Non-streptomycete actinomycetes as biocontrol agents of soil-borne fungal plant pathogens and as plant growth promoters

Among soil microorganisms, bacteria and fungi and to a lesser extent actinomycetes, have received considerable attention as biocontrol agents of soil-borne fungal plant pathogens and as plant growth promoters. Within actinomycetes, Streptomyces spp. have been investigated predominantly, mainly because of their dominance on, and the ease of isolation from, dilution plates and because of the commercial interest shown on the antibiotics produced by certain Streptomyces spp. Many of non-streptomycete actinomycetes (NSA) taxa are therefore rarely reported in literature dealing with routine isolations of biocontrol agents and/or plant growth promoters from plant and soil. It is clear that special isolation methods need to be employed in routine isolations to selectively isolate NSA. Some interesting information exists, albeit in relatively few reports compared to that on other microorganisms, on the biological activities of NSA, especially in relation to their mechanisms of action in the biological control of soil-borne fungal plant pathogens and plant growth promotion. This review presents an overview of this information and seeks to encourage further investigations into what may be considered a relatively unexplored area of research. Certain soil environmental factors, especially in horticultural systems, could be manipulated to render the soil conducive for the biological activities of NSA. A variety of NSA isolated by selective methods have not only shown to be rhizosphere competent but also adapted for an endophytic life in root cortices. Some of the NSA, including endophytic strains that have shown potential to suppress soil-borne fungal plant pathogens, are able to employ one or more mechanisms of antagonism including antibiosis, hyperparasitism and the production of cell-wall degrading enzymes. Strains of NSA promote plant growth by producing plant growth regulators. Enhancement of plant growth by the antagonists are considered to help the host by producing compensatory roots that mask the impact of root diseases.     

Multifaceted beneficial effects of rhizosphere microorganisms on plant health and productivity

Plant growth promoting microorganisms (PGPM) and biological control agents (BCA) are shown to possess secondary beneficial effects that would increase their usefulness as bio-inoculants, regardless of the need for their primary function. Indeed, PGPM, such as Rhizobium and Glomus spp., can promote plant growth and productivity (primary effect) but have now been shown to also play a role in reducing disease (secondary effect). Conversely, BCA, such as Trichoderma and Pseudomonas spp., can control disease (primary effect) but have recently demonstrated stimulation of plant growth (secondary effect) in the absence of a pathogen. Further work shedding light onto the precise mode of action and ecophysiology of these microorganisms would assist with their timely and appropriate use and potentially unleash their full promise as beneficial rhizosphere bio-inoculants for improved growth and health of plants. The potential increased use of these microorganisms afforded by their multifaceted beneficial effects may further help in reducing problems associated with the use of synthetic chemicals in agriculture.     

Biological control: a sustainable and practical approach for plant disease management

ABSTRACT

This review deals with the mechanism of antagonistic action of bacterial and fungal biocontrol agents such as the production of antibiotics, siderophores, enzyme secretion, competition for nutrition, plant growth promotion by rhizosphere microorganism. The utilisation of synthetic pesticides has been the predominant control processor for diseases brought about by phytopathogenic microorganisms. Notwithstanding, their open and improper application in intensive agriculture has realised issues that have prompted ecological contamination, considerable residues in agricultural products and phytopathogen resistance. They are likewise disrupting the quantity of beneficial microorganism which is available in the soil and capable of expanding soil fertility. Along these lines, there is a need to look through the option of synthetic pesticides that are safe, environmental and monetarily feasible to confront this problem. Biocontrol agent’s utilisation is the best alternative method to control the different kinds of diseases, such as nematode infestation, fungal pathogen and bacterial pathogen. Nowadays, biocontrol agents assume a significant role in the field of agriculture. It is a financially savvy, environment-friendly and inhibits the advancement of pathogenic microorganism sustainably. This review emphasises the role of biocontrol agents against different pathogenic microorganisms and their significance potentiality to improve plant growth and enhance defence system of plants.     

Isotope ratio mass spectrometry identifies soil microbial biocontrol agents having trophic relations with the plant pathogen Armillaria mellea

An understanding of the types of interactions that take place between plant pathogens and other microorganisms in the natural environment is crucial in order to identify new potential biocontrol agents. The use of microorganisms labelled with stable isotopes is a potentially useful method for studying direct parasitisation of a given pathogen or assimilation of the pathogen's metabolites by microorganisms. A microorganism labelled with a stable isotope can be monitored in the environment and isotope ratio mass spectrometry can detect whether it is directly parasitised or its metabolites are used by other microorganisms. In this study, we isolated 158 different species of fungi and bacteria from soil and assayed their biocontrol potential against a plant pathogen (Armillaria mellea) by coupling a dual-culture test with mass spectrometry analysis of the ¹³C isotope in the microorganisms in presence of ¹³C-labelled A. mellea. The microorganisms affected the pathogen by means of antibiosis phenomena (total or partial inhibition of pathogen growth, alteration of its morphology) and by antagonism, probably resulting from competition for space and nutrients or from mycoparasitism. Isotope ratio mass spectrometry was used to identify direct trophic interactions between microorganisms and the pathogen as in dual cultures as in soil microcosms. Six fungi and one bacterium were found to display the best active trophic behaviour against the pathogenin dual cultures; three microorganisms were discarded due to their plant pathogen potential. Trichoderma harzianum, Pseudomonas fluorescens and Rhodosporidium babjevae were selected to carry out the experiments. T. harzianum inhibited pathogen development (rate of inhibition 80 ± 0.19%) and its δ ¹³C values increased (244.03 ± 36.70‰) in contact with ¹³C-labelled A. mellea. Lower levels of antagonism and correspondingly lower assimilation of ¹³C were detected in P. fluorescens and R. babjevae. Only T. harzianum maintained mycoparasitic activity in the soil microcosm, showing a δ ¹³C value of 1.97 ± 2.24‰ after one month in co-presence with the labelled pathogen. This study provides support for the use of isotope ratio mass spectrometry as an additional tool in screening for potential biocontrol agents.     

Manipulating the soil microbiome to increase soil health and plant fertility

A variety of soil factors are known to increase nutrient availability and plant productivity. The most influential might be the organisms comprising the soil microbial community of the rhizosphere, which is the soil surrounding the roots of plants where complex interactions occur between the roots, soil, and microorganisms. Root exudates act as substrates and signaling molecules for microbes creating a complex and interwoven relationship between plants and the microbiome. While individual microorganisms such as endophytes, symbionts, pathogens, and plant growth promoting rhizobacteria are increasingly featured in the literature, the larger community of soil microorganisms, or soil microbiome, may have more far-reaching effects. Each microorganism functions in coordination with the overall soil microbiome to influence plant health and crop productivity. Increasing evidence indicates that plants can shape the soil microbiome through the secretion of root exudates. The molecular communication fluctuates according to the plant development stage, proximity to neighboring species, management techniques, and many other factors. This review seeks to summarize the current knowledge on this topic.     

Effect of Foliar Applied Plant Elicitors on Microbial and Nematode Populations in the Root Zone of Potato

Abstract

Systemic acquired resistance (SAR) is a process whereby a plant that successfully resists a pathogen becomes highly resistant to subsequent infection not only by the original pathogen but also by a wide variety of pathogens. Most SAR research has focused on resistance in leaves, so much less is known about the effectiveness of foliar applications of SAR compounds in the protection of plant roots and associated microorganisms in soil. This study was conducted to determine if foliar SAR‐inducing applications (BTH or harpin) negatively impact the potato root system beneficial rhizosphere microbial populations and activity or influence pathogenic nematode populations. Foliar applications of benzo (1,2,3) thiadiazole‐7‐carbothioic acid S‐methyl ester (BTH) and the microbial protein harpin applied in various combinations, timings, and rates showed no effects on microbial biomass, culturable bacteria, Pseudomonas populations, or N‐mineralization potentials over 2 years. No stimulatory or inhibitory effects on major bacterial populations were observed, indicating that SAR induction does not have a negative effect on general microbial populations or activities. BTH and harpin both reduced the numbers of lesion nematodes (Pratylenchus spp.) by potato harvest. BTH reduced root knot nematodes, Meloidogyne chitwoodi at the end of the season. In addition, BTH and high‐dose harpin (applied at the 4× rate) reduced the nematode infection index in comparison to the control. The SAR elicitors increased the population densities of nontarget free‐living nematodes in the soil compared to the control. Potato yields were not affected by plant elicitors but BTH and harpin both reduced the number of culled potatoes 26% compared to the control. Future studies are designed to determine if these plant elicitors have any direct effect on rhizosphere diversity or if plants with active defense pathways alter carbon flow and root exudates into the soil.     

连作根系分泌物加剧土传病害的机制和缓解措施研究进展

基于植物-土壤反馈理论,连作体系中的根系分泌物必然在加剧土传病害发生中起重要作用,但相关研究证据尚缺少系统总结。本文梳理了连作导致土传病害加剧的现象以及连作对典型根系分泌物组分的累积。从有利于土传病原菌由土体向根际迁移、增殖和致病（“利病”）、破坏根际有益微生物群落防线（“压益”）和毒害根系免疫系统（“自毒”）等三个方面,揭示连作根系分泌物中某些物质促进土传病原菌入侵的机制。从根系分泌物角度阐述轮作、间作、套作、伴生和嫁接等多样性种植方式缓解连作土传病害的机制。提出鉴定“利病”、“压益”和“自毒”物质以及构建对应的消减技术途径,可为土传病害绿色高效综合防控提供理论和技术支撑。     

基于西部地区园艺植物常见病虫害特点的生物防治措施

我国西部地区近年来园艺产业发展迅速。为了有效防治园艺植物常见的病虫害,根据中国西部的气候特点及虫种群特点,阐述了园艺植物病虫害生物防治技术的优势,并根据当前西部部分地区园艺植物病虫害的实际情况,提出了一些园艺植物病虫害生物防治技术的应用策略。实际案例表明,科学防治病虫害对提高园艺植物的产量具有显著的优势。     

Status of applied biological control of soil-borne plant pathogens

During the last 15 years a great amount of information has been accumulated on the ecology and physiology of soil-borne plant pathogens around the world. Despite this, very few cases of applied biological or cultural control have been reported. Although soil-borne plant pathogens are widely spread and economically important, only a small fraction of plant pathologists and soil microbiologists has devoted full time in the applied phases of biological control. The number of publications on applied control during the last 10 years in two journals surveyed is exceptionally small. The status of applied biological control of root diseases is considerably less favorable than the situation in entomology where much progress has been made during the last 10 years. The status of biological control (narrowly or broadly defined) of soil-borne pathogens is described here and several examples of applied control are cited. Future prospects for research on the ecology and biological control are also discussed.     

The role of N form supply for PGPM‐host plant interactions in maize

The form of nitrogen (N) supply has a significant impact on rhizosphere chemistry and root growth responses of higher plants. The respective effects are also employed as management options to improve nutrient acquisition and to minimize nutrient losses in cropping systems. However, surprisingly little is known concerning the interactions with rhizosphere biota. In this study, we investigated the effects of selected bacterial and fungal inoculants with proven plant growth‐promoting and phosphate (P)‐solubilizing potential (plant growth‐promoting microorganisms, PGPM) in maize with nitrate or stabilized ammonium supply, on soils with limited P availability and sparingly soluble rock phosphate (Rock‐P) applied as P fertilizer. The combination of the bacterial inoculants Pseudomonas sp. DSMZ 13134 (Proradix) and Bacillus amyloliquefaciens FZB42 with ammonium sulphate fertilization, stabilized with the nitrification inhibitor 3,4‐dimethylpyrazole‐phosphate (DMPP), resulted in a superior shoot biomass production (79–111%) and shoot P accumulation (109–235%) as compared with nitrate supply. This effect could be partially attributed to (1) ammonium‐induced rhizosphere acidification via increased root extrusion of protons, (2) promotion of root hair elongation, and (3) increased shoot concentrations of hormonal growth regulators (indole‐3‐acetic acid, zeatin, gibberellic acid). The effects, induced by the microbial inoculants were mainly related to increased root length development (43–44%), associated with a 60% increase in auxin production potential. No inoculant effects were detected on root hair elongation or on chemical modifications of the rhizosphere involved in P solubilisation, such as rhizosphere acidification, release of carboxylates or secretory phosphohydrolases. However, the ammonium‐induced stimulation of root hair elongation increased preferential sites for root colonization by the selected inoculants, which may explain the increase in rhizosphere abundance of PGPMs, exemplarily recorded for the fungal inoculant Trichoderma harzianum OMG16 (210%). The presented data suggest a network of positive interactions between stabilized ammonium fertilization and plant growth‐promoting functions of various bacterial and fungal PGPM inoculants. This offers perspectives to increase the efficiency and the reproducibility of PGPM‐assisted fertilization strategies.     

Enhanced root uptake of acibenzolar-S-methyl (ASM) by tomato plants inoculated with selected Bacillus plant growth-promoting rhizobacteria (PGPR)

The combination of plant growth-promoting rhizobacteria (PGPR) and plant resistance inducers is an alternative crop protection approach in modern agricultural systems. Despite the numerous reports regarding the improved suppression of plant pathogens by their combined application, little is known about the interactions among these components. In the present study, the persistence behavior of the plant activator acibenzolar-S-methyl (ASM) in the rhizosphere of tomato plants and its root uptake as well as systemic translocation ability in aboveground parts after combined use with certain Bacillus PGPR strains (B. amyloliquefaciens IN937a, B. pumilus SE34, B. subtilis FZB24 and GB03) were investigated. Additionally, the population dynamics of the PGPR strain B. subtilis GB03 at the tomato root system and rhizosphere soil applied with or without the pesticide were studied. The results showed that the addition of PGPR inocula did not affect the dissipation rate of ASM in rhizosphere soil. Also, the formation of its major metabolite CGA 210007 in soil was rapid, since it was detected one hour after root drench and it was maintained at high levels during the sampling period without considerable variations among the bacterial treatments compared to the control. The uptake and systemic translocation of ASM and its metabolite CGA 210007 from root to shoot was rapid and maximum concentrations were observed at 48–96 h after its application. It was revealed that in plants treated with the PGPR strains B. subtilis GB03 and B. pumilus SE34 the uptake and systemic translocation of ASM and CGA 210007 in the aerial parts of the tomato plants was significantly higher compared to the control receiving no bacterial treatment. Also, the populations of the strain B. subtilis GB03 showed high colonizing ability in the root system and the rhizosphere soil. PGPR strains that lead to enhanced pesticide uptake by plants should be further evaluated as components in integrated management systems.     

Hydrogen cyanide production by soil bacteria: Biological control of pests and promotion of plant growth in sustainable agriculture

Currently, plant diseases and insect infestations are mainly controlled by the extraneous application of pesticides. Unfortunately, the indiscriminate use of such agrochemicals can cause ecological and environmental problems, as well as human health hazards. To obviate the potential pollution arising from the application of agrochemicals, biological control of soilborne pathogens or insect pests using antagonistic microorganisms may be employed. Certain soil bacteria, algae, fungi, plants and insects possess the unique ability to produce hydrogen cyanide(HCN), which plays an important role in the biotic interactions of those organisms. In particular, cyanogenic bacteria have been found to inhibit the growth of various pathogenic fungi, weeds, insects, termites and nematodes. Thus, the use of HCN-producing bacteria as biopesticides offers an ecofriendly approach for sustainable agriculture. The enzyme, HCN synthase,involved in the synthesis of HCN, is encoded by the hcnABC gene cluster. The biosynthetic regulation of HCN, antibiotics and fluorescent insecticidal toxins through the conserved global regulatory GacS/GacA system is elaborated in this review, including approaches that may optimize cyanogenesis for enhanced pest control. In addition, the effects of bacterially synthesized HCN on the production of indole acetic acid, antibiotics and fluorescent insecticidal toxins, 1-aminocyclopropane-1-carboxylate deaminase utilization and phosphate solubilization may result in the stimulation of plant growth. A more detailed understanding of HCN biosynthesis and regulation may help to elaborate the precise role of this compound in biotic interactions and sustainable agriculture.     

Fertilizers and soil-borne diseases

Abstract. Nutrient manipulation through fertilization or modification of the soil environment to influence nutrient availability is an important cultural control for plant disease and an integral component of production agriculture. Fertilization decreases soil-borne diseases by maximizing the inherent disease resistance of plants, by facilitating disease escape through increased nutrient availability or stimulated plant growth, and by altering the external environment to influence the survival, germination and penetration of pathogens. The flexibility in most disease-nutrient interactions permits a much broader utilization of this cultural control in decreasing disease severity than is presently practised. It is clear that the severity of most diseases can be decreased and the chemical, biological or genetic control of many plant pathogens enhanced by proper fertilization. Breeding nutrient-efficient or disease-tolerant crops and establishing cultivar requirements should further improve production efficiency.     

Compost Amendments Decrease Verticillium dahliae Infection on Potato

Indigenous soil microorganisms contribute to disease suppression in cropping systems by reducing and competing with pathogen populations, thereby limiting disease severity. Various communities of indigenous microorganisms in any particular soil have adapted to the specific environmental conditions. If the soil around the plant roots could be altered to favor the indigenous soil microorganisms relative to the plant pathogen, the survival and proliferation of indigenous soil microorganisms, and thus effectiveness of biological control, may be increased. Wood chippolyacrylamide (PAM) cores were used to alter the soil environment in a greenhouse study to favor indigenous soil microorganisms in vegetable and manure compost to reduce Verticillium dahliae infection of potato (Solanum tuberosum L.) plants. Potato plants growing in soils amended with vegetable compost-wood chip-PAM cores had significantly lower visible (V_vis) and isolation (V_iso) V. dahliae infection rates than control soils and soils with dairy or vegetable compost alone. Soils amended with wood chip-PAM-dairy compost cores had significantly lower V_vis and isolation V_iso than control soils and soils with dairy compost. Soils with wood chip-PAM cores and soils with wood chip-PAM-vegetable compost had greater microbial biomass/Verticillium dahliae biomass (MB/VB) ratios in soil than control soils or in soils amended with compost alone. MB/VB ratios in wood chip-PAM cores and wood chip- PAM-vegetable compost were greater than in wood chip-PAM-dairy compost cores. V_vis correlated in a quadratic relationship with the MB/VB ratio (r²=0.76). As MB/VB ratio increased V_vis decreased. Although field studies with several crops and economic evaluations are necessary, this greenhouse study provides evidence that a wood chip-PAM or wood chip- PAM-vegetable compost soil amendment may be a viable method to control some soil diseases in high value crops.     

Trichoderma–plant–pathogen interactions

Biological control involves the use of beneficial organisms, their genes, and/or products, such as metabolites, that reduce the negative effects of plant pathogens and promote positive responses by the plant. Disease suppression, as mediated by biocontrol agents, is the consequence of the interactions between the plant, pathogens, and the microbial community. Antagonists belonging to the genus Trichoderma are among the most commonly isolated soil fungi. Due to their ability to protect plants and contain pathogen populations under different soil conditions, these fungi have been widely studied and commercially marketed as biopesticides, biofertilizers and soil amendments. Trichoderma spp. also produce numerous biologically active compounds, including cell wall degrading enzymes, and secondary metabolites. Studies of the three-way relationship established with Trichoderma, the plant and the pathogen are aimed at unravelling the mechanisms involved in partner recognition and the cross-talk used to maintain the beneficial association between the fungal antagonist and the plant. Several strategies have been used to identify the molecular factors involved in this complex tripartite interaction including genomics, proteomics and, more recently, metabolomics, in order to enhance our understanding. This review presents recent advances and findings regarding the biocontrol-resulting events that take place during the Trichoderma–plant–pathogen interaction. We focus our attention on the biological aspects of this topic, highlighting the novel findings concerning the role of Trichoderma in disease suppression. A better understanding of these factors is expected to enhance not only the rapid identification of effective strains and their applications but also indicate the potentials for improvement of natural strains of Trichoderma.     

The invasive plant Solidago canadensis L. suppresses local soil pathogens through allelopathy

Recent studies suggest that invasive plants pose a significant effect on local soil pathogens, which in turn affects on the plant invasion. However, the mechanisms by which invasive plants affect soil pathogens were less well known. We conducted four experiments to test the hypothesis that the invasive plant species Solidago canadensis L. may affect soilborne pathogens through exudation of allelochemicals. Two common soilborne pathogens Pythium ultimum and Rhizoctonia solani were used in the study. Tomato (Lycopersicon esculentum Mill) variety Qianhong No.1 which is sensitive to soil pathogens P. ultimum and R. solani was used to indicate pathogenic activity (in terms of seedling mortality and damping-off). Extracts from root and rhizome of S. canadensis significantly suppressed the growth and pathogenic activity of both pathogens under Petri dish culture and sand culture (experiments 1 and 2), providing direct evidence that S. canadensis exerts allelopathic effects on these pathogens. Subsequently, a pathogen inoculation experiment under sand culture showed that pathogenic activity of both P. ultimum and R. solani was lower under the soil with S. canadensis compared to that under the soil with a common native plant Kummerowia striata (Thunb.) Schindl (experiment 3), implying that invasive S. canadensis had but native K. striata did not have allelopathic effects on soil pathogens through root and rhizome exudation. Finally, results from field soil tests showed that mortality and damping-off rate of tomato seedlings were significantly lower under the soils collected from the fields dominated by S. canadensis than that dominated by native plants at both sampling sites, suggesting that suppression of pathogens also occurs in the field. From the present experimental results we suggest that invasive S. canadensis may acquire spreading advantage in non-native habitat by using “novel weapons” to inhibit not only local plants but also soilborne pathogens.     

根系分泌物与土传病害的关系研究进展

根系分泌物是植物-土壤-病原微生物相互作用的桥梁,是决定病原菌-作物关系的关键生态因子,影响着土传病害的发生与发展.本文阐述了根系分泌物的定义、分类及产生机理;重点从根系分泌物的化感自毒效应,根系分泌物诱导根际微生物群落,根系分泌物影响病原菌丰度,根系分泌物影响根际土壤环境4个方面阐述了根系分泌物与土传病害的关系;并从...     

Compost Tea: Principles and Prospects For Plant Disease Control

An increasing body of experimental evidence indicates that plant disease can be suppressed by treating plant surfaces with a variety of water-based compost preparations, referred to in the literature as watery fermented compost extracts or compost teas. The terms nonaerated compost teas (NCT) and aerated compost teas (ACT) are used in this review to refer to the common production methods that diverge in the intent to actively aerate. Very little data directly compares the efficacy of NCT and ACT for plant disease suppression. A variety of foliar plant pathogens and/or diseases have been suppressed by applications of NCT while few controlled studies have examined ACT. For some diseases the level of control would be considered inadequate for conventional agriculture; organic producers with limited control options consider partial disease control to be an important improvement. For both compost tea production methods, decisions that influence pathogen suppression include choice of compost feedstocks, compost age, water ratio, fermentation time, added nutrients, temperature and pH. Application technology choices include the dilution ratio, application equipment, timing, rates, spray adjuncts and adding specific microbial antagonists. Increased understanding of compost tea microbiology and the survival and interactions of microbes on plants surfaces should make it possible to modify compost tea production practices and application technology to optimize delivery of a microflora with multiple modes of pathogen suppression. Innovative growers and practitioners are leading the development of new compost tea production methods and uses, generating many potential research opportunities. The use of compost tea as part of an integrated plant health management strategy will require much additional whole systems research by a cohesive team of farmers and experts in composting, plant pathology, phyllosphere biology, molecular microbial ecology, fermentation science, plant physiology, plant breeding, soil science, and horticulture.     

Identifying the characteristics of organic soil amendments that suppress soilborne plant diseases

Application of organic amendments has been proposed as a strategy for the management of diseases caused by soilborne pathogens. However, inconsistent results seriously hinder their practical use. In this work we use an extensive data set of 2423 studies derived from 252 papers to explore this strategy. First, we assess the capability of a specific organic amendment to control different diseases; second, we investigate the influence of organic matter (OM) decomposition on disease suppressiveness; and third, we search for physical, chemical and biological parameters able to identify suppressive OM. OM was found to be consistently suppressive to different pathogens in only a few studies where a limited number of pathogens were tested. In the majority of studies a material suppressive to a pathogen was ineffective or even conducive to other pathogens, suggesting that OM suppressiveness is often pathogen-specific. OM decomposition in many studies (73%, n = 426) emerged as a crucial process affecting suppressiveness. During decomposition, disease suppression either increased, decreased, was unchanged or showed more complex responses, such as ‘hump-shaped’ dynamics. Peat suppressiveness generally decreased during decomposition, while responses of composts and crop residues were more complex. However, due to the many interactions of contributing factors (OM quality, microbial community composition, pathosystem tested and decomposition time), it was difficult to identify specific predictors of disease suppression. Among the 81 parameters analysed, only some of the 643 correlations showed a consistent relationship with disease suppression. The response of pathogen populations to OM amendments was a reliable feature only for some organic matter types (e.g. crop residues and organic wastes with C-to-N ratio lower than ∼15) and for pathogens with a limited saprophytic ability (e.g., Thielaviopsis basicola and Verticillium dahliae). Instead, population responses of the pathogenic fungi Phytophthora spp., Rhizoctonia solani and Pythium spp. appeared unrelated to disease suppression. Overall, enzymatic and microbiological parameters, rather than chemical ones, were much more informative for predicting suppressiveness. The most useful features were FDA activity, substrate respiration, microbial biomass, total culturable bacteria, fluorescent pseudomonads and Trichoderma populations. We conclude that the integration of different parameters (e.g. FDA hydrolysis and chemical composition by ¹³C NMR) may be a promising approach for identification of suppressive amendments.     

Soil fertility management and insect pests: harmonizing soil and plant health in agroecosystems

Cultural methods such as crop fertilization can affect susceptibility of plants to insect pests by altering plant tissue nutrient levels. Research shows that the ability of a crop plant to resist or tolerate insect pests and diseases is tied to optimal physical, chemical and mainly biological properties of soils. Soils with high organic matter and active soil biology generally exhibit good soil fertility. Crops grown in such soils generally exhibit lower abundance of several insect herbivores, reductions that may be attributed to a lower nitrogen content in organically farmed crops. On the other hand, farming practices, such as excessive use of inorganic fertilizers, can cause nutrient imbalances and lower pest resistance. More studies comparing pest populations on plants treated with synthetic versus organic fertilizers are needed. Understanding the underlying effects of why organic fertilization appears to improve plant health may lead us to new and better integrated pest management and integrated soil fertility management designs.