Antagonistic potential of endophytic fungal isolates of tomato (Solanum lycopersicum L.) fruits against post-harvest disease-causing pathogens of tomatoes: An in vitro investigation

Post-harvest decay of fresh agricultural produce is a major threat to food security globally. Synthetic fungicides, commonly used in practice for managing the post-harvest losses, have negative impacts on consumers' health. Studies have reported the effectiveness of fungal isolates from plants as biocontrol agents of post-harvest diseases, although this is still poorly established in tomatoes (Solanum lycopersicum L. cv. Jasmine). In this study, 800 endophytic fungi were isolated from mature green and ripe untreated and fungicide-treated tomato fruits grown in open soil and hydroponics systems. Of these, five isolates (Aureobasidium pullulans SUG4.1, Coprinellus micaceus SUG4.3, Epicoccum nigrum SGT8.6, Fusarium oxysporum HTR8.4, Preussia africana SUG3.1) showed antagonistic properties against selected post-harvest pathogens of tomatoes (Alternaria alternata, Fusarium solani, Fusarium oxysporum, Geotrichum candidum, Rhizopus stolonifera, Rhizoctonia solani), with Lactiplantibacillus plantarum as a positive control. P. africana SUG3.1 and C. micaceus SUG4.3 significantly inhibited growth of all the pathogens, with antagonistic capabilities comparable to that exhibited by L. plantarum. Furthermore, the isolates produced an array of enzymes, including among others, amylase, cellulose and protease; and were able to utilize several carbohydrates (glucose, lactose, maltose, mannitol, sucrose). In conclusion, P. africana SUG3.1 and C. micaceus SUG4.3 may complement L. plantarum as biocontrol agents against post-harvest pathogens of tomatoes.

Major Soilborne Pathogens of Field Processing Tomatoes and Management Strategies

Globally, tomato is the second most cultivated vegetable crop next to potato, preferentially grown in temperate climates. Processing tomatoes are generally produced in field conditions, in which soilborne pathogens have serious impacts on tomato yield and quality by causing diseases of the tomato root system. Major processing tomato-producing countries have documented soilborne diseases caused by a variety of pathogens including bacteria, fungi, nematodes, and oomycetes, which are of economic importance and may threaten food security. Recent field surveys in the Australian processing tomato industry showed that plant growth and yield were significantly affected by soilborne pathogens, especially Fusarium oxysporum and Pythium species. Globally, different management methods have been used to control diseases such as the use of resistant tomato cultivars, the application of fungicides, and biological control. Among these methods, biocontrol has received increasing attention due to its high efficiency, target-specificity, sustainability and public acceptance. The application of biocontrol is a mix of different strategies, such as applying antagonistic microorganisms to the field, and using the beneficial metabolites synthesized by these microorganisms. This review provides a broad review of the major soilborne fungal/oomycete pathogens of the field processing tomato industry affecting major global producers, the traditional and biological management practices for the control of the pathogens, and the various strategies of the biological control for tomato soilborne diseases. The advantages and disadvantages of the management strategies are discussed, and highlighted is the importance of biological control in managing the diseases in field processing tomatoes under the pressure of global climate change.

Induced resistance to Fusarium wilt of banana caused by Tropical Race 4 in Cavendish cv Grand Naine bananas after challenging with avirulent Fusarium spp

In the last century, Fusarium wilt of banana (FWB) destroyed the banana cultivar Gros Michel. The Cavendish cultivars saved the global banana industry, and currently they dominate global production (~50%) and the export trade (~95%). However, a new strain called Tropical Race 4 (TR4) surfaced in the late 1960’s, spread globally and greatly damages Cavendish plantations as well as manifold local varieties that are primarily grown by small holders. Presently, there is no commercially available replacement for Cavendish and hence control strategies must be developed and implemented to manage FWB. Here, we studied whether it is possible to induce resistance to TR4 by pre-inoculations with different Fusarium spp. Only pre-treatments with an avirulent Race 1 strain significantly reduced disease development of TR4 in a Cavendish genotype and this effect was stable at various nutritional and pH conditions. We then used transcriptome analysis to study the molecular basis of this response. Several genes involved in plant defence responses were up-regulated during the initial stages of individual infections with TR4 and Race 1, as well as in combined treatments. In addition, a number of genes in the ethylene and jasmonate response pathways as well as several gibberellin synthesis associated genes were induced. We observed upregulation of RGA2 like genes in all treatments. Hence, RGA2 could be a key factor involved in both R1 and TR4 resistance. The data support the hypothesis that activating resistance to Race 1 in Cavendish bananas affects TR4 development and provide a first insight of gene expression during the interaction between various Fusarium spp. and banana.

Biocontrol Activity of Nonpathogenic Strains of Fusarium oxysporum: Colonization on the Root Surface to Overcome Nutritional Competition

Fusarium oxysporum is a soil-borne fungal pathogen that causes vascular wilts in a wide variety of crops. Certain nonpathogenic strains of F. oxysporum are known to protect crops against F. oxysporum pathogens. We assessed the biocontrol activities of nonpathogenic mutants of F. oxysporum ff. spp. melonis and lycopersici generated by disruption of the FOW2 gene, which encodes a Zn(II)2Cys6-type transcriptional regulator essential for their pathogenicity. Pre-inoculation of melon or tomato roots with strain ΔFOW2 conidia markedly reduced disease incidence caused by the parental wild-type strain in a concentration-dependent manner of conidial suspensions of ΔFOW2 strains. The biocontrol effect caused by the ΔFOW2 pre-inoculation lasted for at least 7 days. Pre-inoculation of melon roots with the wild-type or ΔFOW2 strain of F. oxysporum f. sp. lycopersici and nonpathogenic F. oxysporum strain also led to biocontrol activity against F. oxysporum f. sp. melonis, indicating that the biocontrol activity of ΔFOW2 strains is due to its nonpathogenic nature, not to the FOW2 disfunction. Conidial germination and hyphal elongation of only the wild-type strain were inhibited on melon root surface pre-inoculated with conidia of strains nonpathogenic to melon plants. Expression of defense-related genes was not significantly induced in roots and aboveground parts of melon seedlings preinoculated with ΔFOW2 conidia. Carbon source competition assay showed that nonpathogenic strains competed with the wild-type strain for a carbon source in soil. Strain ΔFOW2 also competed with the oomycete pathogen Pythium aphanidermatum for carbon source and protected melon plants from P. aphanidermatum. Our results suggest that the biocontrol activity of the nonpathogenic F. oxysporum strains used in this study mainly depends on their extensive colonization of the root surface and outcompeting pathogens for nutrients.

Disease-Suppressive Soils-Beyond Food Production: a Critical Review

In the pursuit of higher food production and economic growth and increasing population, we have often jeopardized natural resources such as soil, water, vegetation, and biodiversity at an alarming rate. In this process, wider adoption of intensive farming practices, namely changes in land use, imbalanced fertilizer application, minimum addition of organic residue/manure, and non-adoption of site-specific conservation measures, has led to declining in soil health and land degradation in an irreversible manner. In addition, increasing use of pesticides, coupled with soil and water pollution, has led the researchers to search for an environmental-friendly and cost-effective alternatives to controlling soil-borne diseases that are difficult to control, and which significantly limit agricultural productivity. Since the 1960s, disease-suppressive soils (DSS) have been identified and studied around the world. Soil disease suppression is the reduction in the incidence of soil-borne diseases even in the presence of a host plant and inoculum in the soil. The disease-suppressive capacity is mainly attributed to diverse microbial communities present in the soil that could act against soil-borne pathogens in multifaceted ways. The beneficial microorganisms employ some specific functions such as antibiosis, parasitism, competition for resources, and predation. However, there has been increasing evidence on the role of soil abiotic factors that largely influence the disease suppression. The intricate interactions of the soil, plant, and environmental components in a disease triangle make this process complex yet crucial to study to reduce disease incidence. Increasing resistance of the pathogen to presently available chemicals has led to the shift from culturable microbes to unexplored and unculturable microbes. Agricultural management practices such as tillage, fertilization, manures, irrigation, and amendment applications significantly alter the soil physicochemical environment and influence the growth and behaviour of antagonistic microbes. Plant factors such as age, type of crop, and root behaviour of the plant could stimulate or limit the diversity and structure of soil microorganisms in the rhizosphere. Further, identification and in-depth of disease-suppressive soils could lead to the discovery of more beneficial microorganisms with novel anti-microbial and plant promoting traits. To date, several microbial species have been isolated and proposed as key contributors in disease suppression, but the complexities as well as the mechanisms of the microbial and abiotic interactions remain elusive for most of the disease-suppressive soils. Thus, this review critically explores disease-suppressive attributes in soils, mechanisms involved, and biotic and abiotic factors affecting DSS and also briefly reviewing soil microbiome for anti-microbial drugs, in fact, a consequence of DSS phenomenon.

Soil Microbiome Manipulation Gives New Insights in Plant Disease-Suppressive Soils from the Perspective of a Circular Economy: A Critical Review

This review pays attention to the newest insights on the soil microbiome in plant disease-suppressive soil (DSS) for sustainable plant health management from the perspective of a circular economy that provides beneficial microbiota by recycling agro-wastes into the soil. In order to increase suppression of soil-borne plant pathogens, the main goal of this paper is to critically discuss and compare the potential use of reshaped soil microbiomes by assembling different agricultural practices such as crop selection; land use and conservative agriculture; crop rotation, diversification, intercropping and cover cropping; compost and chitosan application; and soil pre-fumigation combined with organic amendments and bio-organic fertilizers. This review is seen mostly as a comprehensive understanding of the main findings regarding DSS, starting from the oldest concepts to the newest challenges, based on the assumption that sustainability for soil quality and plant health is increasingly viable and supported by microbiome-assisted strategies based on the next-generation sequencing (NGS) methods that characterize in depth the soil bacterial and fungal communities. This approach, together with the virtuous reuse of agro-wastes to produce in situ green composts and organic bio-fertilizers, is the best way to design new sustainable cropping systems in a circular economy system. The current knowledge on soil-borne pathogens and soil microbiota is summarized. How microbiota determine soil suppression and what NGS strategies are available to understand soil microbiomes in DSS are presented. Disturbance of soil microbiota based on combined agricultural practices is deeply considered. Sustainable soil microbiome management by recycling in situ agro-wastes is presented. Afterwards, how the resulting new insights can drive the progress in sustainable microbiome-based disease management is discussed.

Sphinganine-Analog Mycotoxins (SAMs): Chemical Structures, Bioactivities, and Genetic Controls

Sphinganine-analog mycotoxins (SAMs) including fumonisins and A. alternata f. sp. Lycopersici (AAL) toxins are a group of related mycotoxins produced by plant pathogenic fungi in the Fusarium genus and in Alternaria alternata f. sp. Lycopersici, respectively. SAMs have shown diverse cytotoxicity and phytotoxicity, causing adverse impacts on plants, animals, and humans, and are a destructive force to crop production worldwide. This review summarizes the structural diversity of SAMs and encapsulates the relationships between their structures and biological activities. The toxicity of SAMs on plants and animals is mainly attributed to their inhibitory activity against the ceramide biosynthesis enzyme, influencing the sphingolipid metabolism and causing programmed cell death. We also reviewed the detoxification methods against SAMs and how plants develop resistance to SAMs. Genetic and evolutionary analyses revealed that the FUM (fumonisins biosynthetic) gene cluster was responsible for fumonisin biosynthesis in Fusarium spp. Sequence comparisons among species within the genus Fusarium suggested that mutations and multiple horizontal gene transfers involving the FUM gene cluster were responsible for the interspecific difference in fumonisin synthesis. We finish by describing methods for monitoring and quantifying SAMs in food and agricultural products.

Breeding, Genetics, and Genomics Approaches for Improving Fusarium Wilt Resistance in Major Grain Legumes

Fusarium wilt (FW) disease is the key constraint to grain legume production worldwide. The projected climate change is likely to exacerbate the current scenario. Of the various plant protection measures, genetic improvement of the disease resistance of crop cultivars remains the most economic, straightforward and environmental-friendly option to mitigate the risk. We begin with a brief recap of the classical genetic efforts that provided first insights into the genetic determinants controlling plant response to different races of FW pathogen in grain legumes. Subsequent technological breakthroughs like sequencing technologies have enhanced our understanding of the genetic basis of both plant resistance and pathogenicity. We present noteworthy examples of targeted improvement of plant resistance using genomics-assisted approaches. In parallel, modern functional genomic tools like RNA-seq are playing a greater role in illuminating the various aspects of plant-pathogen interaction. Further, proteomics and metabolomics have also been leveraged in recent years to reveal molecular players and various signaling pathways and complex networks participating in host-pathogen interaction. Finally, we present a perspective on the challenges and limitations of high-throughput phenotyping and emerging breeding approaches to expeditiously develop FW-resistant cultivars under the changing climate.

Complete genome sequences of Streptomyces spp. isolated from disease-suppressive soils

Background

Bacteria within the genus Streptomyces remain a major source of new natural product discovery and as soil inoculants in agriculture where they promote plant growth and protect from disease. Recently, Streptomyces spp. have been implicated as important members of naturally disease-suppressive soils. To shine more light on the ecology and evolution of disease-suppressive microbial communities, we have sequenced the genome of three Streptomyces strains isolated from disease-suppressive soils and compared them to previously sequenced isolates. Strains selected for sequencing had previously showed strong phenotypes in competition or signaling assays.

Results

Here we present the de novo sequencing of three strains of the genus Streptomyces isolated from disease-suppressive soils to produce high-quality complete genomes. Streptomyces sp. GS93–23, Streptomyces sp. 3211–3, and Streptomyces sp. S3–4 were found to have linear chromosomes of 8.24 Mb, 8.23 Mb, and greater than 7.5 Mb, respectively. In addition, two of the strains were found to have large, linear plasmids. Each strain harbors between 26 and 38 natural product biosynthetic gene clusters, on par with previously sequenced Streptomyces spp. We compared these newly sequenced genomes with those of previously sequenced organisms. We see substantial natural product biosynthetic diversity between closely related strains, with the gain/loss of episomal DNA elements being a primary driver of genome evolution.

Conclusions

Long read sequencing data facilitates large contig assembly for high-GC Streptomyces genomes. While the sample number is too small for a definitive conclusion, we do not see evidence that disease suppressive soil isolates are particularly privileged in terms of numbers of biosynthetic gene clusters. The strong sequence similarity between GS93–23 and previously isolated Streptomyces lydicus suggests that species recruitment may contribute to the evolution of disease-suppressive microbial communities.

Biocontrol Potential of Fungal Endophytes against Fusarium oxysporum f. sp. cucumerinum Causing Wilt in Cucumber

Endophytic fungi have received much attention as plant growth promoters as well as biological control agents against many plant pathogens. In this study, 30 endophytic fungal species, isolated from various plants in China, were evaluated using in vitro dual culture assay against Fusarium oxysporum f. sp. cucumerinum, causing wilt in cucumber. The results of the present study clearly showed that all the 30 endophytic fungal isolates were highly capable of inhibiting the mycelial colony growth of Fusarium oxysporum f. sp. cucumerinum with inhibition % over 66% as compared to control treatments. Among all of them, 5 isolates were highly effective such as, Penicillium sp., Guignardia mangiferae, Hypocrea sp., Neurospora sp., Eupenicillium javanicum, and Lasiodiplodia theobromae, respectively. The Penicillium sp. and Hypocrea sp. were highly effective as compared to other isolates. From in vitro results 10 best isolates were selected for greenhouse studies. The results of the greenhouse studies showed that among all of them 3 endophytic fungal isolates successfully suppressed wilt severity when co-inoculation with pathogen Fusarium. oxysporum f. sp. cucumerinum. The endophytic fungi also enhanced plant growth parameters of the host plants, the antagonistic fungal isolates increased over all plant height, aerial fresh, and dry weight as compared to control.

Fumonisins: Impact on Agriculture, Food, and Human Health and their Management Strategies

The fumonisins producing fungi, Fusarium spp., are ubiquitous in nature and contaminate several food matrices that pose detrimental health hazards on humans as well as on animals. This has necessitated profound research for the control and management of the toxins to guarantee better health of consumers. This review highlights the chemistry and biosynthesis process of the fumonisins, their occurrence, effect on agriculture and food, along with their associated health issues. In addition, the focus has been put on the detection and management of fumonisins to ensure safe and healthy food. The main focus of the review is to provide insights to the readers regarding their health-associated food consumption and possible outbreaks. Furthermore, the consumers' knowledge and an attempt will ensure food safety and security and the farmers' knowledge for healthy agricultural practices, processing, and management, important to reduce the mycotoxin outbreaks due to fumonisins.

Function and Distribution of a Lantipeptide in Strawberry Fusarium Wilt Disease-Suppressive Soils

Streptomyces griseus S4-7 is representative of strains responsible for the specific soil suppressiveness of Fusarium wilt of strawberry caused by Fusarium oxysporum f. sp. fragariae. Members of the genus Streptomyces secrete diverse secondary metabolites including lantipeptides, heat-stable lanthionine-containing compounds that can exhibit antibiotic activity. In this study, a class II lantipeptide provisionally named grisin, of previously unknown biological function, was shown to inhibit F. oxysporum. The inhibitory activity of grisin distinguishes it from other class II lantipeptides from Streptomyces spp. Results of quantitative reverse transcription-polymerase chain reaction with lanM-specific primers showed that the density of grisin-producing Streptomyces spp. in the rhizosphere of strawberry was positively correlated with the number of years of monoculture and a minimum of seven years was required for development of specific soil suppressiveness to Fusarium wilt disease. We suggest that lanM can be used as a diagnostic marker of whether a soil is conducive or suppressive to the disease.

Herbaspirillum seropedicae promotes maize growth but fails to control the maize leaf anthracnose

Herbaspirillum seropedicae is an endophytic diazotrophic bacterium and a plant growth promoting bacteria. Colletotrichum graminicola causes the anthracnose, one of the most destructive maize diseases worldwide. The main objective of this work was to evaluate the effects of H. seropedicae SmR1 strain on the plant growth and leaf anthracnose of maize plants grown in substrate amended or not amended with humic substances. In the first assay, plants were pre-treated with H. seropedicae and inoculated with C. graminicola at 7, 14 and 21 days after treatment (DAT). In the second assay, plants were treated with H. seropedicae, grown in substrate amended with humic substances and inoculated at 3 and 7 DAT. The anthracnose severity was assessed by measurement of necrotic and chlorotic leaf area, and bacteria were quantified in leaves by quantitative PCR. H. seropedicae did not affect the disease severity in maize leaves, although it efficiently colonized the leaf tissues and it promoted maize leaf growth. Humic substances improved H. seropedicae colonization in maize.

Evolution and Diversity of Biosynthetic Gene Clusters in Fusarium

Plant pathogenic fungi in the Fusarium genus cause severe damage to crops, resulting in great financial losses and health hazards. Specialized metabolites synthesized by these fungi are known to play key roles in the infection process, and to provide survival advantages inside and outside the host. However, systematic studies of the evolution of specialized metabolite-coding potential across Fusarium have been scarce. Here, we apply a combination of bioinformatic approaches to identify biosynthetic gene clusters (BGCs) across publicly available genomes from Fusarium, to group them into annotated families and to study gain/loss events of BGC families throughout the history of the genus. Comparison with MIBiG reference BGCs allowed assignment of 29 gene cluster families (GCFs) to pathways responsible for the production of known compounds, while for 57 GCFs, the molecular products remain unknown. Comparative analysis of BGC repertoires using ancestral state reconstruction raised several new hypotheses on how BGCs contribute to Fusarium pathogenicity or host specificity, sometimes surprisingly so: for example, a gene cluster for the biosynthesis of hexadehydro-astechrome was identified in the genome of the biocontrol strain Fusarium oxysporum Fo47, while being absent in that of the tomato pathogen F. oxysporum f.sp. lycopersici. Several BGCs were also identified on supernumerary chromosomes; heterologous expression of genes for three terpene synthases encoded on the Fusarium poae supernumerary chromosome and subsequent GC/MS analysis showed that these genes are functional and encode enzymes that each are able to synthesize koraiol; this observed functional redundancy supports the hypothesis that localization of copies of BGCs on supernumerary chromosomes provides freedom for evolutionary innovations to occur, while the original function remains conserved. Altogether, this systematic overview of biosynthetic diversity in Fusarium paves the way for targeted natural product discovery based on automated identification of species-specific pathways as well as for connecting species ecology to the taxonomic distributions of BGCs.

Effect of land use and soil organic matter quality on the structure and function of microbial communities in pastoral soils: Implications for disease suppression

Cropping soils vary in extent of natural suppression of soil-borne plant diseases. However, it is unknown whether similar variation occurs across pastoral agricultural systems. We examined soil microbial community properties known to be associated with disease suppression across 50 pastoral fields varying in management intensity. The composition and abundance of the disease-suppressive community were assessed from both taxonomic and functional perspectives. Pseudomonas bacteria were selected as a general taxonomic indicator of disease suppressive potential, while genes associated with the biosynthesis of a suite of secondary metabolites provided functional markers (GeoChip 5.0 microarray analysis). The composition of both the Pseudomonas communities and disease suppressive functional genes were responsive to land use. Underlying soil properties explained 37% of the variation in Pseudomonas community structure and up to 61% of the variation in the abundance of disease suppressive functional genes. Notably, measures of soil organic matter quality, C:P ratio, and aromaticity of the dissolved organic matter content (carbon recalcitrance), influenced both the taxonomic and functional disease suppressive potential of the pasture soils. Our results suggest that key components of the soil microbial community may be managed on-farm to enhance disease suppression and plant productivity.

Isolation and characterization of wetland VSW-3, a novel lytic cold-active bacteriophage of Pseudomonas fluorescens

Wetlands are often called the “kidneys of the Earth” and contribute substantially to environmental improvement. Pseudomonas fluorescens is a major contaminant of milk products and causes the spoilage of refrigerated foods and fresh poultry. In this study, we isolated and characterized a lytic cold-active bacteriophage named VSW-3 together with P. fluorescens SW-3 cells from the Napahai wetland in China. Electron microscopy showed that VSW-3 had an icosahedral head (56 nm) and a tapering tail (20 nm × 12 nm) and a genome size of approximate 40 kb. On the basis of the top-scoring hits in the BLASTP analysis, VSW-3 showed a high degree of module similarity to the Pseudomonas phages Andromeda and Bf7. The latent and burst periods were 45 and 20 min, respectively, with an average burst size of 90 phage particles per infected cell. The pH and thermal stability of VSW-3 were also explored. The optimal pH was found to be 7.0 and the activity decreased rapidly when the temperature exceeded 60 °C. VSW-3 is a cold-active bacteriophage, hence, it is important to research its ability to prevent product contamination caused by P. fluorescens and to characterize its relationship with its host P. fluorescens in the future.

Factors Associated with Leguminous Green Manure Incorporation and Fusarium Wilt Suppression in Watermelon

Fall-planted Vicia villosa or Trifolium incarnatum cover crops, incorporated in spring as a green manure, can suppress Fusarium wilt (Fusarium oxysporum f. sp. niveum) of watermelon. During cover crop growth, termination, and incorporation into the soil, many factors such as arbuscular mycorrhizae colonization, leachate, and soil respiration differ. How these cover-crop-associated factors affect Fusarium wilt suppression is not fully understood. Experiments were conducted to evaluate how leachate, soil respiration, and other green-manure-associated changes affected Fusarium wilt suppression, and to evaluate the efficacy of the biocontrol product Actinovate AG (Streptomyces lydicus WYEC 108). General and specific suppression was examined in the field by assessing the effects of cover crop green manures (V. villosa, T. incarnatum, Secale cereale, and Brassica juncea) on soil respiration, presence of F. oxysporum spp., and arbuscular mycorrhizal colonization of watermelon. Cover crop treatments V. villosa, T. incarnatum, and S. cereale and no cover crop were evaluated both alone and in combination with Actinovate AG in the greenhouse. Additionally, in vitro experiments were conducted to measure the effects of cover crop leachate on the mycelial growth rates of F. oxysporum f. sp. niveum race 1 and Trichoderma harzianum. Soil microbial respiration was significantly elevated in V. villosa and Trifolium incarnatum treatments both preceding and following green manure incorporation, and was significantly negatively correlated with Fusarium wilt, suggesting that microbial activity was higher under the legumes, indicative of general suppression. Parallel to this, in vitro growth rates of F. oxysporum f. sp. niveum and Trichoderma harzianum on V. villosa leachate amended media were 66 and 213% greater, respectively, than on nonamended plates. The F. oxysporum spp. population (based on CFU and not differentiated into formae specialis or races) significantly increased in V. villosa-amended field plots. Additionally, the percentage of watermelon roots colonized by arbuscular mycorrhizae following V. villosa and Trifolium incarnatum green manures was significantly higher than in watermelon following bare ground (58 and 44% higher, respectively). In greenhouse trials where cover crops were amended to soil, Actinovate AG did not consistently reduce Fusarium wilt. Both general and specific disease suppression play a role in reducing Fusarium wilt on watermelon.

Polymicrobial Multi-functional Approach for Enhancement of Crop Productivity

There is an increasing global need for enhancing the food production to meet the needs of the fast-growing human population. Traditional approach to increasing agricultural productivity through high inputs of chemical nitrogen and phosphate fertilizers and pesticides is not sustainable because of high costs and concerns about global warming, environmental pollution, and safety concerns. Therefore, the use of naturally occurring soil microbes for increasing productivity of food crops is an attractive eco-friendly, cost-effective, and sustainable alternative to the use of chemical fertilizers and pesticides. There is a vast body of published literature on microbial symbiotic and nonsymbiotic nitrogen fixation, multiple beneficial mechanisms used by plant growth-promoting rhizobacteria (PGPR), the nature and significance of mycorrhiza-plant symbiosis, and the growing technology on production of efficacious microbial inoculants. These areas are briefly reviewed here. The construction of an inoculant with a consortium of microbes with multiple beneficial functions such as N(2) fixation, biocontrol, phosphate solubilization, and other plant growth-promoting properties is a positive new development in this area in that a single inoculant can be used effectively for increasing the productivity of a broad spectrum of crops including legumes, cereals, vegetables, and grasses. Such a polymicrobial inoculant containing several microorganisms for each major function involved in promoting the plant growth and productivity gives it greater stability and wider applications for a range of major crops. Intensifying research in this area leading to further advances in our understanding of biochemical/molecular mechanisms involved in plant-microbe-soil interactions coupled with rapid advances in the genomics-proteomics of beneficial microbes should lead to the design and development of inoculants with greater efficacy for increasing the productivity of a wide range of crops.

Analysis of the defence-related mechanism in cucumber seedlings in relation to root colonization by nonpathogenic Fusarium oxysporum CS-20

A defence response can be induced by nonpathogenic Fusarium oxysporum CS-20 in several crops, but the molecular mechanism has not been clearly demonstrated. In the present study, we analysed the defence mechanism of a susceptible cucumber cultivar (Cucumis sativus L. 9930) against a pathogen (F. oxysporum f. sp. cucumerinum) through the root precolonization of CS-20. A challenge inoculation assay indicated that the disease severity index (DSI) was reduced, ranging from 18.83 to 61.67 in comparison with the pathogen control. Root colonization analysis indicated that CS-20 clearly did not appear to influence the growth of cucumber seedlings. Quantitative reverse-transcriptase polymerase chain reaction (qRT-PCR) revealed that CS-20-mediated defence response was activated by PR3, LOX1 and PAL1 and the pathogen-mediated resistance response was regulated by PR1 and PR3. Moreover, both nonpathogenic and pathogenic F. oxysporum were able to upregulate NPR1 expression. In contrast to a pathogen, CS-20 can activate the Ca(2+) /CaM signal transduction pathway, and the gene expression of both CsCam7 and CsCam12 increased significantly. The gene expression analysis indicated that CS-20 strongly enhanced the expression of PR3, LOX1, PAL1, NPR1, CsCam7 and CsCam12 after inoculation. Overall, the defence response induced by CS-20 can be controlled by multiple genes in the cucumber plant.

Biological and chemical dependent systemic resistance and their significance for the control of root-knot nematodes

Inducing host plant-based systemic resistance is one of the modes of action involved in tri-trophic interactions between host plants, pests and mutualistic microorganisms. Two different types of systemic resistance – systemic acquired resistance (SAR) and induced systemic resistance (ISR) – were found to be functional against pathogens and plant-parasitic nematodes. In this study, the ability of Trichoderma harzianum isolate T10 and insecticidal active neem powder (NP) to induce systemic resistance in tomato against the root-knot nematode Meloidogyne javanica was compared with salicylic acid (SA) and jasmonic acid (JA) as standard elicitors for SAR and ISR, respectively. Results showed that, when the biotic and abiotic elicitors were applied to the inducer side of a split root plant system, a significant reduction in nematode infection was observed on the responder side. Physiological changes in the tomato plant due to the induction of SAR or ISA by these biotic and abiotic elicitors were further investigated using HPLC. Results demonstrated that T10 significantly increased the accumulation of different metabolites in the shoot of the tomato over the NP, JA and SA elicitors. Furthermore, the results demonstrated that several metabolic, physical and biochemical changes occurred in the shoots of the treated plants with both the biotic and abiotic elicitors. The percentage of membrane leakage (Ml) at nematode-infected tomato roots was significantly high, but the differences in percentage leakage were not significant in other treatments compared to the non-infested control. The best results were recorded with SA, T10 and NP, which gave the lowest MI% compared to the infested plants.

Development of a strain-specific real-time PCR assay for the detection and quantification of the biological control agent Fo47 in root tissues

Being able to identify specifically a biological control agent at the strain level is not the only requirement set by regulations (EC)1107/2009, it is also necessary to study the interactions of the agent with the plant and the pathogen in the rhizosphere. Fo47 is a soil-borne strain of Fusarium oxysporum which has the capacity to protect several plant species against the pathogenic formae speciales of F. oxysporum inducing wilts. A strain-specific sequence-characterized amplified region marker has been designed which makes it possible to distinguish Fo47 from other strains of F. oxysporum. In addition, a real-time PCR assay has been developed to quantify Fo47 in root tissues. The proposed assay has been validated by following the dynamics of root colonization of tomato plants grown in soil infested with Fo47. Results showed that with the method it is possible to quantify Fo47 in roots in the absence or presence of the pathogen and in the absence or in presence of the native microbial communities.

Application of inorganic carrier-based formulations of fluorescent pseudomonads and Piriformospora indica on tomato plants and evaluation of their efficacy

AIMS

Fluorescent pseudomonads are widely used as bioinoculants for improving plant growth and controlling phytopathogenic fungi. Piriformospora indica (Pi), a symbiotic root endophyte, also has beneficial effects on a number of plants. The present study focuses on the improvement of growth yields of tomato plants and control of Fusarium wilt using inorganic carrier-based formulations of two fluorescent pseudomonad strains (R62 and R81) and Pi.

METHODS AND RESULTS

The inorganic carrier-based formulations of pseudomonad strains and Pi were tested for plant growth promotion of tomato plants under glass house and field conditions. In controlled glass house experiments, 8·8-fold increase in dry root weight and 8·6-fold increase in dry shoot weight were observed with talcum powder-based consortium formulation of R81 and Pi. Field trial experiments ascertained the glfass house results with a considerable amount of increase in plant growth responses, and amongst all the treatments, R81 + Pi treatment performed consistently well in field conditions with an increase of 2·6-, 3·1- and 3·9-fold increase in dry root weight, shoot weight and fruit yield, respectively. The fluorescent pseudomonad R81 and Pi also acted as biocontrol agents, as their treatments could control the incidence of wilt disease caused by Fusarium oxysporum f.sp. lycopersici in tomato plants under glass house conditions.

CONCLUSIONS

The culture broths of pseudomonads R62, R81 and Pi were successfully used for development of talcum- and vermiculite-based bioinoculant formulations. In controlled glasshouse experiments, the talcum-based bioinoculant formulations performed significantly better over vermiculite-based formulations. In field experiments the talcum-based consortium formulation of pseudomonad R81 and Pi was most effective.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study suggests that the formulations of pseudomonad strains (R62 and R81) and Pi can be used as bioinoculants for improving the productivity of tomato plants. The application of such formulations is a step forward towards sustainable agriculture.

A coevolutionary framework for managing disease-suppressive soils

This review explores a coevolutionary framework for the study and management of disease-suppressive soil microbial communities. Because antagonistic microbial interactions are especially important to disease suppression, conceptual, theoretical, and empirical work on antagonistic coevolution and its relevance to disease suppression is reviewed. In addition, principles of coevolution are used to develop specific predictions regarding the drivers of disease-suppressive potential in soil microbial communities and to highlight important areas for future research. This approach brings an evolutionary perspective to microbial community management and emphasizes the role of species interactions among indigenous nonpathogenic microbes in developing and maintaining disease-suppressive activities in soil.

Control of Fusarium verticillioides, cause of ear rot of maize, by Pseudomonas fluorescens

BACKGROUND

Maize is one of the staple food crops grown in India. Fusarium verticillioides (Sacc.) Nirenberg is the most important fungal pathogen of maize, associated with diseases such as ear rot and kernel rot. Apart from the disease, it is capable of producing fumonisins, which have elicited considerable attention over the past decade owing to their association with animal disease syndromes. Hence, the present study was conducted to evaluate ecofriendly approaches by using a maize rhizosphere isolate of Pseudomonas fluorescens (Trev.) Mig. and its formulation to control ear rot disease and fumonisin accumulation, and also to study the capacity to promote growth and yield of maize. In vitro assays were conducted to test the efficacy of P. fluorescens as a seed treatment on seed germination, seedling vigour and also the incidence of F. verticillioides in different maize cultivars. The field trials included both seed treatment and foliar spray. For all the experiments, P. fluorescens was formulated using corn starch, wheat bran and talc powder. In each case there were three different treatments of P. fluorescens, a non-treated control and chemical control.

RESULTS

Pure culture and the formulations, in comparison with the control, increased plant growth and vigour as measured by seed germination, seedling vigour, plant height, 1000 seed weight and yield. P. fluorescens pure culture used as seed treatment and as spray treatment enhanced the growth parameters and reduced the incidence of F. verticillioides and the level of fumonisins to a maximum extent compared with the other treatments.

CONCLUSION

The study demonstrates the potential role of P. fluorescens and its formulations in ear rot disease management. The biocontrol potential of this isolate is more suited for fumonisin reduction in maize kernels intended for human and animal feed.

Role of 2,4-diacetylphloroglucinol-producing fluorescent Pseudomonas spp. in the defense of plant roots

Plants have evolved strategies of stimulating and supporting specific groups of antagonistic microorganisms in the rhizosphere as a defense against diseases caused by soilborne plant pathogens owing to a lack of genetic resistance to some of the most common and widespread soilborne pathogens. Some of the best examples of natural microbial defense of plant roots occur in disease suppressive soils. Soil suppressiveness against many different diseases has been described. Take-all is an important root disease of wheat, and soils become suppressive to take-all when wheat or barley is grown continuously in a field following a disease outbreak; this phenomenon is known as take-all decline (TAD). In Washington State, USA and The Netherlands, TAD results from the enrichment during monoculture of populations of 2,4-diacetylphloroglucinol (2,4-DAPG)-producing Pseudomonas fluorescens to a density of 10 (5) CFU/g of root, the threshold required to suppress the take-all pathogen, Gaeumannomyces graminis var. tritici. 2,4-DAPG-producing P. fluorescens also are enriched by monoculture of other crops such as pea and flax, and evidence is accumulating that 2,4-DAPG producers contribute to the defense of plant roots in many different agroecosystems. At this time, 22 distinct genotypes of 2,4-DAPG producers (designated A - T, PfY and PfZ) have been defined by whole-cell repetitive sequence-based (rep)-PCR analysis, restriction fragment length polymorphism (RFLP) analysis of PHLD, and phylogenetic analysis of PHLD, but the number of genotypes is expected to increase. The genotype of an isolate is predictive of its rhizosphere competence on wheat and pea. Multiple genotypes often occur in a single soil and the crop species grown modulates the outcome of the competition among these genotypes in the rhizosphere. 2,4-DAPG producers are highly effective biocontrol agents against a variety of plant diseases and ideally suited for serving as vectors for expressing other biocontrol traits in the rhizosphere.

Biological Control Efficiency of Fusarium Wilt of Tomato by Nonpathogenic Fusarium oxysporum Fo-B2 in Different Environments

ABSTRACT Efficiency of nonpathogenic Fusarium oxysporum Fo-B2 for the biological control of Fusarium wilt of tomato, caused by F. oxysporum f. sp. lycopersici CU1, was examined in different environments: a growth chamber with sterile soil-less medium, a greenhouse with fumigated or nonfumigated soil, and nonfumigated field plots. Inoculation of Fo-B2 onto tomato roots significantly reduced the severity of disease, but the efficiency of disease suppression decreased as the experimental environment became less controlled. Relationships between the recovery of Fo-B2 from hypocotyls and the disease severity indicated that the biocontrol agent was most effective when it colonized vascular tissues intensively. Moreover, the degree of Fo-B2 colonization was greatly reduced when the seedlings were grown in nonfumigated soil. Dose-response models (negative exponential, hyperbolic saturation, and logistic) were fit to observed data collected over a range of inoculum densities of the pathogen and the antagonist; the logistic model provided the best fit in all environments. The ratios of an 50% effective dose parameter for Fo-B2 to that of CU1 increased as the environment became less controlled, suggesting that environmentally related efficiency reduction impacted the antagonist more than the pathogen. The results suggest that indigenous soil microbes were a primary factor negatively influencing the efficiency of Fo-B2. Therefore, early establishment of the antagonist in a noncompetitive environment prior to outplanting could improve the efficacy of biological control.

Visualization of interactions between a pathogenic and a beneficial Fusarium strain during biocontrol of tomato foot and root rot

The soilborne fungus Fusarium oxysporum f. sp. radicis-lycopersici causes tomato foot and root rot (TFRR), which can be controlled by the addition of the nonpathogenic fungus F. oxysporum Fo47 to the soil. To improve our understanding of the interactions between the two Fusarium strains on tomato roots during biocontrol, the fungi were labeled using different autofluorescent proteins as markers and subsequently visualized using confocal laser scanning microscopy. The results were as follows. i) An at least 50-fold excess of Fo47over F. oxysporum f. sp. radicis-lycopersici was required to obtain control of TFRR. ii) When seedlings were planted in sand infested with spores of a single fungus, Fo47 hyphae attached to the root earlier than those of F. oxysporum f. sp. radicis-lycopersici. iii) Subsequent root colonization by F. oxysporum f. sp. radicis-lycopersici was faster and to a larger extent than that by Fo47. iv) Under disease-controlling conditions, colonization of tomato roots by the pathogenic fungus was significantly reduced. v) When the inoculum concentration of Fo47 was increased, root colonization by the pathogen was arrested at the stage of initial attachment to the root. vi) The percentage of spores of Fo47 that germinates in tomato root exudate in vitro is higher than that of the pathogen F. oxysporum f. sp. radicis-lycopersici. Based on these results, the mechanisms by which Fo47 controls TFRR are discussed in terms of i) rate of spore germination and competition for nutrients before the two fungi reach the rhizoplane; ii) competition for initial sites of attachment, intercellular junctions, and nutrients on the tomato root surface; and iii) inducing systemic resistance.

Pseudomonas fluorescens infection by bacteriophage PhiS1: the influence of temperature, host growth phase and media

The influence of host growth temperature, phase and media, together with the effect of infection temperature on bacteriophage PhiS1 infection of Pseudomonas fluorescens were examined. The rates of cell lysis and phage release were determined and showed that the efficacy of phage infection was optimal with host cells grown and infected at 26 degrees C. The host physiological state also affected these rates. Infection was dependent on the presence of cell wall proteins with molecular weights of 17.5+/-1 and 99+/-5 kDa.

Integrated management of fusarium wilt of chickpea with sowing date, host resistance, and biological control

ABSTRACT A 3-year experiment was conducted in field microplots infested with Fusarium oxysporum f. sp. ciceris race 5 at Córdoba, Spain, in order to assess efficacy of an integrated management strategy for Fusarium wilt of chickpea that combined the choice of sowing date, use of partially resistant chickpea genotypes, and seed and soil treatments with biocontrol agents Bacillus megaterium RGAF 51, B. subtilis GB03, nonpathogenic F. oxysporum Fo 90105, and Pseudomonas fluorescens RG 26. Advancing the sowing date from early spring to winter significantly delayed disease onset, reduced the final disease intensity (amount of disease in a microplot that combines disease incidence and severity, expressed as a percentage of the maximum possible amount of disease in that microplot), and increased chickpea seed yield. A significant linear relationship was found between disease development over time and weather variables at the experimental site, with epidemics developing earlier and faster as mean temperature increased and accumulated rainfall decreased. Under conditions highly conducive for Fusarium wilt development, the degree of disease control depended primarily on choice of sowing date, and to a lesser extent on level of resistance of chickpea genotypes to F. oxysporum f. sp. ciceris race 5, and the biocontrol treatments. The main effects of sowing date, partially resistant genotypes, and biocontrol agents were a reduction in the rate of epidemic development over time, a reduction of disease intensity, and an increase in chickpea seedling emergence, respectively. Chickpea seed yield was influenced by all three factors in the study. The increase in chickpea seed yield was the most consistent effect of the biocontrol agents. However, that effect was primarily influenced by sowing date, which also determined disease development. Effectiveness of biocontrol treatments in disease management was lowest in January sowings, which were least favorable for Fusarium wilt. Sowing in February, which was moderately favorable for wilt development, resulted in the greatest increase in seed yield by the biocontrol agents. In March sowings, which were most conducive for the disease, the biocontrol agents delayed disease onset and increased seedling emergence. B. subtilis GB03 and P. fluorescens RG 26, applied either alone or each in combination with nonpathogenic F. oxysporum Fo 90105, were the most effective treatments at suppressing Fusarium wilt, or delaying disease onset and increasing seed yield, respectively. The importance of integrating existing control practices, partially effective by themselves, with other control measures to achieve appropriate management of Fusarium wilt and increase of seed yield in chickpea in Mediterranean-type environments is demonstrated by the results of this study.

Microbial diversity in soil: selection microbial populations by plant and soil type and implications for disease suppressiveness

An increasing interest has emerged with respect to the importance of microbial diversity in soil habitats. The extent of the diversity of microorganisms in soil is seen to be critical to the maintenance of soil health and quality, as a wide range of microorganisms is involved in important soil functions. This review focuses on recent data relating how plant type, soil type, and soil management regime affect the microbial diversity of soil and the implication for the soil's disease suppressiveness. The two main drivers of soil microbial community structure, i.e., plant type and soil type, are thought to exert their function in a complex manner. We propose that the fact that in some situations the soil and in others the plant type is the key factor determining soil microbial diversity is related to the complexity of the microbial interactions in soil, including interactions between microorganisms and soil and microorganisms and plants. A conceptual framework, based on the relative strengths of the shaping forces exerted by plant and soil versus the ecological behavior of microorganisms, is proposed.

Fusarium oxysporum and its biocontrol

Fusarium oxysporum is well represented among the rhizosphere microflora. While all strains exist saprophytically, some are well-known for inducing wilt or root rots on plants whereas others are considered as nonpathogenic. Several methods based on phenotypic and genetic traits have been developed to characterize F. oxysporum strains. Results showed the great diversity affecting the soil-borne populations of F. oxysporum. In suppressive soils, interactions between pathogenic and nonpathogenic strains result in the control of the disease. Therefore nonpathogenic strains are developed as biocontrol agents. The nonpathogenic F. oxysporum strains show several modes of action contributing to their biocontrol capacity. They are able to compete for nutrients in the soil, affecting the rate of chlamydospore germination of the pathogen. They can also compete for infection sites on the root, and can trigger plant defence reactions, inducing systemic resistance. These mechanisms are more or less important depending on the strain. The nonpathogenic F. oxysporum are easy to mass produce and formulate, but application conditions for biocontrol efficacy under field conditions have still to be determined.

Effects of Varying Environmental Conditions on Biological Control of Fusarium Wilt of Tomato by Nonpathogenic Fusarium spp

ABSTRACT The influence of varying environmental and cropping conditions including temperature, light, soil type, pathogen isolate and race, and cultivar of tomato on biological control of Fusarium wilt of tomato by isolates of nonpathogenic Fusarium oxysporum (CS-20 and CS-24) and F. solani (CS-1) was evaluated in greenhouse and growth chamber experiments. Liquid spore suspensions (10(6)/ml) of the biocontrol isolates were applied to soilless potting mix at the time of tomato seeding, and the seedlings were transplanted into pathogen-infested field soil 2 weeks later. Temperature regimes ranging from 22 to 32 degrees C significantly affected disease development and plant physiological parameters. Biocontrol isolate CS-20 significantly reduced disease at all temperature regimes tested, yielding reductions of disease incidence of 59 to 100% relative to pathogen control treatments. Isolates CS-24 and CS-1 reduced disease incidence in the greenhouse and at high temperatures, but were less effective at the optimum temperature for disease development (27 degrees C). Growing plants under shade (50% of full light) versus full light affected some plant growth parameters, but did not affect the efficacy of biocontrol of any of the three bio-control isolates. Isolate CS-20 effectively reduced disease incidence (56 to 79% reduction) in four different field soils varying in texture (sandy to clayey) and organic matter content (0 to 3.2%). Isolate CS-1 reduced disease in the sandy and loamy soils (49 to 66% reduction), but was not effective in a heavy clay soil. Both CS-1 and CS-20 were equally effective against all three races of the pathogen, as well as multiple isolates of each race (48 to 66% reduction in disease incidence). Both isolates, CS-1 and CS-20, were equally effective in reducing disease incidence (66 to 80% reduction) by pathogenic races 1, 2, and 3 on eight different tomato cultivars containing varying levels of inherent resistance to Fusarium wilt (susceptible, resistant to race 1, or resistant to races 1 and 2). These results demonstrate that both these Fusarium isolates, and particularly CS-20, can effectively reduce Fusarium wilt disease of tomato under a variety of environmental conditions and have potential for further development.

Novel Use of a Pyrenomycetous Mycoparasite for Management of Fusarium Wilt of Watermelon

Fusarium wilt of watermelon, caused by Fusarium oxysporum f. sp. niveum, is a destructive disease that limits watermelon production in many areas of the world. The discovery of several pyrenomycetous ascomycetes occurring naturally in association with different formae speciales of F. oxysporum identified these fungi as potential biological control organisms for watermelon wilt. One such mycoparasitic isolate, identified as Sphaerodes retispora var. retispora, was chosen for biological control and ecological trials in the greenhouse. Four different methods to inoculate the mycoparasite were evaluated, three of which utilized the parasite encapsulated into sodium alginate pellets. The other method employed root-dipping plants with mycoparasite ascospore suspensions. Ecological factors also were investigated, including the ability of S. retispora var. retispora to colonize watermelon roots, and its ability to survive in soil over time and reduce propagules of F. oxysporum f. sp. niveum. In the biological control studies, the use of the mycoparasite significantly reduced plant mortality and increased dry weights of watermelon plants after being challenged with F. oxysporum f. sp. niveum, compared with pathogen-inoculated controls. It appears that the incorporation of the parasite into alginate pellets in the planting mix at seeding may be the most practical method for future field evaluations of transplant-grown vegetable crops. In the ecological studies, the mycoparasite was recovered from infested soil after 9 months, but was only isolated from watermelon roots when applied in the presence of F. oxysporum. S. retispora var. retispora had no effect on F. oxysporum f. sp. niveum propagules after being applied to soils in the greenhouse.

Microbial populations responsible for specific soil suppressiveness to plant pathogens

Agricultural soils suppressive to soilborne plant pathogens occur worldwide, and for several of these soils the biological basis of suppressiveness has been described. Two classical types of suppressiveness are known. General suppression owes its activity to the total microbial biomass in soil and is not transferable between soils. Specific suppression owes its activity to the effects of individual or select groups of microorganisms and is transferable. The microbial basis of specific suppression to four diseases, Fusarium wilts, potato scab, apple replant disease, and take-all, is discussed. One of the best-described examples occurs in take-all decline soils. In Washington State, take-all decline results from the buildup of fluorescent Pseudomonas spp. that produce the antifungal metabolite 2,4-diacetylphloroglucinol. Producers of this metabolite may have a broader role in disease-suppressive soils worldwide. By coupling molecular technologies with traditional approaches used in plant pathology and microbiology, it is possible to dissect the microbial composition and complex interactions in suppressive soils.

Genetic diversity of Fusarium oxysporum populations isolated from different soils in France

The genetic diversity of soil-borne populations of Fusarium oxysporum was assessed using 350 isolates collected from six different French soils. All isolates were characterised by restriction fragment analysis of the PCR-amplified ribosomal intergenic spacer (IGS). Twenty-six IGS types were identified among the 350 isolates analysed. Five to nine different IGS types were detected in each soil. None of the IGS types was common to all of the soils. An analysis of the molecular variance based on IGS type relationships and frequency revealed that the genetic structure of the populations of F. oxysporum varied widely among the soils. Some populations were both highly diverse within the soils and differentiated between the soils. A possible relationship between the intrapopulation or interpopulation level of diversity and some external factors such as the soil type or the crop history was evaluated. A subsample representative of the diversity of the six populations was further characterised by analysing the genomic distribution of two transposable elements, impala and Fot1. One to 10 copies of the impala element were present in most of the isolates, irrespective of their soil of origin. The Fot1 element was only detected in 40% of the isolates originating from the three populations less diverse in terms of IGS types, but in 82.6% of the isolates originating from the three more diverse populations.

Influence of growth and environmental conditions on cell surface hydrophobicity ofPseudomonas fluorescensin non-specific adhesion

The relative cell surface hydrophobicity (CSH) of 18 soil isolates of Pseudomonas fluorescens, determined by phase exclusion, hydrophobic interaction chromatography (HIC), electrostatic interaction chromatography (ESIC), and contact angle, revealed large degrees of variability. Variation in the adhesion efficiency to Macrophomina phaseolina of the hyphae/sclerotia of these isolates was also examined. Two such isolates with maximum (32.8%; isolate 12-94) and minimum (12%; isolate 30-94) CSH were selected for further study. Early- to mid-log exponential cells of these isolates were more hydrophobic than those in stationary phase, and the CSH of these isolates was also influenced by fluctuations in temperatures and pH. Isolate 12-94 exhibited high CSH (32.3%) at 30°C, compared to lower values (28-24%) in the higher temperature range (35-40°C). Increasing concentrations of either Zn2+, Fe3+, K+, and Mg2+in the growth medium were associated with the increased CSH. Trypsin, pepsin, and proteinase K (75 to 150 μg·mL-1) reduced the CSH of isolate 12-94 cells. CSH was reduced, following exposure to DTT, SDS, Triton X-100, or Tween 80. Prolonged exposure of cells to starvation (60 days) also caused a significant decline in CSH. Several protein bands (18, 21, 23, 26 kDa) of the outer cell membrane were absent in 60-day starved cells compared to unstarved cells. In conclusion, our findings demonstrate that CSH of P. fluorescens isolates may contribute to non-specific attachment/adhesion onto M. phaseolina hyphae/sclerotia, and the efficiency of adhesion is regulated by growth and other environmental conditions. Key words: adhesion, hydrophobicity, Pseudomonas fluorescens, Macrophomina phaseolina

Mechanisms of Action and Dose-Response Relationships Governing Biological Control of Fusarium Wilt of Tomato by Nonpathogenic Fusarium spp

ABSTRACT Three isolates of nonpathogenic Fusarium spp. (CS-1, CS-20, and Fo47), previously shown to reduce the incidence of Fusarium wilt diseases of multiple crops, were evaluated to determine their mechanisms of action and antagonist-pathogen inoculum density relationships. Competition for nutrients, as represented by a reduction in pathogen saprophytic growth in the presence of the biocontrol isolates, was observed to be an important mechanism of action for isolate Fo47, but not for isolates CS-1 and CS-20. All three biocontrol isolates demonstrated some degree of induced systemic resistance in tomato (Lycopersicon esculentum) and watermelon (Citrullus lanatus) plants, as determined by split-root tests, but varied in their relative abilities to reduce disease. Isolate CS-20 provided the most effective control (39 to 53% disease reduction), while Fo47 provided the least effective control (23 to 25% reduction) in split-root tests. Dose-response relationships also differed considerably among the three biocon-trol isolates, with CS-20 significantly reducing disease incidence at antagonist doses as low as 100 chlamydospores per g of soil (cgs) and at pathogen densities up to 10(5) cgs. Isolate CS-1 also was generally effective at antagonist densities of 100 to 5,000 cgs, but only when pathogen densities were below 10(4) cgs. Isolate Fo47 was effective only at antagonist densities of 10(4) to 10(5) cgs, regardless of pathogen density. Epidemiological dose-response models (described by linear, negative exponential, hyperbolic saturation [HS], and logistic [LG] functions) fit to the observed data were used to quantify differences among the biocontrol isolates and establish biocontrol characteristics. Each isolate required a different model to best describe its dose-response characteristics, with the HS/HS, LG/HS, and LG/LG models (pathogen/biocontrol components) providing the best fit for isolates CS-1, CS-20, and Fo47, respectively. Model parameters (defining effective biocontrol dose (ED(50)) indicated an ED(50) of 2.6, 36.3, and 2.1 x 10(6) cgs and estimates of biocontrol efficiency of 0.229, 0.539, and 0.774 for isolates CS-1, CS-20, and Fo47, respectively. Differences in dose-response relationships among the biocontrol isolates were attributed to differences in their mechanisms of action, with CS-20 and CS-1 functioning primarily by induced resistance and Fo47 functioning primarily by competition for nutrients.

Microbial Antagonism at the Root Level Is Involved in the Suppression of Fusarium Wilt by the Combination of Nonpathogenic Fusarium oxysporum Fo47 and Pseudomonas putida WCS358

ABSTRACT Two biological control agents, nonpathogenic Fusarium oxysporum Fo47 and Pseudomonas putida WCS358, were evaluated for suppression of Fusarium wilt of flax grown in nutrient solution and for suppression of the population density and metabolic activity of the causal organism F. oxysporum f. sp. lini strain Foln3GUS on root surfaces. Due to the presence of an introduced gusA reporter gene construct in Foln3GUS, the pathogen expressed beta-glucuronidase activity that was related to its carbon metabolism. At a Fo47 to Foln3GUS inoculum ratio of 100:1, both the population density of the pathogen and the beta-glucuronidase activity on and in flax roots were reduced by the nonpathogenic strain, and Fusarium wilt was suppressed. At a Fo47 to Foln3GUS inoculum ratio of 10:1, Fo47 decreased the severity of Fusarium wilt to a smaller extent and it also reduced beta-glucuronidase activity without reducing the density of Foln3GUS on flax roots. At a nonpathogenic to pathogenic Fusarium strains ratio of 10:1, the addition of P. putida WCS358 further suppressed Fusarium wilt and the density of the pathogen at the root level, whereas a mutant of WCS358 deficient in pseudobactin production had no significant effect. Iron availability to WCS358 on flax roots, assessed by ice-nucleation activity conferred from a transcriptional fusion (pvd-inaZ) of an ice-nucleation reporter gene to an iron-regulated promoter, was sufficiently low to allow pseudobactin production. P. putida WCS358 did not reduce the severity of Fusarium wilt of flax when inoculated without Fo47, and it did not improve disease suppression achieved by high inoculum doses of Fo47 (a Fo47 to Foln3GUS ratio of 100:1). Together, these data provide evidence that (i) suppression of Fusarium wilt of flax by Fo47 is related to reductions in the population density and metabolic activity of the pathogen on the root surface; (ii) WCS358 can enhance the biological control activity of Fo47, but this enhancement depends on the population of Fo47 relative to the pathogen; and (iii) pseudobactin contributes to suppression of Fusarium wilt by the combination of Fo47 and WCS358 on roots in which conditions are conducive to pseudobactin production by the bacterium.