Suppression Of Plant Pathogens Research Articles

Plant–microbe interactions promoting plant growth and health: perspectives for controlled use of microorganisms in agriculture

Plant-associated microorganisms fulfill important functions for plant growth and health. Direct plant growth promotion by microbes is based on improved nutrient acquisition and hormonal stimulation. Diverse mechanisms are involved in the suppression of plant pathogens, which is often indirectly connected with plant growth. Whereas members of the bacterial genera Azospirillum and Rhizobium are well-studied examples for plant growth promotion, Bacillus, Pseudomonas, Serratia, Stenotrophomonas, and Streptomyces and the fungal genera Ampelomyces, Coniothyrium, and Trichoderma are model organisms to demonstrate influence on plant health. Based on these beneficial plant-microbe interactions, it is possible to develop microbial inoculants for use in agricultural biotechnology. Dependent on their mode of action and effects, these products can be used as biofertilizers, plant strengtheners, phytostimulators, and biopesticides. There is a strong growing market for microbial inoculants worldwide with an annual growth rate of approximately 10%. The use of genomic technologies leads to products with more predictable and consistent effects. The future success of the biological control industry will benefit from interdisciplinary research, e.g., on mass production, formulation, interactions, and signaling with the environment, as well as on innovative business management, product marketing, and education. Altogether, the use of microorganisms and the exploitation of beneficial plant-microbe interactions offer promising and environmentally friendly strategies for conventional and organic agriculture worldwide.

Predictive Capacity of Visible-Near Infrared Spectroscopy for Quality Parameter Assessment of Compost

Screening tests are basic procedures commonly used to assess compost quality. Important parameters for quality assessment are the germination capacity and the suppression of plant pathogens which have to be measured by time-consuming laboratory methods. The objective was to test whether visible (vis) and near infrared (NIR) spectroscopy (vis-NIR) is useful to analyse parameters important for compost quality. Ninety seven compost samples from Switzerland were analysed by conventional methods and by vis-NIR. The content of organic (Corg) and inorganic C (Cinorg), total N (Ntot), mineralisable N after 56 days (Nmin_d56), total P (Ptot), K, Ca and salt, the C/N ratio, pH and microbiological characteristics [hydrolysis of fluorescein diacetate (FDA-hydrolysis) as indicator of total enzyme activity and cellulase activity] were determined. Furthermore, plant tolerance and the suppression of pathogens were tested using germination tests with salad, cress, ryegrass and bean or a Rhizoctonia solani bioassay, respectively. The samples were scanned in the range of 400–2500 nm (visible light and NIR) using a Foss NIRSystems spectrometer 6500. A modified partial least squares regression method and the whole spectrum were used to develop cross-validation equations for all constituents. For this, the first to third derivative was calculated. The prediction accuracy was evaluated as excellent for Corg and good for N, and the C/N ratio based on the RSC values (ratio of standard deviation of laboratory results to standard error of cross-validation) and the coefficients of determination ( r2). Approximate quantitative predictions were possible for the contents of Ptot, K, Ca and salt, whereas for the constituents Cinorg, Nmin_d56, FDA-hydrolysis and the germination tests with cress and salad only between high and low values could be discriminated. Unsuccessful predictions as indicated by RSC values lower than 1.5 and r2 values below 0.50 were obtained for pH, cellulase activity, germination tests with ryegrass and bean and the disease suppression test using R. solani. Overall the results of the present study indicate that vis-NIR spectroscopy has the potential to be used for quality assessment of composts and to replace time-consuming methods such as germination tests using salad and cress. However, the use for monitoring purposes requires further research to clarify whether other complex quality parameters such as disease suppression indicators may also be predicted successfully.

Preparation of New Biofungicides Using Antagonistic Bacteria and Mineral Compounds for Controlling Cotton Seedling Damping-Off Disease

The overuse of chemical pesticides in agriculture has caused serious environmental problems and thus the demand for safer pesticides is increasing. One alternative is microbial pesticides that suppress plant pathogens via their microbial activities. As microbial pesticides are eco-friendly products, in this study we prepared four biological fungicides using two isolates of Pseudomonas fluorescens that included a talc-based powder and bentonite-based powder as mineral carriers. Then we evaluated the efficacy of these products in controlling cotton seedlings, damping-off, a fungal disease caused by Rhizoctonia solani at four intervals of 15, 30, 45 and 60 days after sowing the cotton seeds under greenhouse conditions. The results of greenhouse experiment on application of biofun- gicides showed that the efficacy of Bentonite-B 1 treatment to control R. solani was promising as it increased the number of healthy seedlings 3.42 to 3.57 - fold and was much more effective than the carboxin/thiram fungicide in all stages.

A novel role for Trichoderma secondary metabolites in the interactions with plants

Secondary metabolites play a pivotal role in the antagonistic activities of some biocontrol species of Trichoderma resulting in the suppression of plant pathogens, but their involvement in complex interactions with plants has not been specifically studied. In this work the major secondary metabolites produced by biocontrol strains of Trichoderma ( T. harzianum strains T22, T39 and A6, and T. atroviride strain P1) have been investigated for their effect on plant growth promotion. An auxin-like activity was observed on etiolated pea ( Pisum sativum) stems treated with harzianolide and 6- n-pentyl-6 H-pyran-2-one (6PP), which also affected the growth of tomato ( Lycopersicum esculentum) and canola ( Brassica napus) seedlings. The ability of these molecules to induce systemic defence responses in planta was also investigated. Tomato and oil-seed rape seedlings were treated with the metabolites and then inoculated with a spore suspension of Botrytis cinerea or Leptosphaeria maculans, respectively. In both cases, a reduction of disease symptoms was observed, particularly on 6PP-treated plants. Moreover an over-expression of pathogenesis-related (PR) proteins was also detected in treated plants. These results clearly indicate that secondary metabolites of Trichoderma spp. may have a role in both plant growth regulation and activation of plant defence responses.

Interference of quorum sensing and virulence of the rice pathogen Burkholderia glumae by an engineered endophytic bacterium

Many bacterial species are known to thrive within plants. Among these bacteria, a group referred to as endophytes provide beneficial effects to the host plants by the promotion of plant growth and the suppression of plant pathogens. Among 44 putative endophytic isolates isolated from surface-sterilized rice roots, Burkholderia sp. KJ006 was selected for further study because of a lack of pathogenicity to rice, a broad spectrum of antifungal properties, and the presence of the nifH gene, which is an indicator for nitrogen fixation. In an attempt to control Burkholderia glumae, a casual pathogen of seedling rot and grain rot of rice, an N-acyl-homoserine lactonase (aiiA) gene from Bacillus thuringiensis was introduced into Burkholderia sp. KJ006 given that the major virulence factor of Burkholderia glumae is controlled in a population-dependent manner (quorum sensing). The engineered strain KJ006 (pKPE-aiiA) inhibited production of quorum-sensing signals by Burkholderia glumae in vitro and reduced the disease incidence of rice seedling rot caused by Burkholderia glumae in situ. Our results indicate the possibility that a bacterial endophyte transformed with the aiiA gene can be used as a novel biological control agent against pathogenic Burkholderia glumae that are known to occupy the same ecological niche.

Interactions among endophytic bacteria and fungi: Effects and potentials

Plants benefit extensively by harbouring endophytic microbes. They promote plant growth and confer enhanced resistance to various pathogens. However, the way the interactions among endophytes influence the plant productivity has not been explained. Present study experimentally showed that endophytes isolated from rice (Oryza sativa) used as the test plant produced two types of interactions; biofilms (bacteria attached to mycelia) and mixed cultures with no such attachments. Acidity, as measured by pH in cultures with biofilms was higher than that of fungi alone, bacteria alone or the mixed cultures. Production of indoleacetic acid like substances (IAAS) of biofilms was higher than that of mixed cultures, fungi or bacteria. Bacteria and fungi produced higher quantities of IAAS than mixed cultures. In mixed cultures, the potential of IAAS production of resident microbes was reduced considerably. There was a negative relationship between IAAS and pH of the biofilms, indicating that IAAS was the main contributor to the acidity. However, such a relationship was not observed in mixed cultures. Microbial acid production is important for suppressing plant pathogens. Thus the biofilm formation in endophytic environment seems to be very important for healthy and improved plant growth. However, it is unlikely that an interaction among endophytes takes place naturally in the endophytic environment, due to physical barriers of plant tissues. Further, critical cell density dependant quorum sensing that leads to biofilm formation may not occur in the endophytic environment as there is a limited space. As such in vitro production and application of beneficial biofilmed inocula of endophytes are important for improved plant production in any agro-ecosystem. The conventional practice of plant inoculation with monocultures or mixed cultures of effective microbes may not give the highest microbial effect, which may only be achieved by biofilm formation.

A polyphasic approach for assessing maturity and stability in compost intended for suppression of plant pathogens

One of the most debated parameters surrounding the use of compost is the ability to evaluate maturity. Accurately determining maturity of composts is important in assessing application potential, including when considering compost as a suppressive agent to plant pathogens. In this study, methods to assess compost maturity and stability were evaluated on statically aerated composts known to be suppressants to turfgrass pathogens. Since disease suppressive capabilities of composts are related to microbial activity and microbial community structure, it is beneficial to monitor these components both during the composting process and in storage. Temperature and oxygen levels were useful in tracking progression of composting and assessment of compost maturity and stability. Physico-chemical variables monitored during composting and storage (nutrient levels (N, P, K, C), C:N ratio, dry matter, pH, and electrical conductivity) were indicative of some patterns. During storage of mature composts, oxygen consumption and carbon dioxide production were, indicative of maintenance of an active microbial population. Fluorescein diacetate (FDA) hydrolysis, monitored throughout composting and during storage, was indicative of maintenance of high rates of microbial activity, even during storage. Phospholipid fatty acid (PLFA) analysis was a valuable method for tracking microbial community profiles and metabolic rates during composting and in storage. Results were similar to respiration and FDA testing, in that they demonstrated a high level of microbial activity in composts. Development of composts for end-product uses such as plant disease and pathogen suppression requires monitoring of the composting processes to better understand progression of feedstocks into value-added materials. In addition, evaluation of storage capabilities of this ‘living’ material may aid in prediction of its useful life span as a disease suppressant.

Ecology of Bacillus and Paenibacillus spp. in Agricultural Systems

ABSTRACT Diverse populations of aerobic endospore-forming bacteria occur in agricultural fields and may directly and indirectly contribute to crop productivity. This paper describes recent advances in our understanding of the ecology of Bacillus and Paenibacillus spp. and how different subpopulations of these two genera can promote crop health. The abundance, diversity, and distribution of native populations and inoculant strains in agricultural fields have been characterized using a variety of methods. While native populations of these two genera occur abundantly in most agricultural soils, plant tissues are differentially colonized by distinct subpopulations. Multiple Bacillus and Paenibacillus spp. can promote crop health in a variety of ways. Some populations suppress plant pathogens and pests by producing antibiotic metabolites, while others may directly stimulate plant host defenses prior to infection. Some strains can also stimulate nutrient uptake by plants, either by promoting rhizobial and mycorrhizal symbioses or by fixing atmospheric nitrogen directly. Despite a wealth of new information on the genetics and physiology of Bacillus and related species, a better understanding of the microbial ecology of these two genera must be developed. To this end, several important, but unanswered, questions related to the ecological significance and potential for managing the beneficial activities of these bacteria are discussed.

Overview of the Nature and Application of Biocontrol Microbes: Bacillus spp.

While diverse microbes may contribute to the biological control of plant pathogens, most research and development efforts have focused on isolates of three genera, Bacillus, Trichoderma, and Pseudomonas. Continued studies of these three genera will be needed to further advance our understanding of the nature of biological control microbes and to improve our ability to successfully integrate biological control strategies into pest management systems. While many presentations given at The American Phytopathological Society Annual Meetings cover advances in biological control of plant pathogens with various agents, no comprehensive review of a particular genus has been conducted since the 1998 symposium focused on Burkholderia spp. Given the recent advances in microbial genomics, as well as the on-going preeminence of these three genera in commerce and research, the Biological Control Committee proposed a series of symposia to intensively review our current knowledge of these three genera. The first of these, co-sponsored by the Soil Microbiology and Phyllosphere Microbiology committees, was entitled “The Nature and Application of Biocontrol Microbes: Bacillus spp.” and convened on 12 August 2003 in Charlotte, NC. The following series of papers present a thorough review of the biology of Bacillus spp., focusing attention to details most relevant to the development and application of biological control strategies involving various strains of the genus. A comprehensive treatise of the biology of Bacillus sp. and related microbes was published about 10 years ago (3) and recently updated (4). The contents of the latter volume indicate the tremendous advances achieved in our understanding of the cellular and molecular biology of the genus Bacillus, based largely on studies of the model system B. subtilis strain 168. Reviews by Stahly et al. (5), Slepecky and Hemphill (2), and more recently, Nicholson (1) provide excellent introductions to the biology of the genus Bacillus and closely related genera. However, additional work has been done that further describes the microbial ecology of the genus and the capacity of different strains to improve crop health and productivity. The following papers expand upon the presentations given by scientists from academia, government, and industry with a common interest in Bacillus spp. These presentations helped to link otherwise separate fields and helped to catalyze multidisciplinary research on the genus. Much remains to be learned if we are to improve biocontrol methodologies. Refinements in the taxonomy, with particular emphasis on distinguishing strains with and without biocontrol potential, will be needed. The molecular basis for biological control and the regulation of such activities has yet to be fully explored, even in some of the best studied strains. The environmental factors that promote and constrain biocontrol activities of natural and inoculant strains must also be better characterized. Central to these efforts will be genomic approaches. For example, the ability of biocontrol strains of B. subtilis (e.g., GB03), but not standard laboratory strains (i.e., those derived from strain 168), to suppress plant pathogens raises the possibility of using genome sequence comparisons in conjunction with genetic approaches to discover and characterize the genes critical to those interactions leading to improvements in plant health. Such analyses would require only a relatively modest investment and may lead to the development of powerful new tools for study and application of biological control mediated by Bacillus spp. Clearly, these are exciting times for both basic studies of plant–microbe interactions and bacterial ecology, as well as for efforts to improve agricultural technologies. It is our hope that such information will stimulate new research that will eventually lead to more widespread application of safe and effective Bacillus inoculants that will promote plant health.

Abiotic and Biological Suppression of Phytophthora parasitica in a Horticultural Medium Containing Composted Swine Waste

ABSTRACT Horticultural potting media have been amended with compost to enhance biological suppression and with Al(2)(SO(4))(3) to enhance abiotic suppression of plant pathogens, but these factors have not been simultaneously incorporated into the same medium. In this study, the efficacy of aluminum (Al)-amended potting medium containing 20% composted swine waste (CSW) was assessed for control of Phytophthora parasitica (syn. P. nicotianae), a soilborne pathogen causing damping-off of many horticultural bedding plants. Steamed and unsteamed media were amended with no Al or Al at 0.0079 g of Al g(-1) of medium with an Al(2)(SO(4))(3) solution at either pH 4 or pH 6. Infested leaf disks were buried for 2-day durations beginning 0, 6, 13, and 21 days after Al amendment. The number of sporangia produced on infested leaf disks was assessed. A similar experiment was conducted to determine the effect of steaming and Al amendments on pathogen populations. Medium treated with the pH 4 solution consistently reduced sporangia production between 38 and 65% on day 0, but no Al effect was noted at subsequent time points. The pH 6 amendment did not consistently affect sporangia production. Exchangeable Al levels decreased over time, and abiotic suppression was only observed at >2 muM Al g(-1) of medium. Pathogen populations were occasionally affected by steaming and Al. Sporangia production in unsteamed medium was reduced by 50% on leaf disks buried on days 6, 13, and 21, but not on day 0. Al amendment of a 20% CSW potting medium enhanced suppression of P. parasitica and abiotic suppression occurred before biological suppression developed.

Effect of 2,4-Diacetylphloroglucinol on Pythium: Cellular Responses and Variation in Sensitivity Among Propagules and Species

ABSTRACT The antibiotic 2,4-diacetylphloroglucinol (2,4-DAPG) plays an important role in the suppression of plant pathogens by several strains of Pseudomonas spp. Based on the results of this study, there is variation within and among Pythium spp. to 2,4-DAPG. Also, various propagules of Pythium ultimum var. sporangiiferum, that are part of the asexual stage of the life cycle, differ considerably in their sensitivity to 2,4-DAPG. Mycelium was the most resistant structure, followed by zoosporangia, zoospore cysts, and zoospores. Additionally, we report for the first time that pH has a significant effect on the activity of 2,4-DAPG, with a higher activity at low pH. Furthermore, the level of acetylation of phloroglucinols is also a major determinant of their activity. Transmission electron microscopy studies revealed that 2,4-DAPG causes different stages of disorganization in hyphal tips of Pythium ultimum var. sporangiiferum, including alteration (proliferation, retraction, and disruption) of the plasma membrane, vacuolization, and cell content disintegration. The implications of these results for the efficacy and consistency of biological control of plant-pathogenic Pythium spp. by 2,4-DAPG-producing Pseudomonas spp. are discussed.

Molecular characterization of Legionella populations present within slow sand filters used for fungal plant pathogen suppression in horticultural crops.

The total bacterial community of an experimental slow sand filter (SSF) was analyzed by denaturing gradient gel electrophoresis (DGGE) of partial 16S rRNA gene PCR products. One dominant band had sequence homology to Legionella species, indicating that these bacteria were a large component of the SSF bacterial community. Populations within experimental and commercial SSF units were studied by using Legionella-specific PCR primers, and products were studied by DGGE and quantitative PCR analyses. In the experimental SSF unit, the DGGE profiles for sand column, reservoir, storage tank, and headwater tank samples each contained at least one intense band, indicating that a single Legionella strain was predominant in each sample. Greater numbers of DGGE bands of equal intensity were detected in the outflow water sample. Sequence analysis of these PCR products showed that several Legionella species were present and that the organisms exhibited similarity to strains isolated from environmental and clinical samples. Quantitative PCR analysis of the SSF samples showed that from the headwater sample through the sand column, the number of Legionella cells decreased, resulting in a lower number of cells in the outflow water. In the commercial SSF, legionellae were also detected in the sand column samples. Storing prefilter water or locating SSF units within greenhouses, which are often maintained at temperatures that are higher than the ambient temperature, increases the risk of growth of Legionella and should be avoided. Care should also be taken when used filter sand is handled or replaced, and regular monitoring of outflow water would be useful, especially if the water is used for misting or overhead irrigation.

Nonphytotoxic Aluminum-Peat Complexes Suppress Phytophthora parasitica

ABSTRACT Amendment of peat-based potting media with Al(2)(SO(4))(3) suppresses damping-off of Vinca (Catharanthus roseus) caused by Phytophthora parasitica. The species of aluminum (Al) responsible for disease suppression have not been identified. The objective of this study was to determine the effects of amount and pH of Al(2)(SO(4))(3) amendment solutions on survival of P. parasitica. In separate experiments, peat was amended with Al(2)(SO(4))(3) solutions adjusted to pH 4 or 6 at either 0.0158 or 0.0079 g of Al per gram of peat. Amended peat was placed in Büchner funnels maintained at -2.5 kPa matric potential. Peat was infested with P. parasitica by placing zero, two, or five colonized Vinca leaf disks in each funnel, and 15 Vinca seeds were placed in each funnel. After 24 h, the matric potential was brought to 0 kPa to induce zoospore release and returned to -2.5 kPa after 24 h. Pathogen populations and stand counts were assessed after 2-week incubation. Al amendment solutions at both pH 4 and 6 reduced pathogen populations at 0.0158 g of Al per gram of peat. Solutions at pH 4 reduced pathogen populations by more than 90% at both inoculum levels; amendment solutions at pH 6 reduced populations by 95% at the low inoculum level and 65% at the high inoculum level. The prevalence of Al(OH)(2)(+) in peat amended with Al(2)(SO(4))(3) solution at pH 6 suggests that ions other than Al(3+) may be responsible for pathogen suppression. Based on the difference in chemical conditions of Al-amended peat and suppressive mineral soils, the mechanism of Al-mediated suppression of plant pathogens is speculated to be different in the two systems. Peat containing Al-peat complexes was chemically suppressive to P. parasitica and may confer Al-mediated suppression of plant pathogens with a nonphytotoxic form of Al.

Modelling: the basis for rational disease management

Adequate suppression of plant pathogens is achieved by integration of diverse control measures, which may differ in efficacy, duration of effectiveness and cost. Since the prevalence of pathogens and the intensity of the diseases they cause vary from year to year and from location to location, many factors should be taken into account for rational and cost-effective disease management. Several types of model can be used to achieve that goal. Reliable estimation of disease intensity is crucial for implementation of action thresholds, and modelling of the spatial distribution of the disease is needed for developing disease-monitoring procedures. Forecast or warning systems are models that use past and future weather data for predicting the likelihood of disease outbreaks. Yield loss models that relate the expected reduction in yield to each level of disease intensity are useful for cost–benefit calculations. The complexity of the system forces the decision-makers (growers, extension personnel, and farmer consultants) to seek assistance in their decision-making procedure. Decision support systems are computerized models developed for this purpose, and used not only to time the application of control measures, but also in choosing the appropriate measure to be applied.

Crop rotation and N fertilization effects on growth, yield, and disease incidence in potato

Crop rotation can be an effective mechanism for reducing disease incidence and contributing nitrogen (N) to succeeding crops. Interactions of plant pathogen suppression and soil nutrient availability may also exist, adding to the cropping system complexity. This study examined the impact of crop rotation, N fertilization, and their interaction on growth, yield, andRhizoctonia solani incidence in potato (Solanum tuberosum L. Norwis). Potato was grown continuously and in two-year rotations with annual alfalfa (Medicago sativa L. Nitro), hairy vetch (Vicia villosa Roth), white lupin (Lupinus albus L. Ultra), and oat (Avena sativa Astro). Fertilizer was banded at potato planting with 0, 45, 90, 135, 180, or 225 kg N ha−1 as (NH4)2SO4. Approximately 58% of continuous potato possessed stem lesions caused byR. solani, but only 12 to 22% of potato stems from other rotations possessed lesions. Tuber dry weight was affected by crop rotation in 1989, a dry year, but not in 1990. Apparent N fertilizer replacement values for hairy vetch, Nitro alfalfa, white lupin, and oat were 65, 43, 26, and 11 kg N ha−1, respectively. All crop rotations studied appeared to enhance potato production by reducing stem infection byR. solani. Vetch and alfalfa provide additional benefits through their N contributions.

Response of the imported willow leaf beetle to Bacillus thuringiensis var. san diego on poplar and willow

Response of the imported willow leaf beetle to Bacillus thuringiensis var. san diego on poplar and willow

Suppression of Pythium aphanidermatum damping-off in container media containing composted liquorice roots

Pythium damping-off is a major problem in nurseries using peat in container media. Composted liquorice ( Glycyrrhiza glabra) roots (CLR) was used as a peat substitute in container media and was suppressive to P. aphanidermatum. CLR and peat media were inoculated with the pathogen and planted with cucumber. The inoculum level was low, so that no seedlings were killed in the first planting. Damping-off occurred in the peat medium during the second and third cycle, but not in the CLR medium. Disease development in the peat medium was highest after three plantings, when 85% disease was recorded, compared with 10% in the CLR medium. CLR therefore was effective in preventing disease build-up. Compost taken from a heap during the thermophilic stage was not suppressive and sterilized compost lost its ability to suppress Pythium damping-off. Apparently the suppression of plant pathogens in compost is due to a biological mechanism developed during the final stage of composting, i.e. during maturation.

The Mycolytic Phenomenon and Biological Control of Fusarium in Soil

THERE have been many attempts to make use of biological control measures for the suppression of plant pathogens, borne by soil. Frequently, these attempts involve the inoculation into soil of organisms which antagonize the pathogens in vitro ; such procedures try to disturb the natural microbiological equilibrium by the simple introduction into the environment of individual isolates. The frequent failure of the added micro-organisms to become established is not surprising because the biological associations and antagonisms within the ecosystem determine the composition of the microflora, the climax population being a reflexion of the physical and chemical characteristics of the habitat. By modifying the habitat, however, it is possible to alter the composition of the microflora, and such microbiological alterations may destroy plant pathogens borne by soil.