Enhanced activity of introduced biocontrol agents in solarized soils and its implications on the integrated control of tomato damping-off caused by Pythium spp

Soil solarization in combination with introduction of biocontrol agents (BCA) was evaluated as a potential disease management strategy for tomato damping-off caused by Pythium spp. A rifampicin resistant Pseudomonas fluorescens strain (PfT-8) and a carbendazim resistant Trichoderma harzianum strain (ThM-1) were introduced into soil following solarization. Tomato seeds were planted into treated field plots. The influence of soil solarization and application of biocontrol agents on damping-off incidence, plant biomass, rhizosphere population of introduced antagonists, and native Pythium spp. was assessed by two consecutive field trials. Damping-off incidence was significantly reduced in solarized plots compared to control. Soil inoculation of biocontrol agents into solarized plots resulted in the highest suppression of damping-off incidence (PfT-8 up to 92%; ThM-1 up to 83%), and increase in plant biomass (PfT-8 up to 66%; ThM-1 up to 48%) when compared to un-solarized control plots. Rhizosphere population of introduced biocontrol agents gradually increased (PfT-8 up to 102% and ThM-1 up to 84%) in solarized soils when compared to unsolarized control. The population of Pythium spp in rhizosphere soil was reduced up to 55% in solarized plots; whereas, application of BCA to solarized soils reduced the rhizosphere population of Pythium spp. by 86 and 82% in P. fluorescens and T. harzianum applied plots respectively.     

Development of new formulations of Bacillus subtilis for management of tomato damping-off caused by Pythium aphanidermatum

Formulations of a strain of Bacillus subtilis AUBS-1 inhibitory to the growth of the damping-off pathogen, Pythium aphanidermatum, were developed for seed treatment. The formulations included a talc-based powder, lignite-based powder, lignite+fly ash-based powder, wettable powder, bentonite-paste, polyethylene glycol (PEG) paste and a water-dispersible tablet. Formulations were stored at room temperature for 2 years and frequently sampled to test their shelf life. Populations of bacteria in the formulations were stable for up to 2 years storage at room temperature (28°C). Viability of propagules in lignite, lignite+fly ash, bentonite paste, wettable powder and water dispersible tablet formulations was 100% for up to 1 year. However, the viability of propagules was significantly reduced in talc, wettable powder, PEG paste and tablet formulations beyond 1 year of storage. Seed treatment of tomato with these formulations resulted in effective control of damping-off caused by P. aphanidermatum, and also enhanced plant biomass under glasshouse and field conditions. Active rhizosphere colonization by the bacterium was observed on tomato plants grown from seeds treated with the above formulations.     

Quorum-sensing system influences root colonization and biological control ability in Pseudomonas fluorescens 2P24

Pseudomonas fluorescens 2P24 is a biocontrol agent isolated from a wheat take-all decline soil in China. This strain produces several antifungal compounds, such as 2,4-diacetylphloroglucinol (2,4-DAPG), hydrogen cyanide and siderophore(s). Our recent work revealed that strain 2P24 employs a quorum-sensing system to regulate its biocontrol activity. In this study, we identified a quorum-sensing system consisting of PcoR and PcoI of the LuxR–LuxI family from strain 2P24. Deletion of pcoI from 2P24 abolishes the production of the quorum-sensing signals, but does not detectably affect the production of antifungal metabolites. However, the mutant is significantly defective in biofilm formation, colonization on wheat rhizosphere and biocontrol ability against wheat take-all, whilst complementation of pcoI restores the biocontrol activity to the wild-type level. Our data indicate that quorum sensing is involved in regulation of biocontrol activity in P. fluorescens 2P24.     

Sustaining productivity of wheat–soybean cropping system through integrated nutrient management practices on the Vertisols of central India

Wheat–soybean is one of the most dominant cropping systems on the Vertisols of central India. Cultivation of durum wheat in winter season (November to April) has a considerable potential due to congenial climate, while soybean in rainy season (June to October) has witnessed a phenomenal growth in the last two decades in the region. Beside including a legume (soybean) in sequence with a cereal crop (wheat), combined use of available organic sources along with chemical fertilizers may prove beneficial for long-term productivity and sustainability of the system. A long-term experiment was conducted during 1995–2000 on the fine-textured Vertisols at Indore, India to study the effect of combined use of farmyard manure (FYM), poultry manure, vermicompost and biofertilizers (Azotobacter + phosphate solubilizing bacteria) with 0.5 and 1.0 NPK (120 kg N + 26.2 kg P + 33.3 kg K ha⁻¹) on wheat, and residual effect on following soybean. Grain yield of aestivum wheat in the initial 2 years and durum wheat in the later 3 years was significantly increased with 0.5 NPK + poultry manure at 2.5 t ha⁻¹ or FYM at 10 t ha⁻¹ compared with 0.5 NPK alone, and was on par with 1.0 NPK. However, the highest productivity was obtained when these organic sources were applied along with 1.0 NPK. Quality parameters of durum wheat viz protein content, hectolitre weight and sedimentation value showed improvement, and yellow berry content was significantly lower with combined use of NPK + organic sources compared with NPK alone and control. Soybean did not show much response to residual effect of nutrient management treatments applied to wheat. Wheat gave higher profit than soybean, particularly in the later years due to lower grain yields and market price of soybean. However, the superiority of FYM as well as poultry manure along with 1.0 NPK was evident on the overall profitability of the system. Various soil fertility parameters including chemical and biological properties showed conspicuous improvement over the initial status under the treatments of FYM and poultry manure. Sustainability yield index was maximum under 1.0 NPK, followed by 1.0 NPK + poultry manure or FYM. It was concluded that application of available organic sources, particularly FYM and poultry manure along with full recommended dose of NPK fertilizers to wheat was essential for improving productivity, grain quality, profitability, soil health and sustainability of wheat–soybean system.     

Role of Organic Amendments to Mitigate Cd Toxicity and Its Assimilation in <i>Triticum aestivum</i> L.

In soil biota, higher and enduring concentration of heavy metals like cadmium (Cd) is hazardous and associated with great loss in growth, yield, and quality parameters of most of the crop plants. Recently, in-situ applications of eco-friendly stabilizing agents in the form of organic modifications have been utilized to mitigate the adverse effects of Cd-toxicity. This controlled experiment was laid down to appraise the imprints of various applied organic amendments namely poultry manure (PM), farmyard manure (FYM), and sugarcane press mud (PS) to immobilize Cd in polluted soil. Moreover, phytoavailability of Cd in wheat was also accessed under an alkaline environment. Results revealed that the addition of FYM (5–10 ton ha^-1 ) in Cd-contaminated soil significantly increased germination rate, leaf chlorophyll content, plant height, spike length, biological and grain yield amongst all applied organic amendments. Moreover, the addition of FYM (5–10 ton ha^-1 ) also reduced the phytoavailability of Cd by 73–85% in the roots, 57–83% in the shoots, and 81–90% in grains of wheat crop. Thus, it is affirmed that incorporation of FYM (5–10 ton ha^-1 ) performed better to enhance wheat growth and yield by remediating Cd. Thus, the application of FYM (5–10 ton ha^-1 ) reduced the toxicity induced by Cd to plants by declining its uptake and translocation as compared to all other applied organic amendments to immobilize Cd under sandy alkaline polluted soil.     

Enhancing the biocontrol efficacy of Pseudomonas fluorescens F113 by altering the regulation and production of 2,4-diacetylphloroglucinol

Pseudomonas fluorescens F113 is an effective biocontrol agent against Pythium ultimum, the causative agent of damping-off of sugarbeet seedlings. Biocontrol is mediated via the production of the anti-fungal metabolite 2,4-diacetylphloroglucinol (Phl). A genetic approach was used to further enhance the biocontrol ability of F113. Two genetically modified (GM) strains, P. fluorescens F113Rif (pCU8.3) and P. fluorescens F113Rif (pCUP9), were developed for enhanced Phl production and assessed for biocontrol efficacy and impact on sugarbeet in microcosm experiments. The multicopy plasmid pCU8.3 contains the biosynthetic genes (phlA, C, B and D) and the putative permease gene (phlE) of F113 cloned into the rhizosphere stable plasmid pME6010, independent of external promoters. The plasmid pCUP9 consists of the Phl biosynthetic genes cloned downstream of the constitutive Plac promoter in pBBR1MCS. Introduction of pCU8.3 and pCUP9 into P. fluorescens F113 significantly altered the kinetics of Phl biosynthesis when grown in SA medium. A significant and substantial increase in Phl production by the GM strains was observed in the early logarithmic phase and stationary phase of growth compared with the wild-type strain. In microcosm, the two Phl overproducing strains proved to be as effective at controlling damping-off disease as the proprietary fungicide treatment, indicating the potential of genetic modification for plant disease control.     

Poultry Manure as an Organic Fertilizer with or without Biochar Amendment: Influence on Growth and Heavy Metal Accumulation in Lettuce and Spinach and Soil Nutrients

This pot-based study investigated the influence of poultry manure and 1:1 mixture of poultry manure + biochar (produced from farmyard manure [FYM] or wood), on the biomass production and concentration of heavy metals in leaves of lettuce and spinach. The concentration of mineral nitrogen (N) and soluble inorganic phosphorus (P) of soils cultivated with these vegetables was also investigated. The application of poultry manure or FYM biochar in soil as 10% (equivalent to 60 t ha^–1 , an estimated 1726.8 kg ha^–1 N in poultry manure and 1353.9 kg ha^–1 N in FYM) and 15% amendment (equivalent to 90 t ha^–1 , an estimated 2590.2 kg ha^–1 N in poultry manure and 2030.8 kg ha^–1 N in FYM) significantly decreased biomass production of lettuce as compared to control (no fertilizer added) treatment. However, mixture of poultry manure with wood-derived biochar at both application rates (i.e., 10% and 15%) and with FYM biochar at lower application rate (i.e., 10%) caused 2–3-fold increase in aboveground plant biomass and 2–14-fold increase in root biomass (p < 0.05). Furthermore, as compared to control treatment, a significant ~2–3-fold increase in aboveground plant biomass was also observed in response to mixture of poultry manure with wood-derived and FYM derived biochars at 10% amendment rates. As compared to control treatment, concentration of mineral N and soluble inorganic P were higher in soils of all other treatments. In spinach, amendment of poultry manure or its co-amendment with biochar of FYM significantly increased aboveground plant biomass at 7% (equivalent to 42 t ha^–1 ) as compared to 3% and 5% amendment rates (equivalent to 18 and 30 t ha^–1 respectively). The concentration of soil mineral N and soil soluble mineral P was not different between treatments. In lettuce, wood-derived biochar did not reduce concentration of heavy metals (i.e., manganese (Mn), copper (Cu), iron, (Fe), cadmium (Cd), lead (Pb), nickel (Ni) and cobalt (Co) than FYM-derived biochar while in spinach, as compared to poultry manure, co-amendment of poultry manure with wood-derived biochar reduced concentration of heavy metals, indicating differential responses of crops to organic amendments.     

Formulation of a Streptomyces Biocontrol Agent for the Suppression of Rhizoctonia Damping-off in Tomato Transplants

Formulations of a Streptomyces biological control agent for Rhizoctonia damping-off in tomato seedlings were developed for the first time from vegetative propagules obtained from actively growing, nonsporulating liquid cultures. Alginate beads, durum flour (starch) granules, and talcum powder formulation of this new actinomycetous antagonist (Streptomyces sp. Di-944) isolated from the rhizosphere of field-grown tomato (Lycopersicon esculentum) suppressed damping-off caused by Rhizoctonia solani in tomato plug transplants (cv. Bonny Best) in a peat-based, soilless potting mix under greenhouse conditions. For formulations, vegetative biomass of Streptomyces sp. Di-944 from 3-day-old liquid fermentation in yeast extract–malt extract–glucose broth was lyophilized and pulverized to obtain fragments of viable vegetative filaments. The pulverized biomass had an initial viable count of 2 × 10⁷colony forming units/g and retained 100% viability for 2 weeks when stored at 4°C. Formulations stored at 4°C had a longer shelf life than those stored at 24°C based on viability at 2-week intervals over a 6-month storage period. In addition, dual culture tests showed declining efficacy for surviving Streptomyces propagules in formulations during this storage period. At 4°C, the powder and granular formulations were found to be the most stable and were shown to be 100% viable after 14 and 10 weeks of storage, respectively. However, at the end of 24 weeks, the number of viable propagules in the powder and granular formulations declined to 1.2 × 10⁵ and 7 × 10³ colony forming units/g, respectively. Alginate beads were the least stable in storage. Even at 4°C, 6.9 × 10⁴ and 7.3 × 10² viable propagules/g formulation were detected at the end of 12 and 24 weeks, respectively. The talcum powder formulation delivered to tomato seeds as a seed-coating was the most effective biocontrol treatment. It suppressed damping-off in 10-day-old tomato transplants by almost 90% compared to 30 and 22% damping-off reduction when alginate beads or starch granules were delivered concomitantly with tomato seeds. Seed-coating with powder formulation of the biocontrol agent was as effective as drench application of the fungicide, oxine benzoate (No-Damp), in controlling Rhizoctonia damping-off and superior to the commercial biocontrol agent, Streptomyces griseoviridis (Mycostop), applied to tomato seeds as seed-coating.     

Effects of organic amendments to soil on the rhizosphere microflora of antirrhinum infected withVerticillium dahliae Kleb.

Summary Organic (e.g. chitin, green manure, cellulose) amendments to soil induced quantitative and qualitative changes in the rhizosphere microflora of antirrhinum plants infected withVerticillium dahliae Kleb. Whereas reduction in disease severity occurred with chitin and green manure amendments, an increase in disease severity was observed with the application of cellulose. The reduction of the disease severity with chitin and green manure may be correlated with the increased population of actinomycetes in the antirrhinum rhizosphere.     

Biocontrol of <Emphasis Type="Italic">Pythium</Emphasis> damping-off in cucumber by arbuscular mycorrhiza-associated bacteria from the genus <Emphasis Type="Italic">Paenibacillus</Emphasis>

The Pythium biocontrol features of 17 Paenibacillus strains, all previously isolated from the rhizosphere, hyphosphere or bulk soil from mycorrhizal and non-mycorrhizal cucumber plants, were examined using a cucumber seedling emergence bioassay. Thirteen strains – four strains of Paenibacillus polymyxa, eight strains of P. macerans and one strain of Paenibacillus sp. – significantly increased the percentage of seedling emergence of seeds inoculated with agar plugs of Pythium aphanidermatum FC42. Overall, the efficacy of Pythium biocontrol did not seem to differ between isolates of Paenibacillus originating from either mycorrhizal or non-mycorrhizal systems. No strains significantly reduced the damping-off incidence caused by the aggressive isolate Pythium sp. B5. Two strains of P. macerans not only reduced the incidence of pre-emergence damping-off by 73%, but they also counteracted the plant growth-depressing effect of P. aphanidermatum FC42, so that 68–82% of the emerged seedlings remained healthy 7 days after sowing. Two strains of P. macerans and one strain of P. polymyxa also significantly increased the percentage of seedling emergence following inoculation with approximately 10⁵ zoospores of P. aphanidermatum FC42. There was no significant difference between the dry weight of three selected bacteria-inoculated and -uninoculated plants in the absence of Pythium; however, the dry weight of bacteria-inoculated plants was significantly higher than that of the uninoculated control plants with bacteria in the presence of P. aphanidermatum FC42.     

Influence of Host Plant Genotype, Presence of a Pathogen, and Coinoculation with Pseudomonas fluorescens Strains on the Rhizosphere Expression of Hydrogen Cyanide- and 2,4-Diacetylphloroglucinol Biosynthetic Genes in P. fluorescens Biocontrol Strain CHA0

The production of hydrogen cyanide (HCN) and 2,4-diacetylphloroglucinol (DAPG) is a major factor in the control of soil-borne diseases by Pseudomonas fluorescens CHA0. We investigated the impact of different biotic factors on the expression of HCN–in comparison to DAPG biosynthetic genes in the rhizosphere. To this end, the influence of plant cultivar, pathogen infection, and coinoculation with other biocontrol strains on the expression of hcnA-lacZ and phlA-lacZ fusion in strain CHA0 was monitored on the roots of bean. Interestingly, all the tested factors influenced the expression of the two biocontrol traits in a similar way. For both genes, we observed a several-fold higher expression in the rhizosphere of cv. Derakhshan compared with cvs. Goli and Naz, although bacterial rhizosphere colonization levels were similar on all cultivars tested. Root infection by Rhizoctonia solani stimulated total phlA and hcnA gene expression in the bean rhizosphere. Coinoculation of strain CHA0 with DAPG-producing P. fluorescens biocontrol strains Pf-68 and Pf-100 did neither result in a substantial alteration of hcnA nor of phlA expression in CHA0 on bean roots. To our best knowledge, this is the first study investigating the impact of biotic factors on HCN production by a bacterial biocontrol strain in the rhizosphere.     

Effect of organic soil amendments on the incidence of stalk rot of maize

Five organic materials, Calopogonium sp. leaves, rice straw, mixed wood saw dust of Terminalia sp. Triplochyton sp. and Kaya sp., fresh guinea grass and poultry manure were added to soil with a recent history of high stalk rot of maize caused by Macrophomina phaseolina and Fusarium moniliforme. The two stalk rot organisms reacted differently to the organic amendments. All the amendments produced significantly less Fusarium stalk rot disease than the unamended control. Amendment of soil with fresh Calopogonium leaves was almost ineffective in controlling the incidence of Macrophomina phaseolina. Organic amendment with poultry manure increased the disease casued by M. phaseolina. The other amendments appeared to be equally effective in checking the disease. Increased microbial population due to amendment may play some role in disease suppression.     

The use of <Emphasis Type="Italic">Pseudomonas fluorescens</Emphasis> P13 to control sclerotinia stem rot (<Emphasis Type="Italic">Sclerotinia sclerotiorum</Emphasis>) of oilseed rape

Sclerotinia stem rot (SSR) caused by the fungus Sclerotinia sclerotiorum has been an increasing threat to oilseed rape (Brassica napus L.) cultivation. Efficient and environment—friendly treatments are much needed. Here we focus on microbial control. The Pseudomonas fluorescens P13 that was isolated from oilseed rape cultivation soil, proved to be a useful biocontrol strain for application. Morphology, physiological and biochemical tests and 16S rDNA analysis demonstrated that it was P. fluorescens P13 and that it had a broad antagonistic spectrum, significantly lessening the mycelial growth of S. sclerotiorum by 84.4% and suppressing sclerotial formation by 95–100%. Scanning electron microscopy studies attested that P13 deformed S. sclerotiorum mycelia when they were cultured together. P13 did not produce chitinase but did produce hydrogen cyanide (HCN) which was likely one of the antagonistic mechanisms. The density of P13 remained at a high level (≥10⁶ CFU/ml) during 5 weeks in the rhizosphere soil and roots. P13 reduced SSR severity at least by 59% in field studies and also promoted seedling growth (p<0.05) at the seedling stage. From these data, our work provided evidence that P13 could be a good alternative biological resource for biocontrol of S. sclerotiorum.     

Colonization behaviour of Pseudomonas fluorescens and Sinorhizobium meliloti in the alfalfa (Medicago sativa) rhizosphere

The colonization ability of Pseudomonas fluorescens F113rif in alfalfa rhizosphere and its interactions with the alfalfa microsymbiont Sinorhizobium meliloti EFB1 has been analyzed. Both strains efficiently colonize the alfalfa rhizosphere in gnotobiotic systems and soil microcosms. Colonization dynamics of F113rif on alfalfa were similar to other plant systems previously studied but it is displaced by S. meliloti EFB1, lowering its population by one order of magnitude in co-inoculation experiments. GFP tagged strains used to study the colonization patterns by both strains indicated that P. fluorescens F113rif did not colonize root hairs while S. meliloti EFB1 extensively colonized this niche. Inoculation of F113rif had a deleterious effect on plants grown in gnotobiotic systems, possibly because of the production of HCN and the high populations reached in these systems. This effect was reversed by co-inoculation. Pseudomonas fluorescens F113 derivatives with biocontrol and bioremediation abilities have been developed in recent years. The results obtained support the possibility of using this bacterium in conjunction with alfalfa for biocontrol or rhizoremediation technologies.     

Control of Pythium damping-off of sugar beet by seed treatment with crop straw powders and a biocontrol agent

Seed treatment with non-sterilized powdered straws from 39 crops was tested for the control of Pythium damping-off of sugar beet. Four straws, including flax, coriander, pea, and lentil were effective in controlling the disease in soil artificially infested with Pythium sp. “group G.” Sterilizing flax and pea straws eliminated the efficacy of these straws. Wheat straw powder coated on sugar beet seeds increased the incidence of Pythium damping-off but this effect was reversed by the co-inoculation of wheat straws with the biocontrol agent Pseudomonas fluorescens 708. Coating sugar beet seeds with P. fluorescens 708 and flax or pea straws also increased the efficiency of the bacterial strain for the control of Pythium damping-off. Pea straws and to a lesser extent lentil straws produced volatile substances that affected mycelial growth of Pythium sp. “group G” on potato dextrose agar in Petri plates when the straws were mixed with water and left to ferment for two days. Fermentation of pea straws led to the accumulation of volatile ammonia, which was produced by the reduction of the large amount of nitrate stored in the straw. Reduction of nitrate and therefore the release of volatile ammonia did not occur in sterilized pea straws. However, fermenting sterile pea straws with bacteria from different genera restored nitrate reduction and the release of volatile ammonia, suggesting that microorganisms associated with pea straws are responsible for the conversion of nitrate into volatile ammonia which in turn control Pythium damping-off disease in sugar beet.     

Biological control of root-knot nematode,Meloidogyne incognita infesting tomato

Talc based formulations of two antagonistic fungi, Acremonium strictum W. Gams and Aspergillus terreus Thom were tested separately and together for their ability to suppress the development of root-knot disease of tomato caused by the root-knot nematode, Meloidogyne incognita Kofoid & White in two consecutive trials (2007–08). Tomato seedlings were each inoculated with M. incognita at 2 infective second stage juveniles /g of soil. M. incognita caused up to 48% reduction in plant growth parameters compared to un-inoculated control. Control efficacy achieved by combined soil application of both fungi, in terms of galls/root system and soil population/50 ml of soil, was 66 and 69% respectively at 60 days of inoculation compared to control. Soil application by individual fungus did not achieve as much effectiveness as the biocontrol agents applied together. The combined treatment was found to have antagonistic effect on M. incognita development and increased plant vigor. Incorporation of fine powder of chickpea pod waste with talc powder was beneficial in providing additional nutrients to both plant and biocontrol agents and increased the activity of the nematophagous fungi in soil. A. strictum and A. terreus were successfully established in the rhizosphere of tomato plants up to the termination of the experiment.     

Plant growth promotion abilities and formulation of <Emphasis Type="Italic">Bacillus megaterium</Emphasis> strain B 388 (MTCC6521) isolated from a temperate Himalayan location

Bacillus megaterium strain B388, isolated from rhizosphere soil of pine belonging to a temperate Himalayan location has been characterized. The plant growth promotion and biocontrol properties of the bacterium have been evaluated through petridish and broth based assays. The isolate solubilized tricalcium phosphate under in vitro conditions; maximum activity (166 μg/ml) was recorded at 28°C after 15 days of incubation. Production of indole acetic acid demonstrated in broth assays was another important plant growth promoting character. The bacterium produced diffusible and volatile compounds that inhibited the growth of two phytopathogens viz. Alternaria alternata and Fusarium oxysporum. The carrier based formulations of the bacterium resulted in increased plant growth in bioassays. The rhizosphere colonization and the viability of the cells entrapped in alginate beads were greater in comparison to coal or broth based formulations. The bacterium showed maximum similarity with Bacillus megaterium by 16S rRNA analysis.     

Comparison of the effects of poultry manure and its biochar on barley growth in petroleum-contaminated soils

This study was conducted to evaluate and compare the effectiveness of two organic amendments [poultry manure (PM) and poultry manure biochar (PMB)] for the degradation of petroleum hydrocarbons in contaminated soils by barley plant at three levels of total petroleum hydrocarbons (TPHs) during 5 months under greenhouse conditions. TPHs removal efficiency and microbial respiration were shown to be higher at soil-cultivated plant than at uncultivated soil and in lowest level of contamination rather than other levels of contamination and at organic amendment treatment than unamended soil. Soil microbial respiration and TPHs degradation in the rhizosphere of barley increased by 15.64 and 12.74% for PM-amended treatment and 28.07 and 26.83% for PMB-amended treatment, respectively, in the 4% TPHs level compared with unamended treatment. Comparison of two amendments showed that in PMB treatment soil, highest dry weight, microbial respiration, and TPHs degradation potential were observed.     

Development of formulations based on Streptomyces rochei strain PTL2 spores for biocontrol of Rhizoctonia solani damping-off of tomato seedlings

Rhizoctonia solani is one of the most problematic soil-borne pathogenic fungi for several crop cultures worldwide. This study highlights the effectiveness of high-antagonistic Streptomyces rochei strain PTL2, isolated from root tissues of Panicum turgidum, in controlling the R. solani damping-off and growth promotion of tomato (cv. Marmande) seedlings. The isolate PTL2 was characterised for in vitro biocontrol and plant growth-promoting traits. It exhibited remarkable positive results in all trials, including production of hydrogen cyanide, siderophores, 1-aminocyclopropane-1-carboxylate deaminase and phytohormones, chitinolytic activity and inorganic phosphate solubilisation. PTL2 spores were formulated as wettable talcum powder, sodium alginate pellets and sodium alginate-clay pellets. Their abilities in the biocontrol of R. solani and plant growth promotion were investigated in autoclaved and non-autoclaved soils. Talcum powder and sodium alginate pellets significantly reduced the damping-off severity index compared to a positive control. The talcum powder exhibited the highest protective activity, reducing the disease incidence from 89.3% to 14.1%, whereas chemical seed treatment with Thiram^® provided a disease incidence of 16.7%. Furthermore, the talc-based powder formulation resulted in greatest increases in the root length, shoot length and dry weight of seedlings. The interesting biocontrol potential and growth enhancement of tomato seedlings open up promising perspectives for the possible application of talcum powder formulation based on PTL2 spores in crop improvement.     

Three native Pseudomonas fluorescens strains tested under growth chamber and field conditions as biocontrol agents against damping-off in alfalfa

Alfalfa (Medicago sativa) is one of the most important crops used in Uruguay for livestock feeding. Seedling diseases, particularly damping-off, are a critical factor which limits its establishment. Three native Pseudomonas fluorescens strains, UP61.2, UP143.8 and UP148.2, previously isolated from Lotus corniculatus, were evaluated to determine their efficacy as biological control agents for alfalfa seedling diseases in the field. Their compatibility with the alfalfa-Sinorhizobium meliloti symbiosis was also assessed. In growth chamber conditions seed inoculation with Pseudomonas strains did not affect different parameters of alfalfa-rhizobium symbiosis as shown by nodulation rate and shoot dry weight of plants. The presence of the commercial inoculant strains of S. meliloti did not impair colonization by the P. fluorescens and vice versa. In field trials the dynamics of rhizobial rhizospheric populations were not affected by the presence of P. fluorescens. Each P. fluorescens strain successfully colonized alfalfa roots at adequate densities for biocontrol activity. Results showed that P. fluorescens strains provided a 10–13% increase in the number of established plants relative to the control, an intermediate result compared to the fungicide treatment (24%). The alfalfa above-ground biomass was increased by 13% and 15–18% in the presence of the fungicide and P. fluorescens strains, respectively. Therefore, results from this study demonstrated that the three P. fluorescens strains provided effective control against soil-borne pathogens and suggest a potential use in the development of a commercial inoculant to be applied for the control of legume seedling diseases.