Combination of Pseudomonas aeruginosa and Pochonia chlamydosporia for Control of Root-Infecting Fungi in Tomato

A plant growth‐promoting rhizobacterium, Pseudomonas aeruginosa strain IE‐6, and a fungal antagonist, Pochonia chlamydosporia, were tested for their ability to inhibit mycelial growth of root‐infecting fungi under laboratory conditions including Macrophomina phaseolina, Fusarium oxysporum, F. solani and Rhizoctonia solani. Biocontrol effectiveness of the bacterium and the fungus alone or in combination was also determined for the control of root‐infecting fungi under field conditions. In a dual‐culture plate assay, the colonies of P. chlamydosporia and P. aeruginosa met each other and no further growth of either organism occurred. Against M. phaseolina, F. solani and R. solani, an ethyl acetate extract of the culture filtrates of P. aeruginosa inhibited fungal growth greater than the hexane extract, but against F. oxysporum the hexane extract caused greater inhibition of fungal growth. By contrast, against M. phaseolina, F. oxysporum and F. solani, the hexane extract of P. chlamydosporia was more effective in the inhibition of fungal growth than the ethyl acetate fraction. Ethyl acetate extracts of P. aeruginosa at 1.0 mg/ml not only inhibited the radial colony growth of R. solani but also lysed the fungal mycelium. P. aeruginosa produced siderophores and hydrogen cyanide under laboratory conditions. Field experiments conducted in 1997 and repeated in 1998 revealed that Pochonia chlamydosporia and P. aeruginosa significantly suppressed the root‐infecting fungi M. phaseolina, F. oxysporum, F. solani and R. solani and that the combination of the two caused greater inhibition of the fungal pathogens than either alone. Application of P. chlamydosporia and P. aeruginosa as a soil drench also resulted in enhanced growth of tomato plants.     

Role of Zinc in Rhizobacteria-Mediated Suppression of Root-Infecting Fungi and Root-Knot Nematode

Understanding the environmental factors that influence the rhizosphere and inner root colonization of the disease‐suppressive strains of fluorescent pseudomonads is an essential step towards improving the level and reliability of their biocontrol activity. Soil amendment with Zn at 0.8 or 1.6 mg/kg of soil alone or in combination with Pseudomonas aeruginosa IE‐6S⁺significantly reduced nematode penetration in tomato roots. Zn applied alone did not reduce root infection caused by Macrophomina phaseolina or Fusarium solani but did reduce when used in combination with IE‐6S⁺. Soil amendment with Zn at 0.8 or 1.6 mg/kg of soil alone or in conjunction with IE‐6S⁺ markedly suppressed Rhizoctonia solani infection. Plant height, fresh weight of shoot and protein contents of the leaves substantially improved when used with Zn, however, plants growing in the soil treated with 1.6 mg/kg of Zn in the absence of IE‐6S⁺ not only reduced plant growth but also showed necrotic symptoms on the leaves. Zn application in the soil decreased populations of IE‐6S⁺ both in the rhizosphere and root. A positive correlation between bacterial rhizosphere and inner root colonization was also observed. With an increase in nematode densities in the soil, nematode penetration and subsequent galling due to Meloidogyne javanica increased. Regardless of the nematode densities, Zn applied alone or in combination with IE‐6S⁺ caused marked suppression of M. javanica. At all the population densities of M. javanica, Zn enhanced the efficacy of IE‐6S⁺ to reduce nematode invasion and subsequent gall development. IE‐6S⁺ caused significant suppression of soil‐borne root‐infecting fungi both in Zn‐sufficient and Zn‐deficient soil although this suppressive effect accentuated in Zn‐sufficient soils. In the absence of IE‐6S⁺ and/or Zn, increased nematode densities in the soil significantly reduced plant height, fresh weight of shoot and protein contents of the shoots. With an increase in nematode densities, populations of IE‐6S⁺ in the rhizosphere and root increased regardless of the Zn application. However, Zn‐deficient soils supported larger populations of IE‐6S⁺ compared with those of Zn‐sufficient soils.     

Impact of biocontrol agents in combination with Prosopis juliflora (Swartz) DC. in controlling the root-infecting fungi of leguminous crops

Biocontrol agents, viz., Rhizobium meliloti, Pseudomonas aeruginosa, Aspergillus niger and Trichoderma harzianum, are used as seed dressing and soil is amended with Prsosopis juliflora (Swartz) DC. plant parts like stem, leaves and flower at 1% w/w for the control of root-rot fungi. All antagonists suppressed the infection of root-rot fungi viz., Fusarium spp., Rhizoctonia solani and Macrophomina phaseolina whereas the infection of R. solani and M. phaseolina was controlled when cowpea (Vigna unguiculata L.) and mungbean (Vigna radiata L.) seeds were treated with P. aeruginosa and T. harzianum and the soil was amended with P. juliflora leaves’ powder at 1% w/w. However, germination of both the crops was observed in all treatments. Growth parameters like shoot and root length, shoot and root weight, and leaf area significantly increased in all the treatments as compared to the control parameters. P. aeruginosa and T. harzianum in combination with soil amendment with P. juliflora plant parts at 1% w/w were the most effective for the control of root-rot fungi of leguminous plants.     

Genetic Diversity of Thanatephorus cucumeris Infecting Tomato in Iran

The necrotrophic fungus Thanatephorus cucumeris (anamorph Rhizoctonia solani) is among the most important soil‐borne pathogens which causes tomato foot and root rot worldwide. We investigated virulence and genetic relationships among and within different taxonomic groups of R. solani from the tomato‐growing regions in the north‐east of Iran. Characterization of R. solani taxonomic groups revealed that, of 56 isolates, four were AG‐2‐1, 16 were AG‐3 PT, 21 were AG‐4 HG‐I and 15 were AG‐4 HG‐II. Because interprimer binding site (iPBS), which is based on amplification of retrotransposons, is known as novel and powerful DNA fingerprinting technology, we selected four iPBS primers, which can detect polymorphisms of tomato foot root and root rot pathogen, for investigating genotypic variability of the isolates. The iPBS analyses separated various taxonomic groups of R. solani and showed great diversity among the isolates, demonstrating that the R. solani isolates obtained from tomato were not a clonal population. Crop rotation strategies and geographic location seem to be important factors affecting genetic structure of the isolates. Pathogenicity tests on tomato cultivar ‘Mobil’ showed significant differences in the virulence of various isolates. The overall results indicated that isolates of AG‐3 and AG‐4 were more virulent than AG‐2‐1. There was no significant correlation between genetic diversity and virulence of the isolates. This is the first report of R. solani AG‐4 HG‐II, causing tomato foot and root rot. Also, our research is the first in assessment of genetic diversity in fungal populations using iPBS molecular markers.     

Volatiles from soil‐borne fungi affect directional growth of roots

Volatiles play major roles in mediating ecological interactions between soil (micro)organisms and plants. It is well‐established that microbial volatiles can increase root biomass and lateral root formation. To date, however, it is unknown whether microbial volatiles can affect directional root growth. Here, we present a novel method to study belowground volatile‐mediated interactions. As proof‐of‐concept, we designed a root Y‐tube olfactometer, and tested the effects of volatiles from four different soil‐borne fungi on directional growth of Brassica rapa roots in soil. Subsequently, we compared the fungal volatile organic compounds (VOCs) previously profiled with Gas Chromatography–Mass Spectrometry (GC–MS). Using our newly designed setup, we show that directional root growth in soil is differentially affected by fungal volatiles. Roots grew more frequently toward volatiles from the root pathogen Rhizoctonia solani, whereas volatiles from the other three saprophytic fungi did not impact directional root growth. GC–MS profiling showed that six VOCs were exclusively emitted by R. solani. These findings verify that this novel method is suitable to unravel the intriguing chemical cross‐talk between roots and soil‐borne fungi and its impact on root growth.     

Nematicidal and allelopathic potential of Argemone mexicana,a tropical weed

Argemone mexicana L. (Papaveraceae), a tropical annual weed, is phytotoxic to many crop species. This study was designed to examine the allelochemical and nematicidal potential of A. mexicana and to better understand the role of this weed in the ecosystem. A methanol-soluble extract of the leaf material caused greater juvenile mortality of Meloidogyne javanica than did ethyl acetate or hexane extracts indicating the polar nature of the toxins. Decomposing tissues of A. mexicana in soil at 50 g kg^–1 were highly deleterious causing 80% mortality of tomato plants. At 10 g kg^–1 plant growth was enhanced, while at 30 g kg^–1 plant growth was substantially retarded. M. javanica population densities in the rhizosphere and in roots, and gall formation were significantly suppressed when 10, 30 or 50 g kg^–1 A. mexicana was allowed to decompose in the soil. To establish whether decomposition was necessary to produce phytotoxic symptoms, or whether the shoot extract alone could interfere with plant growth, an aqueous shoot extract was applied to soil. Whereas a 50% extract promoted plant growth, a 100% (100 g/500 mL distilled water) concentration significantly reduced plant height, and fresh weights of shoot and root. In general, decomposing plant material caused greater phytotoxicity compared to the aqueous extract. Addition of N as NH₄NO₃ partially alleviated the phytotoxic action of A. mexicana,and also reduced severity of root-knot disease. Adding Pseudomonas aeruginosa to soil amended with A. mexicana resulted in decreased density of M. javanicain the rhizosphere and in tomato roots, suppressed galling rates and enhanced plant growth.     

Interactions between a fluorescent pseudomonad,an arbuscular mycorrhizal fungus and a hypovirulent isolate of Rhizoctonia solani affect plant growth and root architecture of tomato plants

Abstract

Although Rhizoctonia solani is a cosmopolitan soilborne pathogen, the genus includes isolates with different pathogenicity ranging from high virulence to avirulence. The biocontrol strain Pseudomonas fluorescens P190r and the arbuscular mycorrhizal (AM) fungus Glomus mosseae BEG12 were inoculated alone or in combination in tomato plants infested by the mildly virulent pathogen R. solani #235. Plant growth as well as root morphometric and topological parameters were evaluated. The infection of R. solani was significantly reduced by all the combinations of the beneficial microorganisms. Root systems of R. solani‐infected plants were weakly developed but highly branched with a herring‐bone pattern, while those inoculated with the AM fungus, alone or in combination with the bacterial strain, were longer and more developed, and displayed a dichotomous pattern. The interactions among these three microorganisms affected plant growth and root architecture of tomato plants.     

Biocontrol of Rhizoctonia Damping-off of Tomato by Bacillus subtilis Combined with Burkholderia cepacia

Bacillus subtilis strain RB14‐C and Burkholderia cepacia strain BY were used in combination to control damping‐off of tomato plants caused by Rhizoctonia solani. Microcosm tests showed complete inhibition of R. solani growth on filter disks buried in soil added with the mixture of both bacteria. Single BY inhibited the fungus, but not completely, and RB14‐C had only slight inhibitory effect on pathogen growth. The efficacy of this combining treatment was checked in pot experiments, where bacteria were applied to the soil in several combinations: RB14‐C and BY together 4 days before seed planting, RB14‐C 4 days and BY 2 days before seed planting, RB14‐C 4 days and BY immediately before seeds. The effect of these treatments on population of R. solani in soil and infection of plants was compared with the activity of single application of each agent. All bacterial treatments significantly decreased damping‐off of tomato plants. The best control was obtained when BY was added 2 days after RB14‐C. In this treatment plant protection was significantly higher than that obtained in other combined applications and obtained by single strains, except BY added to the soil 4 days before seed planting. The lowest suppression indicated BY introduced to the soil before seed planting. RB14‐C only slightly decreased number of R. solani in the soil. In contrast, BY drastically reduced population of the pathogen. However, there was not a clear relation between decrease of pathogen density in soil and the rate of plant infection. The results show that combination of B. subtilis RB14‐C with B. cepacia BY can lead to greater damping‐off suppression than biocontrol exhibited by these strains used separately, but the effect of combining bacterial agents was clearly related to the order in which both agents were introduced.     

Efficacy of dried seed powder of some plant species as soil amendment against Meloidogyne incognita (Tylenchida: Meloidogynidae) on tomato

The nematicidal activity of dried ground seeds of Ammi majus, Matricaria chamomilla, Ricinus communis, Brassica alba, B. oleracea, Peganum harmala, Solanum nigrum, Raphanus sativus and Eucalyptus sp. was assessed against the root-knot nematode, Meloidogyne incognita, infecting tomato in a glasshouse. The powdered seeds of the tested plants were incorporated into the soil at the rate of 5 g/kg and their nematicidal activity was compared with that of the synthetic nematicide carbofuran at the rate of 0.01 g a.i./kg. The effects of the treatments on the growth of tomato were also examined. The populations of M. incognita in the soil and root galling of tomato were significantly suppressed by the powdered seeds of all the plant species tested, with the greatest reduction occurring in soil amended with M. chamomilla, followed by soil treated with powdered seeds of A. majus, S. nigrum, R. communis and Eucalyptus sp. The efficacy of B. oleracea, B. alba, M. chamomilla and R. communis in reducing the number of J₂ in the soil was similar to that of carbofuran. All amendments, except powdered seeds of M. chamomilla and A. majus significantly increased shoot length compared to the untreated inoculated plants. Shoot weight was significantly increased in soil amended with powdered seeds of B. oleracea, B. alba, R. communis, P. harmala and S. nigrum, but not in soil amended with the other seed powders when compared with untreated inoculated soil. Significant increases in root length occurred in pots amended with seed powder of B. alba, R. communis and Eucalyptus and in root weight for P. harmala. None of the tested dried seeds was phytotoxic to tomato plants at the applied rate.     

The Effect of Mode of Application of Bacillus subtilis RB14-C on its Efficacy as a Biocontrol Agent Against Rhizoctonia solani

Bacillus subtilis RB14‐C, which produces the antibiotic iturin A, was investigated for its effectiveness as a biocontrol agent against Rhizoctonia solani infecting tomato using seed coating and/or direct introduction of the bacteria to the soil. The ability of RB14‐C to colonize plant roots and produce iturin A in soil, depending on the method of bacterial application, was also determined. Seed coating and the combined treatment (soil and seed bacterization) did not protect seedlings against damping‐off caused by R. solani. By contrast, RB14 introduced only to the soil controlled the disease. The total number of RB14‐C bacteria on the roots of plants grown from coated seeds was significantly lower than on the roots of plants grown in soil mixed with the bacteria. In the combined treatment, application of B. subtilis with seeds to soil preinoculated with this bacterium, at first suppressed the population of RB14‐C in the soil. Then the colonization was generally uniform. The concentration of iturin A in non‐planted soil was highest at the beginning of the experiment (i.e. after application of the bacterial suspension) but then decreased, and was undetectable 3 days after incubation. However, after seed planting the antibiotic was produced again around young roots. Bacteria introduced to the soil as a seed coating also released the antibiotic around the seeds.     

Interactive effect of root-knot nematode,Meloidogyne incognita and root-rot fungus,Rhizoctonia solani,on okra [Abelmoschus esculentus L.]

The individual, concomitant and sequential inoculation of second stage juveniles (at 2000 J₂/kg soil) of Meloidogyne incognita and Rhizoctonia solani (at 2 g mycelial mat/kg soil) showed significant reduction in plant growth parameters viz. plant length, fresh weight and dry weight as compared to control. The greatest reduction in plant growth parameters was recorded in the plants simultaneously inoculated with M. incognita and R. solani followed by sequential and individual inoculation. In sequential inoculation, plant inoculated with M. incognita 15 days prior to R. solani shows more reduction in comparison to plant inoculated with R. solani 15 days prior to M. incognita. Moreover, the multiplication of nematode and number of galls/root system were significantly reduced in concomitant and sequential inoculation as compared to individual inoculation, whereas the intensity of root-rot/root system caused by R. solani was increased in the presence of root-knot nematode M. incognita as compared to when R. solani was inoculated individually.     

Resistance risk assessment for fludioxonil in Stemphylium solani

An outbreak of grey leaf spot caused by Stemphylium solani was observed on tomato in Shandong Province of China in recent years and brought huge economical losses. Fludioxonil is a phenylpyrrole fungicide with strong antifungal activity against S. solani. To evaluate the risk of S. solani developing fludioxonil resistance, a total of 145 field isolates were examined for sensitivity to fludioxonil by measuring mycelial growth. The baseline sensitivity was distributed as a unimodal curve with a mean EC₅₀ value of 0.0659 (±0.0170) µg mL^?1. Five mutants with high resistance to fludioxonil (RF > 1000) were obtained by successively selecting on fludioxonil‐amended plates in the laboratory. All the resistant mutants associated with strongly reduced fitness in mycelial growth, sporulation and pathogenicity. Fludioxonil had positive cross‐resistance with procymidone and iprodione, but there was no cross‐resistance with other fungicides including boscalid, fluazinam, azoxystrobin and flusilazole. Based on the current results, resistance risk of S. solani to fludioxonil could be moderate. This is the first report of baseline sensitivity of S. solani to fludioxonil and its risk assessment. In order to delay the resistance development, it is recommended that fludioxonil can be used as one component of the mixture or fungicides with different modes of action should be alternatively used for this disease management.     

Suppression of the root rot–root knot disease complex by Pseudomonas aeruginosa in tomato: The influence of inoculum density,nematode populations,moisture and other plant-associated bacteria

The influence of different application rates of the plant growth-promoting rhizobacterium, Pseudomonas aeruginosa, population densities of the root-knot nematode, Meloidogyne javanica, moisture and other plant-associated bacteria in the suppression of root rot–root knot disease complex of tomato are described. The impact of these factors on bacterial rhizosphere and inner root and shoot establishment are also presented. The highest inoculum level of P. aeruginosa (7.4 × 10⁸ cfu ml^–1) in the presence of the lowest population density of M. javanica (500 J2/plant) caused the greatest reduction in gall formation due to M. javanica. The number of root–knot nematodes recovered from soil and roots treated with P. aeruginosa were also significantly reduced. Root infection caused by the soilborne root-infecting fungi Fusarium oxysporum, F. solani and Rhizoctonia solani was also effectively suppressed following application of P. aeruginosa. A P. aeruginosa-Bacillus subtilis treatment was the most effective in the suppression of root-rot disease complex with enhancement of plant growth. Biocontrol and growth promoting potential of the bacterium was enhanced when soil was kept at 50% or 75% moisture holding capacity, whereas a 25% MHC reduced bacterial efficacy. Rhizosphere population of P. aeruginosa declined drastically in P. aeruginosa-Bradyrhizobium japonicum treatments. Rhizosphere colonisation by P. aeruginosa seems to be governed by two factors: Initial inoculum size of the bacterium and severity of the root-knot disease. Endoroot and endoshoot colonisation of the bacterium was dependent on degree of root-colonisation by Fusarium oxysporum. An inoculum level 2.5 × 10⁸ cfu/ml of P. aeruginosa was optimal for the enhancement of plant growth, whereas inoculum below this level reduced plant growth.     

Trichoderma virens 106 inoculation stimulates defence enzyme activities and enhances phenolic levels in tomato plants leading to lowered Rhizoctonia solani infection

Interaction of tomato roots with Trichoderma virens TRS106 provided protection against Rhizoctonia solani-induced disease. In tomato, plants inoculated with R. solani disease symptoms were observed on the roots as brown, necrotic lesions. These symptoms were limited on plants treated with TRS106 and inoculated with R. solani. It was shown that TRS106 did not directly inhibit Rhizoctonia growth in in vitro test. The tested Trichoderma isolate stimulated systemic defence responses in tomato plants, by activating defence enzymes including guaiacol peroxidase (GPX), syringaldazine peroxidase (SPX) and phenylalanine ammonia lyase (PAL). Simultaneously, it enhanced accumulation of phenolics and hydrogen peroxide (H₂O₂) accompanied by decrease in lipid peroxidation in the leaves. HPLC analysis indicated remarkable increases in the concentrations of 22 phenolics in the leaves of Trichoderma-treated tomato, both uninoculated and inoculated with R. solani. Some of the phenolics were present in a free form, the others were accumulated in a bound form as glycosylated conjugates belonging to phenylpropanoids, hydroxybenzoic and cinnamic acid derivatives and flavonoids. Several of the detected phenolics: ferulic and salicylic acids, pyrocatechol and hesperetin were strongly toxic to R. solani in plate tests. The systemic mobilisation of phenolic metabolism might be an element of tomato defence response positively involved in biocontrol of R. solani by TRS106. Based on the results, T. virens TRS106 may have potential to develop a new biofungicide for integrated management of R. solani-induced disease.     

Root Rot and Wilt of Kangaroo Paw (Anigozanthos manglesii) Caused by Pythium myriotylum (Drechs.) in Israel

A root rot and wilt disease of Anigozanthos manglesii (Kangaroo Paw) grown in greenhouses in Israel, for exporting as cut flowers to Europe, was characterized. Pythium myriotylum (Drechs.) and Rhizoctonia solani (Kühn) were the prevalent pathogens in diseased plants collected from commercial greenhouses. Fusarium oxysporum, Fusarium spp. and Myrothecium sp. were also isolated, but P. myriotylum or R. solani were not detected in samples from symptomless plants in tissue cultures (Australian origin) or plants at different stages in the nursery; non‐pathogenic F. oxysporum and Fusarium spp. were detected in several samples. In pathogenicity tests carried out in pots, plant mortality occurred 7 days after inoculation with P. myriotylum. In a field experiment carried out in methyl bromide‐fumigated soil, the incidence of dead plants following inoculation with P. myriotylum alone was 22% 10 days after inoculation, increasing to 78% after an additional 25 days. The incidence of dead plants following inoculation with R. solani alone was only 5% and in plants inoculated simultaneously with both pathogens, disease incidence was 88% 35 days after inoculation. Mortality reached 90–100% in plants inoculated with P. myriotylum, either singly or combined with R. solani 60 days after inoculation, whereas in plants inoculated with R. solani it was 5%. The maximum mortality in plants inoculated with R. solani was 25%, 76 days after inoculation. These results clearly demonstrate that P. myriotylum was the dominant pathogen in the root rot and wilt of A. manglesii.     

Anastomosis Group Determination of Rhizoctonia solani Kühn (Telemorph: Thanatephorus cucumeris) Isolates from Tomatoes Grown in Aydin,Turkey and their Disease Reaction on Various Tomato Cultivars

In the Province of Aydin‐Turkey most frequently Fusarium spp. and secondly Rhizoctonia solani Kühn were isolated from the roots and crown of tomato plants. Based on affinities for hyphal fusion with test isolates, all R. solani isolates were identified as AG‐4. The tomato cultivars which were grown in soil infested with R. solani AG‐4 exhibited different reactions. From the point of symptom expression and the rate of seedling emergence Sunny 6066 F1 was found to be the most resistant cultivar, whereas Rio Grande, Rio Fuego, NDM 725, Interpeel and Konia were the most susceptible cultivars.     

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.     

Influence of the endomycorrhizal fungus Glomus mosseae on the development of peanut pod rot disease in Egypt

Glomus mosseae and the two pod rot pathogens Fusarium solani and Rhizoctonia solani and subsequent effects on growth and yield of peanut (Arachis hypogaea L.) plants were investigated in a greenhouse over a 5-month period. At plant maturity, inoculation with F. solani and/or R. solani significantly reduced shoot and root dry weights, pegs and pod number and seed weight of peanut plants. In contrast, the growth response and biomass of peanut plants inoculated with G. mosseae was significantly higher than that of non-mycorrhizal plants, both in the presence and absence of the pathogens. Plants inoculated with G. mosseae had a lower incidence of root rot, decayed pods, and death than non-mycorrhizal ones. The pathogens either alone or in combination reduced root colonization by the mycorrhizal fungus. Propagule numbers of each pathogen isolated from pod shell, seed, carpophore, lower stem and root were significantly lower in mycorrhizal plants than in the non-mycorrhizal plants. Thus, G. mosseae protected peanut plants from infection by pod rot fungal pathogens. Accepted: 10 February 2000     

Evaluation of Pseudomonas aeruginosa Z5 for biocontrol of cotton seedling disease caused by Fusarium oxysporum

Plant growth-promoting bacteria-mediated biocontrol of plant pathogens is renowned to enhance the growth of the plants using different direct or indirect mechanisms. The goal of the present investigation was the evaluation of Pseudomonas aeruginosa Z5 isolated from cotton grown in Pakistani soils for the suppression of Fusarium oxysporum associated with cotton seedling disease. In dual culturing techniques, four bacterial strains inhibited fungal pathogens, i.e. F. oxysporum, Fusarium moniliforme, Fusarium solani and Rhizoctonia solani, significantly with percent inhibition ranging from 25% to 91.5%. P. aeruginosa Z5 showed maximum suppression of all the tested pathogens. Net-house experiments showed that the application of P. aeruginosa Z5 both separately and in combination with Bacillus fusiformis S10 significantly reduced the disease incidence by suppressing F. oxysporum (the causal agent of cotton seedling disease) up to 64–65% and improved the percent germination as compared to the infected control plants. The production of antibiotics, proteases and siderophores may be the contributing factors for its antagonistic properties. Highest bacterial population (8.9 CFU/g root) observed on roots of cotton plants inoculated with P. aeruginosa Z5 showed its good colonisation aptitudes even in the presence of high inoculation of soil with F. oxysporum. Confocal laser scanning microscopy supported the root colonisation of cotton plants with fluorescently labelled P. aeruginosa Z5. Because of innate fungicidal potential, growth promoting P. aeruginosa Z5 can be used as a bioinoculant and an antagonist to suppress the growth of cotton root-associated fungal pathogen.     

Systemic Resistance in Tomato Induced by Biocontrol Bacteria Against the Root-Knot Nematode, Meloidogyne javanica is Independent of Salicylic Acid Production

Salicylic acid (SA)‐mediated induction of systemic resistance by Pseudomonas aeruginosa strain 7NSK2 and P. fluorescens strain CHA0 against soil‐borne fungi and viruses have been reported. The role of SA biosynthesis in the enhancement of defence mechanism against plant‐parasitic nematodes by these bacterial strains in tomato is not known. To better understand the importance of SA in rhizobacteria‐mediated suppression of root‐knot nematodes, biocontrol potential of SA‐negative or SA‐overproducing mutants against Meloidogyne javanica was evaluated with their respective wild type counter parts. Culture supernatant of 7NSK2, CHA0 and their respective mutants caused significant mortality of M. javanica juveniles in vitro. SA deletion in 7NSK2 and SA overproduction in CHA0 did not influence bacterial efficacy to cause nematode deaths. Similarly, culture supernatants resulting from King's B liquid medium amended with FeCl₃ did not influence nematicidal activity of the bacterial strains. Strain CHA0 induced juvenile deaths more than 7NSK2 did. In pot experiments, the bacterial strains applied in unsterilized sandy loam soil markedly reduced final nematode population densities in roots and subsequent root‐knot infection in tomato seedlings. SA‐negative or overproducing derivatives prevented tomato roots in kinetics similar to those with their respective wild types. When soil iron concentration was lowered by the addition of ethylenediamine di(o‐hydroxyphenylacetic acid), nematode biocontrol by the bacterial strains (both wild type and mutants) remained unaltered. To understand the mechanism involved in rhizobacteria‐mediated suppression of root‐knot nematode in tomato, bacterial performance was assessed in a split root trial in which one‐half of the root system was treated with bacterium while the other inoculated with nematode. Compared with the controls, application of the bacterial cell suspension to one‐half of the root system lowered the populations of root‐knot nematode in non‐bacterized nematode‐treated sections indicating enhanced defence in the non‐bacterized half. With respect to nematode infection, mutants induced systemic resistance to a similar extent as that caused by the wild types in both wild type tomato and NahG tomato plants. It is concluded that fluorescent pseudomonads induce systemic resistance against root‐knot nematode via a signal transduction pathway, which is independent of SA accumulation in roots.