Abstract: Arbuscular mycorrhizal (AM) fungi have been shown to induce the biocontrol of soilborne diseases, to change the composition of root exudates and to modify the bacterial community structure of the rhizosphere, leading to the formation of the mycorrhizosphere. Tomato plants were grown in a compartmentalized soil system and were either submitted to direct mycorrhizal colonization or to enrichment of the soil with exudates collected from mycorrhizal tomato plants, with the corresponding negative controls. Three weeks after planting, the plants were inoculated or not with the soilborne pathogen Phytophthora nicotianae growing through a membrane from an adjacent infected compartment. At harvest, a PCR-Denaturing gradient gel electrophoresis analysis of 16S rRNA gene fragments amplified from the total DNA extracted from each plant rhizosphere was performed. Root colonization with the AM fungi Glomus intraradices or Glomus mosseae induced significant changes in the bacterial community structure of tomato rhizosphere, compared to non-mycorrhizal plants, while enrichment with root exudates collected from mycorrhizal or non-mycorrhizal plants had no effect. Our results support that the effect of AM fungi on rhizosphere bacteria would not be mediated by compounds present in root exudates of mycorrhizal plants but rather by physical or chemical factors associated with the mycelium, volatiles and/or root surface bound substrates. Moreover, infection of mycorrhizal or non-mycorrhizal plants with P. nicotianae did not significantly affect the bacterial community structure suggesting that rhizosphere bacteria would be less sensitive to the pathogen invasion than to mycorrhizal colonization. Of 96 unique sequences detected in the tomato rhizosphere, eight were specific to mycorrhizal fungi, including two Pseudomonas , a Bacillus simplex , an Herbaspirilium and an Acidobacterium. One Verrucomicrobium was common to rhizospheres of mycorrhizal plants and of plants watered with mycorrhizal root exudates.
Abstract: Potato brown rot disease (Ralstonia solanacearum) is a serious economic problem in Egypt, partly due to an European Union requirement that potatoes for export to the EU should be grown in so-called pest free areas (PFAs), where fields are tested and infested fields are put under quarantine measures. To
investigate pathogen survival and to determine the time required to keep infested fields in quarantine, the survival of R. solanacearum race 3 biovar 2 was tested in soils differing in origin (Dutch versus Egyptian soils), soil type (sand versus clay), and management type (organic versus conventional). All eight soils were tested atmoderate (150C) and elevated temperatures (280C). Also the effects of artificial fertilizer and organic (compost and cow manure) amendments on survival of R. solanacearum were tested. In all soils, with and without amendments, the pathogen dropped below the detection limit (102 CFU g-1 d.w. soil) within 5 months. At both temperatures, all Egyptian soils showed a significantly faster decline in pathogen density than the Dutch soils. The decline in colony forming units of R. solanacearum per gram of soil was faster in sandy soils than in clay soils from both countries. Management effects on decline of R. solanacearum were smaller and less consistent: for some soils, organic management resulted in a significantly shorter 50%-reduction-time and/or greater decline rate than conventional management, for other soils the differences were not significant. Survival periods at 150C were longer than at 280C in Dutch soils, but not in Egyptian soils, where survival was slightly shorter at the lower temperature. Amendments with NPK fertilizer to the conventional soils and with cow manure to the organic soils enhanced the decline rate of R. solanacearum in these soils. The decline rate of the pathogen was negatively correlated with total soluble organic matter and positively with bacterial diversity. In conclusion, the overriding factors determining survival of R. solanacearum in soil may be the production of toxic concentrations of ammonia on the one hand, and availability of substrate in combination with microbial competition on the other hand. The sandy desert soils of Egypt are very suitable for production of export potatoes because the pathogenwould survive for only a relatively short period in those soils, if it were accidentally introduced. Addition of ammonia-producing amendments can reduce populations of R. solanacearum, whereas compost addition and organic management do not necessarily result in an enhanced decline of the pathogen.
Abstract: An N-acyl homoserine lactonase gene aiiA, transcribed by a strong and constitutive Escherichia coli promoter Plpp (Accession No. EU723847), was transformed into Lysobacter enzymogenes strain OH11, creating strain OH11A. The N-acyl-homoserine lactone (AHL)-degradation assay showed that transformant OH11A acquired the ability to degrade AHL molecules produced by Agrobacterium tumefaciens, Pectobacterium carotovorum, Pseudomonas syringae pv. tomato strain DC3000 and Acidovorax avenae subsp. citrulli. Pathogenicity tests showed that while the parental strain OH11 did not reduce P. carotovorum infection, the transformant OH11A caused a strong reduction of Pectobacterium virulence on Chinese cabbage and cactus, whereas strain OH11A did not seem to interfere with the normal growth of this pathogen in cabbages. In antimicrobial activity assays, strain OH11A and OH11 showed similar antimicrobial activity against Phytophthora capsici and Sclerotinia sclerotiorum. This work provided a new strategy for developing genetically engineered multi-functional L. enzymogenes strains that possessed the ability to biologically control fungal pathogens and reduce bacterial pathogenicity.
Abstract: Several rhizobacteria play a vital role in plant protection, plant growth promotion and the improvement of soil health. In this study, we have isolated a strain of Lysobacter antibioticus HS124 from rhizosphere and demonstrate its antifungal activity against various pathogens including Phytophthora capsici, a destructive pathogen of pepper plants. L. antibioticus HS124 produced lytic enzymes such as chitinase, b-1,3-glucanase, lipase, protease, and an antibiotic compound. This antibiotic compound was purified by diaion HP-20, silica gel, sephadex LH-20 column chromatography and high performance liquid chromatography. The purified compound was identified as 4-hydroxyphenylacetic acid by gas chromatography-electron ionization (GC-EI) and gas chromatography-chemical ionization (GC-CI) mass spectrometry. This antibiotic exhibited destructive activity toward P. capsici hyphae. In vivo experiments utilizing green house grown pepper plants demonstrated the protective effect of L. antibioticus HS124 against P. capsici. The growth of pepper plants treated with L. antibioticus culture was enhanced, resulting in greater protection from fungal disease. Optimum growth and protection was found when cultures were grown in presence of Fe(III). Additionally, the activities of pathogenesis-related proteins such as chitinase and b-1,3-glucanase decreased in roots, but increased in leaves with time after treatment compared to controls. Our results demonstrate L. antibioticus HS124 as a promising candidate for biocontrol of P. capsici in pepper plants.
Abstract: In this work, effects of soil solarization and vesicular arbuscular mycorrhizal (VAM) fungus, Glomus intraradices on diseases caused by Phytophthora capsici Leonian in pepper plants and crop yields were studied. At the end of the growth season, 82.5% mortality of plants and 47.7% of yield loss caused by P. capsici were obtained in control plots. At the beginning of the vegetative period, plant mortality caused by P. capsici in solarized plots was less than that in non-solarized plots, but it was higher at the end of the experiments. The total crop yield, however, increased to 20.9% by solarization. At the beginning of the growth season, anthocyanin production, early flowering and fruit settings were observed in the seedlings inoculated with VAM. Plant mortality caused by P. capsici was inhibited by 69.4% in plants inoculated with VAM fungus, but this rate decreased to 14.9% at the end of the experiment. On the other hand, total yield increased to 40.4% in plots infested with P. capsici, but treated with VAM. The total yield increased to 49.9% in pathogen free solarized + VAM inoculated grown plants plots in comparison to pathogen free non-solarized + non-VAM inoculated grown plants plots. This increase was 42.8% in solarized + VAM inoculated grown plants plots, which were infested with P. capsici. Total yield was 22.7% in solarized + VAM inoculated grown plants plots without P. capsici in comparison to non-solarized plots + non-VAM inoculated grown plants plots but infected with P. capsici. The yield loss caused by P. capsici in pepper was decreased by means of long-term effect of soil solarization with artificially VAM inoculation.
Abstract: To identify phytoalexins of adzuki bean elicited in response to attempted infection of Phytophthora vignae f. sp. adzukicola, we isolated compounds from adzuki bean and evaluated their antifungal activity. Seven flavonoids (daidzein, genistein, 2-hydroxygenistein, coumestrol, dalbergioidin, kievitone, and phaseol) were identified from epicotyls wound-inoculated with a mycelial suspension of an avirulent race of P. vignae f. sp. adzukicola. Of those compounds, kievitone and dalbergioidin accumulated to higher levels in incompatible interactions compared to compatible interactions 48 h after inoculation. Kievitone strongly inhibited the germination of encysted zoospores, and dalbergioidin were slightly suppressive. From these results, we concluded that kievitone and dalbergioidin are phytoalexins in adzuki bean.
Abstract: Crude root extracts from Mexican avocado trees (Persea americana) were screened for antioomycete activity against Phytophthora cinnamomi. Forty-eight accessions from Mexican avocado trees were selected with potential resistance to P. cinnamomi according to environmental and site descriptors. Crude root extracts from these accessions were obtained and tested in vitro against the oomycete P. cinnamomi. Seven crude root extracts inhibited mycelial growth (>50%) and only root extracts from accessions 765-01 and 773-01 showed 100% of inhibition. Extracts from accessions 765-01 and 773-01 were analysed by preparative thin-layer chromatography and six fractions were detected under UV light. In both extracts, fractions IV (R f 0.85) and V (R f 0.9) showed in vitro inhibition (100%) against mycelial growth of P. cinnamomi. Fraction V was subjected to GC–MS analysis and stigmastan-3,5-diene (C29H48) was identified as the major compound. In vitro assay showed that stigmastan-3,5-diene (100 ppm) inhibited the mycelial growth of P. cinnamomi. The constitutive presence of this compound in avocado roots offers possibilities to identify and to select potentially resistant plants to P. cinnamomi.
Abstract: We studied the role of modification in root exudation induced by colonization with Glomus intraradices and Glomus mosseae in the growth of Phytophthora nicotianae in tomato roots. Plants were grown in a compartmentalized plant growth system and were either inoculated with the AM fungi or received exudates from mycorrhizal plants, with the corresponding controls. Three weeks after planting, the plants were inoculated or not with P. nicotianae growing from an adjacent compartment. At harvest, P. nicotianae biomass was significantly reduced in roots colonized with G. intraradices or G. mosseae in comparison to non-colonized roots. Conversely, pathogen biomass was similar in non-colonized roots supplied with exudates collected from mycorrhizal or non-mycorrhizal roots, or with water. We cannot rule out that a mycorrhiza-mediated modification in root exudation may take place, but our results did not support that a change in pathogen chemotactic responses to host root exudates may be involved in the inhibition of P. nicotianae.
Abstract: Aim: To find sustainable alternatives to the application of synthetic chemicals for oomycete pathogen suppression.
Methods and Results: Here, we present experiments on an Arabidopsis thaliana model system in which we studied the antagonistic properties of rhizobacterium Paenibacillus polymyxa strains towards the oomycete plant pathogens Phytophthora palmivora and Pythium aphanidermatum. We carried out studies on agar plates, in liquid media and in soil. Our results indicate that P. polymyxa strains significantly reduced P. aphanidermatum and P. palmivora colonization in liquid assays. Most plants that had been treated with P. polymyxa survived the P. aphanidermatum inoculations in soil assays.
Conclusions: The antagonistic abilities of both systems correlated well with mycoidal substance production and not with the production of antagonistic substances from the biocontrol bacteria.
Significance and Impact of the Study: Our experiments highlight the need to take biofilm formation and niche exclusion mechanisms into consideration for biocontrol assays performed under natural conditions.
Abstract: Leaf rot of betel vine caused by Phytophothora parasitica var. piperina result in severe loss to the grower. A number of synthetic organic and inorganic compounds have been introduced in the field of disease management (Maheshwari et al.,2007). The leaf of betel vine is used directly for chewing without any treatment. Hence, plant extracts were tested and results are reported herein. Phytophthora parasitica var. piperina was isolated from infected leaves of betel vine and grown on oat meal agar medium, maintained in PDA slant as stock culture. The fresh leaves of different plant were thoroughly washed, extracted and tested as per the procedure described by Sahani and Saxena(2009).
Abstract