Influence of the mycorrhizal fungus,Glomus coronatum,and soil phosphorus on infection and disease caused by binucleate Rhizoctonia and Rhizoctonia solani on mung bean (Vigna radiata)

Root-infecting fungal pathogens and also parasites, which do not cause major disease symptoms cause problems of contamination in pot cultures of arbuscular mycorrhizal (AM) fungi. We investigated the effect of the AM fungus, Glomus coronatum Giovannetti on disease caused by binucleate Rhizoctonia sp. (BNR) and R. solani in mung bean in the absence (P0) and presence (P1) of added soil phosphorus (P). When G. coronatum and BNR or R. solani were inoculated at the same time, G. coronatum improved the growth of the plants and reduced colonization of roots by BNR, but not by R. solani. R. solani reduced the growth of non-mycorrhizal mung bean in P0 soil 6 weeks after inoculation, whereas BNR had no effect on growth. G. coronatum reduced the severity of disease caused by BNR or R. solani on mung bean in both soil P treatments. When G. coronatum was established in the roots 3 weeks before BNR or R. solani was added to the potting mix, there was no significant effect of BNR or R. solani on growth of mung bean. Prior colonization by G. coronatum slightly reduced indices of disease caused by BNR or R. solani. In both experiments, addition of P stimulated plant growth and reduced the colonization of roots by BNR, but had little effect on disease severity. We conclude that the reduction of the effect of BNR or R. solani on mung bean could not be explained by improved P nutrition, but could be attributed to the presence of G. coronatum within and among the roots.     

Fungi and Gram-negative bacteria as soilborne minor pathogens of goat's rue (Galega orientalis Lam.)

Fungi and Gram-negative bacteria were isolated from inside the roots of field-grown goat's rue (Galega orientalis). Fungi were isolated from three plants out of a total of 45 tested. Two multinuclear Rhizoctonia solani isolates were identified to the anastomosis group 5 (R. solani AG-5-Gal) using pairings with known AG test cultures. One fungal isolate was identified to Phoma chrysanthemicola. Gram-negative bacteria were isolated from three plants out of 25 tested. They were identified using classical methods, the BIOLOG identification system based on the utilisation of 95 different carbon sources, and the MIDI system for the analysis of whole cell fatty acids. The two latter systems were computer-associated and utilised an extensive reference library of isolates. One bacterial isolate was identified as Enterobacter agglomerans and two isolates as Pseudomonas marginalis. R. solani AG-5-Gal reduced the emergence of Lupinus luteus, L. polyphyllus and french bean (Phaseolus vulgaris) and the growth of broad bean (Viciafaba), L. luteus and french bean, but did not cause obvious damage in goat's rue and pea (Pisum sativum). However, R. solani AG-5-Gal was re-isolated from the roots of all the test plant species following inoculation. P. chrysanthemicola reduced the emergence of L. polyphyllus and the growth of goat's rue, french bean and broad bean, and it was re-isolated from all of the test plant species (except for french bean) following inoculation. All the bacteria reduced the emergence of french bean, but not that of goat's rue and pea, when applied to the soil. When the roots were dipped into bacterial suspension, all the bacteria damaged french bean and L. polyphyllus. Additionally, P. marginalis JV3 damaged goat's rue and red clover. The pathogenicity of the fungi and bacteria were not changed when they were double-inoculated in pairs, except for R. solani AG-5-Gal and P. marginalis JV2 which reduced the emergence of goat's rue when inoculated together but not when inoculated separately.     

Identification and pathogenicity of Rhizoctonia species isolated from bean and soybean plants in Samsun,Turkey

A total of 434 isolates of Rhizoctonia belonging to 10 anastomosis groups were obtained from the roots and rhizosphere soils of bean and soybean plants grown in Samsun, Turkey. AG-4 was found to be the most common group on bean and soybean plants and AG-5, AG-6, binucleate AG-A, AG-B and R. zeae were other groups isolated from the both plant species. AG-1, AG-7 and AG-K from bean and AG-E from soybean were other groups obtained in the study. The pathogenicity tests on bean and soybean seedlings showed that the highest disease severities were caused by AG-4 isolates, whereas AG-1 and AG-6 isolates were moderately pathogenic. Binucleate Rhizoctonia AG-B isolates were also moderately pathogenic, while other binucleate Rhizoctonia were found to be weakly pathogenic. Rhizoctonia zeae isolates caused moderate disease symptoms on bean, but soybean plants were slightly affected by this group of isolates. This is the first reported observation of R. solani AG-6 and AG-7 and binucleate Rhizoctonia AG-B on bean, and R. solani AG-5 and AG-6 and binucleate Rhizoctonia AG-A, AG-B and AG-E on soybean, in Turkey.     

Growth stimulation in bean (Phaseolus vulgaris L.) by Trichoderma

Trichoderma species are commonly used as biological control agents against phytopathogenic fungi and some strains are able to produce metabolites that enhance plant growth. In the current study we evaluated the production of potential growth-promoting metabolites, rhizosphere competence and endophytism for 101 isolates of Trichoderma from Colombia, and assessed the relationship of these factors to the enhancement of early stages of growth on bean seedlings. Twenty percent of these Trichoderma strains were able to produce soluble forms of phosphate from phosphoric rock. Only 8% of the assessed strains showed consistent ability to produce siderophores to convert ferric iron to soluble forms by chelation. Sixty percent of isolates produced indole-3-acetic acid (IAA) or auxin analogues. The production of any of these metabolites was a characteristic of specific strains, as the ability to produce these metabolites varied greatly within species. Moreover, the production of these substances did not correlate with enhanced growth on bean seedlings, measured as the combined increase in length of roots and aerial parts in the V3 stage of growth. Seven Trichoderma isolates significantly improved the growth of bean seedlings. However, metabolite production varied widely in these seven strains, and some isolates did not produce any of the assessed growth-promoting metabolites. Results indicated that growth was enhanced in the presence of rhizosphere competent and endophytic strains of Trichoderma, and these characteristics were strain-specific and not characteristic for species.     

Developmental Regulation of Susceptibility and Tolerance to Fusarium Root Rot in Beans

The soil-borne fungus, Fusarium solani f. sp. phaseoli, attacks roots and hypocotyls of bean (Phaseolus vulgaris) plants causing a devastating disease called root and foot rot. In a study of the host-pathogen relationship it was found that young bean roots, with the radicle just emerging, were highly tolerant to the pathogen, whereas older bean seedlings, with a fully developed root system, were completely susceptible. Investigations by low-temperature scanning electron microscopy demonstrated that significantly fewer spores and hyphae were present on the root surface of young bean seedlings as compared to older ones. A similar pattern of attachment was found when bean roots were inoculated with spores of F. solani f. sp. pisi, a related pathogen causing disease on peas but not on beans. Light microscopic studies showed that F. solani f. sp. pisi did not penetrate the root but rapidly formed thick-walled resting spores on the root surface. F. solani f. sp. phaseoli on the other hand quickly penetrated the root and formed an extensive network of fungal hyphae. These results demonstrate that the ability of fungal propagules to adhere to and to penetrate host tissues are two distinct processes. Furthermore, the data indicate that young bean roots lack a surface component necessary for attachment of fungal spores which may help explain their tolerance to Fusarium root rot.     

Biological control of Meloidogyne incognita and Fusarium solani in dry common bean in the field

Meloidoyne incognita (root-knot nematode) and Fusarium solani (root-rot pathogen) were the common soil-borne pathogens and cause severe damage to bean plants in newly reclaimed sandy soil in Nubaryia district, Behera Governorate, Egypt. The antagonistic effects of Trichoderma album and Trichoderma viride as well as three commercial products namely Rhizo-N^® (Bacillus subtilis), Bio-Arc^® 6% (Bacillus megaterium) and Bio-Zeid^® 2.5% (T. album) were tested against M. incognita and F. solani under naturally infected field conditions. T. album and T. viride highly reduced the frequency (%) population of pathogenic fungi such as Fusarium spp., F. solani and Rhizoctonia spp., than the commercial products. Results indicated that all the tested bio-control agents reduced, significantly, the nematode criteria as evidenced by the number of juvenile (J₂) in soil and number of galls and egg masses on roots of common bean and Fusarium root-rot incidence (%). Rhizo-N^® highly reduced the number of J₂ in soil, while T. album was the best in reducing the number of galls and egg masses in roots. The bio-control agents also increased the plant growth parameters of common bean plants i.e. plant height, plant weight, branch no./plant, pods no./plant, pod weight/plant, pod weight, seeds no./plant, fresh seeds weight/pod, dry seeds weight/pod and dry weight of 100 seeds.     

Control of fusarium wilt of <Emphasis Type="Italic">Solanum melongena</Emphasis> by <Emphasis Type="Italic">Trichoderma</Emphasis> spp.

Biological control of wilt of egg plant (Solanum melongena L.) caused by Fusarium solani was made with the application of five Trichoderma species, T. harzianum, T. viride, T. lignorum, T. hamatum and T. reesei. The effect of volatile and non-volatile antibiotics of Trichoderma origin on growth inhibition of the wilt pathogen was studied. T. harzianum showed maximum growth inhibition (86.44 %) of the pathogen through mycoparasitism. The non-volatiles produced by the Trichoderma species exhibited 100 % growth inhibition of the pathogen under in vitro condition. Production of siderophores and fungal cell wall degrading enzymes, chitinase and β-1,3-glucanase were found. Treatments with two most efficient Trichoderma species, T. harzianum and T. viride resulted in the decreasing population of Fusarium solani in soil thereby deterring disease incidence in field condition.     

Differential expression of Phaseolus vulgaris genes induced during the interaction with Rhizoctonia solani

Common bean (Phaseolus vulgaris L.) is the most important grain legume for direct human consumption; however, bean production is affected by several diseases such as Rhizoctonia root rot. Few bean cultivars have been identified that effectively resist the attack of this fungus. Herein, we used the P. vulgaris Pv-2094 landrace, which is less susceptible to Rhizoctonia root rot, for the construction of a suppressive subtractive hybridization cDNA library in order to isolate plant defense-related genes. Total RNAs obtained after 8 and 16 h from inoculated and non-inoculated roots with R. solani Kühn, were used as the source of the “tester” and the “driver” samples, respectively. A total of 136 unigenes were obtained and classified into 12 functional categories. Six unigenes were selected to analyze for differential expression by qRT-PCR, including a receptor-like kinase (PvRK20-1), an acid phosphatase associated to defense (PA), a pathogenesis related protein (PR1), an ethylene responsive factor (ERF), a polygalacturonase inhibitor protein (PGIP), and an alpha-dioxygenase (α-DOX). These genes were found to be differentially expressed in a time-dependent manner in bean roots during the interaction with R. solani. Data generated from this study will contribute to the understanding of the molecular mechanisms associated with plant defense against root rot in common bean.     

Optimization of different application methods of multi-facial bacterial and fungal antagonists against sheath blight pathogen of rice,Rhizoctonia solani AG1-IA

Relative effectiveness of seven different application methods of five native bacterial and fungal biocontrol agents (BCAs) and one fungicide (azoxystrobin) was evaluated against Rhizoctonia solani, under pot and field conditions on rice cultivar Pusa Sugandha-5. Plants grown in pots infected with R. solani suffered a 30%–49% decrease in plant growth and yield of grain. However, treatment with BCAs reduced the adverse effect of the pathogen but significantly varied with the treatment schemes. Amongst the treatments, soil application (SA) at 20 + 40 days after planting (DAP) followed by foliar application (FA) at 60 DAP was recorded as most efficacious, and reduced the severity of disease by 42–68%, resulting in a 21–36% plant growth promotion and yield enhancement. Treatment comprising SA 20 + FA 40 DAP was next in effectiveness but statistically equal to seed priming (SP) + SA 40 DAP of treatment. Amongst the BCAs, Pseudomonas putida was shown to be the most efficient, trailed by Trichoderma harzianum, P. fluorescens, T. viride and Bacillus subtilis. Field trials under naturally infested fields have also validated the effectiveness of P. putida. The SA 20 + 40 + FA 60 DAP with P. putida and T. harzianum were found quite effective and decreased the disease severity and incidence (40–81%), and improved the grain yield (42–72%). Relatively lower ShB control was recorded with SA 20 + FA 60 DAP, however, it was statistically at par with SA 20 DAP treatment and equal to SA 20 + FA 40 DAP.     

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.     

Novel strains of Bacillus,isolated from compost and compost-amended soils,as biological control agents against soil-borne phytopathogenic fungi

A stepwise screening strategy made it possible to identify five new Bacillus spp. strains for biocontrol of Rhizoctonia solani, Sclerotinia minor and Fusarium solani. In vitro and in vivo biocontrol activity and M13-PCR DNA-fingerprinting led to the selection of these valuable biological control agents (BCAs) from a wide collection of over 250 candidates. At the end of this selection, the highest potential antagonists were identified at species level by 16S-rRNA gene sequence analysis, and results assigned them to Bacillus subtilis group as Bacillus amyloliquefaciens- and Bacillus methylotrophicus-related strains. In the current study, spore-forming bacteria provided substantial biocontrol of telluric diseases on cress and other different host plants. The strains named 15S and 09C were effective in disease control on Brassica oleracea/R. solani pathosystem, whereas Sclerotinia drop of lettuce was reduced by treatments with the strains 17S and 08C. Finally, the strains 17S and 12S were equally effective to control potato Fusarium rot. The evident zone of inhibition seen in dual culture plates suggested antibiosis-like antagonisms as the main mechanisms used by these bacterial isolates in interaction with the pathogens. Additionally, the API-ZYM method revealed constitutive activity of certain extracellular enzymes that could be involved in plant fortification. Bacillus strains isolated from compost and compost-amended soils are promising BCAs that have potential for practical application as biofungicides.     

Analysis of populations and physiological characterization of microorganisms in rhizospheres of plants with antagonistic properties to phytopathogenic nematodes

Populations of rhizosphere microflora of plants which have demonstrated an antagonism toward phytopathogenic nematodes, including velvet bean (Mucuna deeringiana), castor bean (Ricinus communis), sword bean (Cannavalia ensiformis), and Abruzzi rye (Secale cereale)., were compared to the rhizosphere microflora of soybean. Population densities of total bacteria were significantly lower for young Abruzzi rye, mature velvet bean, and mature castor bean, and fungi from mature velvet bean than for soybean. Population densities of spore-forming bacilli were significantly higher for Abruzzi rye than for soybean. Population densities of coryneform bacteria for mature sword bean and velvet bean were significantly higher than for soybean. All seedling test poants supported significantly higher population densities of chitinolytic fungi than soybean. On mature plants, chitinolytic bacteria were significantly higher on all test plants except velvet bean. Populations of endophytic root bacteria for three of the four test plants were significantly higher than for soybean. Fifty randomly, selected bacterial strains from seedlings and mature plants of soybean and each test plant were characterized for various physiological traits associated with rhizosphere competence, including chitinolytic activity, gelatin hydrolysis, production of hydrogen cyanide, starch hydrolysis, phenol oxidation, siderophore production, and production of antifungal compounds (inhibition ofPythium ultimum and/orRhizoctonia solani). There was a strong trend to increased frequency in each of the physiological tests with bacteria from test plants in comparison to those from soybean. The frequency of starch hydrolysis was up to 24 times greater for strains from test plants than for soybean strains, and siderophore production was up to 22 times more frequent for test plants. These results demonstrate that, compared to soybean, plants with properties antagonistic to phytopathogenic nematodes have a distinct rhizosphere microflora.     

Hyphae-Colonizing <Emphasis Type="Italic">Burkholderia</Emphasis> sp.—A New Source of Biological Control Agents Against Sheath Blight Disease (<Emphasis Type="Italic">Rhizoctonia solani</Emphasis> AG1-IA<Emphasis Type="Italic">)</Emphasis> in Rice

Sheath blight infection of rice by Rhizoctonia solani Kühn AG1-IA often results in serious yield losses in intensive rice cultivation. Biological control agents (BCAs) have previously been isolated but poor efficiency is often observed when applied under field conditions. This study compares a traditional dual-culture plate assay and a new water-surface microcosm assay for isolation of antagonistic soil bacteria. In the water-surface microcosm assay, floating pathogen mycelium is used as a source for isolation of hyphae-colonizing soil bacteria (HCSB), which are subsequently screened for antagonism. Ten antagonistic soil bacteria (ASB) isolated from a variety of Vietnamese rice soils using dual-culture plates were found to be affiliated with Bacillus based on 16S rRNA gene sequencing. However, all the ASB isolates grew poorly and showed no antagonism in the water-surface microcosm assay. In contrast, 11 (out of 13) HCSB isolates affiliated with Burkholderia sp. all grew well by colonizing the hyphae in the microcosms. Two of the Burkholderia sp. isolates, assigned to B. vietnamiensis based on recA gene sequencing, strongly inhibited fungal growth in both the dual-culture and water-surface microcosm assays; HCSB isolates affiliated to other species or species groups showed limited or no inhibition of R. solani in the microcosms. Our results suggest that HCSB obtained from floating pathogen hyphae can be a new source for isolation of efficient BCAs against R. solani, as the isolation assay mimics the natural habitat for fungal-bacterial interaction in the fields.     

Induction of pathogenesis-related proteins during biocontrol of <Emphasis Type="Italic">Rhizoctonia solani</Emphasis> with <Emphasis Type="Italic">Pseudomonas aureofaciens</Emphasis> in soybean (<Emphasis Type="Italic">Glycine max</Emphasis> L. Merr.) plants

To investigate the biocontrol effectiveness of the antibiotic producing bacterium, Pseudomonas aureofaciens 63–28 against the phytopathogen Rhizoctonia solani AG-4 on Petri plates and in soybean roots, growth response and induction of PR-proteins were estimated after inoculation with P. aureofaciens 63–28 (P), with R. solani AG-4 (R), or with P. aureofaciens 63–28 + R. solani AG-4 (P + R). P. aureofaciens 63–28 showed strong antifungal activity against R. solani AG-4 pathogens in Petri plates. Treatment with P. aureofaciens 63–28 alone increased the emergence rate, shoot fresh weight, shoot dry weight and root fresh weight at 7 days after inoculation, when compared to R. solani AG-4; P + R treatment showed similar effects. Peroxidase (POD) and β-1,3-glucanase activity of P. aureofaciens 63–28 treated roots increased by 41.1 and 49.9%, respectively, compared to control roots. POD was 26% greater in P + R treated roots than R. solani treated roots. Two POD isozymes (59 and 27 kDa) were strongly induced in P + R treated roots. The apparent molecular weight of chitinase from treated roots, as determined through SDS-PAGE separation and comparison with standards, was about 29 kDa. Five β-1,3-glucanase isozymes (80, 70, 50, 46 and 19 kDa) were observed in all treatments. These results suggest that inoculation of soybean plants with P. aureofaciens 63–28 elevates plant growth inhibition by R. solani AG-4 and activates PR-proteins, potentially through induction of systemic resistance mechanisms.     

Characterisation of antagonistic Bacillus and Pseudomonas strains for biocontrol potential and suppression of damping‐off and root rot diseases

Novel strains of rhizobacteria, Pseudomonas fluorescens Pf 9A‐14, Pseudomonas sp. Psp. 8D‐45 and Bacillus subtilis Bs 8B‐1, showed broad‐spectrum antagonistic activity and provided suppression of Pythium damping‐off and root rot of cucumber. Their biocontrol potential was further investigated for suppression of additional seedling diseases of cucumber (Phytophthora capsici) and radish (Rhizoctonia solani). Bacterial strains were also characterised for production of antibiotics, metabolites, volatiles, phytohormones and lytic enzymes. Seed and pre‐plant applications of all three antagonistic bacteria as cell suspension and talc or irradiated peat formulations to the infested potting mix provided overall high level of suppression of Phytophthora damping‐off and root rot of cucumber (66–85% healthy seedlings) and relatively low level of suppression of Rhizoctonia damping‐off of radish (18–38% healthy seedlings). Bacterial treatments also resulted in higher plant fresh masses. Seed coating with irradiated peat formulation of a mixture of three bacteria resulted in superior control of Phytophthora damping‐off and root rot of cucumber and much higher plant fresh masses. The presence of genes for biosynthesis of phenazine‐1‐carboxylic acid, 2,4‐diacetylphloroglucinol, pyrrolnitrin and pyoluteorin was confirmed in Pseudomonas strains, and that of fengycin, bacillomycin, bacilysin, surfactin and iturin A in B. subtilis Bs 8B‐1. All three strains produced HCN, salicylic acid, indole‐3‐acetic acid, protease and β‐1,3‐glucanase. Both Pseudomonas strains produced siderophores and only P. fluorescens Pf 9A‐14 showed phosphate solubilisation and chitinase activity. All three strains inhibited pathogen growth by producing volatiles, and gas chromatography–mass spectrometry analysis revealed eight compounds in Pf 9A‐14, 10 in Bs 8B‐1 and 4 in Psp 8D‐45, some with known antifungal activity. The antagonistic and plant‐growth promotion activities of these strains might be due to production of antibiotics, metabolites, lytic enzymes or phytohormones.     

Identification of a stress-induced protein in stem exudates of soybean seedlings root-infected with Fusarium solani f. sp. glycines

Sudden death syndrome of soybean (Glycine max) is caused by the soilborne fungus, Fusarium solani f. sp. glycines, that infects soybean roots. Besides root necrosis, symptoms include interveinal leaf chlorosis, necrosis and premature defoliation. It is proposed that a fungal toxin is produced in soybean roots and translocated to foliage. In this study, we isolated compounds from soybean stem exudates from plants that were either inoculated or not inoculated with F. solani f. sp. glycines. A protein with an estimated molecular mass of 17 kDa and designated as FISP 17 for F. solani f. sp. glycines-induced stress protein was identified using sodium dodecyl sulfate-polyacrylamide gel electrophoresis. This protein occurred only in F. solani f. sp. glycines-infected soybean stem exudates. The N-terminal amino acid sequence of the purified protein had 100 % identity with a starvation-associated message 22 protein, and 80 and 78 % identity with purified bean pathogenesis-related proteins, PvPR1 and PvPR2, respectively. To determine if the protein was of plant or fungal origin, a synthetic peptide was designed based on the N-terminal sequence and used to raise a polyclonal antibody from rabbit. Western blot analysis showed that the antibody only reacted with a 17-kDa protein in F. solani f. sp. glycines-infected plant exudates, but no reaction occurred with healthy plant exudates or with culture filtrates of F. solani f. sp. glycines. This is the first report of the presence of a stress-induced protein in stem exudates of soybean seedlings root-infected with F. solani f. sp. glycines.     

Cultivar response against root-infecting fungi and efficacy of Pseudomonas aeruginosa in controlling soybean root rot

Abstract

A study was conducted in the greenhouse to examine the resistance of three soybean cultivars against root-infecting fungi, and to determine the role of five strains of Pseudomonas aeruginosa in protecting the roots from these fungal pathogens. In this study soybean cv RAWAL was found to be less susceptible against charcoal rot fungus Macrophomina phaseolina than cvs PARC and BRAGG. Most of the strains of P. aeruginosa used as seed dressing significantly reduced M. phaseolina and Rhizoctonia solani infection on all three cvs PARC, BRAGG and RAWAL (p < 0.05). Most of the strains of P. aeruginosa were effective on cv PARC against Fusarium solani infection, while on cv BRAGG P. aeruginosa strain Pa₃, and on cv RAWAL strain Pa₅ were effective. Both strains Pa₃ and Pa₂₂ gave maximum plant height and fresh weight of shoots, respectively on cvs PARC and BRAGG than other strains. These characteristics make these P. aeruginosa strains good candidates for use as biocontrol agents against soil-borne plant pathogens.     

Evaluation of arbuscular mycorrhiza and other biocontrol agents in managing Fusarium oxysporum f. sp. Cubense infection in banana cv. Neypoovan

Panama wilt of banana caused by Fusarium oxysporum f. sp. cubense (race 1) is a serious disease devastating the important cultivar Neypoovan (syn Elakki Bale AB) in southern India. Chemical control methods are not very effective in controlling the disease. The objective of this study was to evaluate biocontrol agents (BCAs) under controlled and field conditions for their efficacy against the pathogen and to detect and quantify the reduction in FOC population. Arbuscular mycorrhizal (AM) fungi, Trichoderma harzianum and Pseudomonas fluorescens were inoculated at the time of planting in single, dual and tripartite combinations allowing colonization up to 0, 45 and 90 days. Plants were challenged thereafter with 50 g of FOC inoculum multiplied on sorghum grains containing 1.5×10⁶ cfu g^?1. Uninoculated plants and those inoculated with pathogen only were controls. Plant growth parameters were measured and structural modifications in the roots were studied. FOC populations in the roots were determined by ELISA every month and final yield was recorded. At the end of 7 months, plants pre-inoculated with BCAs i.e., G. mosseae+T. harzianum and challenged with Fusarium under field conditions could sustain 61 and 70% improvement in plant height and girth, respectively, and 75% in bunch weight over plants not precolonised with BCAs but challenged with FOC which finally succumbed to the disease. ELISA study revealed Fusarium population was reduced to 0.58 OD in 7 months in G. mosseae and T. harzianum treatment compared to a level of 1.9 OD in Fusarium alone treated plants. Beneficial effect of BCAs may be due to the over all protection provided by them by causing physical modifications in the cell wall, growth promotion and through induction of disease resistance.     

Association of the Hydrolytic Enzyme Chitinase against Rhizoctonia solani in Rhizobacteria-treated Rice Plants

Twenty‐two strains of Pseudomonas fluorescens isolated from the rhizosphere soil of nine plant species were screened in vitro for their inhibitory effect on the mycelial growth of the rice sheath blight fungus, Rhizoctonia solani. Of the 22 strains, two promising strains (Pf1 and FP7) were assessed for their effect on seedling vigour and their ability to promote growth in vitro of four cultivars of rice. Both bacterial strains induced systemic resistance in rice cv. IR 50, which is susceptible to sheath blight. After inoculation of the sheaths with the pathogen, Pseudomonas‐treated plants showed an increase in chitinase activity significantly higher than that of untreated control plants. A twofold increase in chitinase activity occurred 2 days after inoculation of plants with the pathogen. Western blot analysis of chitinase indicated the expression of 28 and 38 kDa proteins in rice sheaths against R. solani. Increased induction of the pathogenesis‐related chitinase isoform in Pseudomonas‐treated rice in response to R. solani infection indicates that the induced chitinase has a definite role in suppressing disease development.     

Bacterial antagonists of Sunflower (Helianthus annuus L.) fungal pathogens

Bacteria isolated on nutrient agar and King's medium B from sunflower leaves, crown and roots inhibited in vitro growth of the leaf spot and wilt pathogens Alternaria helianthi, and Sclerotium rolfsii, respectively, and also the root rot pathogensRhizoctonia solani and Macrophomina phaseolina. Antagonistic bacteria from leaves were mainly actinomycetes and pigmented Gram-positive bacteria, while those from roots and crowns were identified asPseudomonas fluorescens-putida, P. maltophilia, P. cepacia, Flavobacterium odoratum andBacillus sp. In soil bioassays, when used as seed inoculum in the presence ofS. rolfsii, P. cepacia strain N24 increased significantly the percentage of seedling emergence. Bacterial strains which exhibited broad spectrum in vitro antagonistic activity were tested for colonisation of sunflower roots, when used as a seed inoculum. Good colonisers (10⁴ to 10⁶ bacteria/g root) were consistent in their ability to reduce disease and fungal wilt. A seedling having a primary root length < 5 cm with fewer lateral roots, necrosed cotyledons or crown and a wilted shoot indicated its diseased status. On an average, only 30% of seedlings were diseased when treated with the antagonistic strains, in the presence of the pathogen, while 60% of the seedlings were diseased in the presence of the pathogen alone. In microplots treated with strain N24, only 1 to 3% of the seedlings were wilted, while 14% of the seedlings were wilted in the presence of the pathogen alone. The results obtained show that bacterial antagonists of sclerotial fungi can be used as seed inocula to improve plant growth through disease suppression