertilizer rates contributed to N accumulation in bacterial biomass. These preliminary findings, which indicate that insect frass can partially replace mineral fertilizers, are promising and can be used in pot and field experiments testing various plant species.

Diversity Studies of Sclerotium rolfsii Sacc. Isolates Associated with Collar Rot of Soybean Through Molecular and Biochemical Approaches

Background: Sclerotium rolfsii Sacc. is one of the potential soil-borne plant pathogens due to its significant loss in terms of yield and quality in several crop plants. In soybean the pathogen is associated with collar rot disease. Methods: Sixteen samples which were showing collar rot symptoms were collected from different soybean growing regions of the country and were subjected for isolation and purification. Molecular detection was carried out using ITS1/4 primers followed by phylogenetic analysis was performed using the MEGA 11 bioinformatics tool. Further, the biochemical test was made for oxalic acid production to determine the variability across the isolate. Result: DNA was amplified for ITS rDNA region at 650-700 bp in all the sixteen isolates. BLAST detection of nucleotide sequence of sixteen isolates framed mainly into two distinct phylogenetic clusters. Cluster B consisted of two isolates (MPSe 1 and MHSa 3) whereas, Cluster A contains the remaining fourteen isolates. There was a variation with respect to oxalic acid production among the isolates which was in the rage of 1.15 to 2.23 mg/ml.

Супресивен ефект на почвата към ризоктонийно кореново гниене, индуциран чрез имитирана монокултура на пшеница

Disease decline and induction of soil suppressiveness by continuous (5-6 years) monoculture with susceptible plant-host have been reported for various crop-pathogen systems. The present study achieves the goal of inducing suppressive effect in soil towards Rhizoctonia root rot (Rhizoctonia solani AG 4) through monoculture of winter wheat (Triticum aestivum L.). Twenty four high-quality wheat (c. Exotic) seeds were planted in plastic containers, each containing 1.1 L Alluvial-Meadow (sandy-clay) soil (Eutric Fluvisols, according to FAO classification). According to the treatment applied, the resulting plants were grown for 0-4 successive short cropping cycles, each lasting three months. Disease suppressiveness was measured in a bioassay in which the experimental units were rectangular, plastic containers (26 x 6 x 8 cm), with drainage holes in the bottom and detachable trays allowing for manual irrigation from the bottom of the container, 7 cm height of the column of tested soil in each container and a two-row crop of wheat (c. Exotic, 2 rows x 12 plants, 2 x 3 cm plant spacing). The spread of the disease was measured from a focus (point) source of inoculum of the pathogen – 20 inoculum grains from a pure culture developed on autoclaved (30 min at 121 °C) barley, introduced into the soil at the beginning of both rows after emergence of the plants. Immediately after termination of the soil suppressiveness test, the abundance of the major groups of soil microorganisms were determined in rhizosphere soil in each treatment by using the method of decimal dilutions and subsequent culturing on agar media. Separately, bacteria of the genus Pseudomonas were isolated from the rhizosphere soil on King’s medium. The antagonistic activity of 10 Pseudomonas spp. isolates from each treatment was tested against R. solani AG 4 in vitro by the double culture method. The monoculture of wheat imitated under controlled conditions resulted in a significant (P<0.05) reduction of the spread of Rhizoctonia root rot from the source of infection. Furthermore, statistically significant differences (P<0.05) were also found between the individual treatments with pre-cultivation of wheat: with the increase in the number of cultivation cycles from one to four, there was a significant reduction in disease spread – by 19.5%, 31.0%, 40.5% and 54.8%, respectively, as compared to the untreated control. Simultaneously, no substantial alterations were observed in the population densities of the monitored groups of microorganisms in the rhizosphere of the experimental plants, including ammonifying and spore-forming bacteria, microscopic fungi, Actinomycetes and bacteria, utilizing mineral N. With the increase in soil suppressiveness from the first to the fifth growing cycle of the crop, a significant increase in the average antagonistic activity of the isolated Pseudomonas spp. against R. solani AG 4 was observed. The thesis is argued that shattering rachis of the ripe ear in wild forms conditioned a continuous “natural” monoculture in which bread wheat and its predecessors grew and evolve during the prehistoric era. We speculate that this condition, along with the lack of genes for resistance to necrotrophic pathogens, have given rise to evolutionary stable defence mechanism and the latter based on mutualistic relationships between the host-plant and bacteria that naturally inhabit rhizosphere and exhibiting high antagonistic (antibiotic) activity against soil-borne plant pathogens. A manifestation of this mechanism is the increasing suppressive response of the soil and the disease decline during or after continuous wheat monoculture. The results obtained in this study are discussed in the light of the microbial eco-evolutionary dynamics in the plant rhizosphere. The progressively increasing pathogen-antagonistic activity of Pseudomonas spp. isolates is discussed as an example of “rapid microbial evolution”, accomplished by possible horizontal gene transfer within one or more successive cultivation cycles of the wheat crop.