Claude Alabouvette

The rhizosphere: a playground and battlefield for soilborne pathogens and beneficial microorganisms

The rhizosphere is a hot spot of microbial interactions as exudates released by plant roots are a main food source for microorganisms and a driving force of their population density and activities. The rhizosphere harbors many organisms that have a neutral effect on the plant, but also attracts organisms that exert deleterious or beneficial effects on the plant. Microorganisms that adversely affect plant growth and health are the pathogenic fungi, oomycetes, bacteria and nematodes. Most of the soilborne pathogens are adapted to grow and survive in the bulk soil, but the rhizosphere is the playground and infection court where the pathogen establishes a parasitic relationship with the plant. The rhizosphere is also a battlefield where the complex rhizosphere community, both microflora and microfauna, interact with pathogens and influence the outcome of pathogen infection. A wide range of microorganisms are beneficial to the plant and include nitrogen-fixing bacteria, endo- and ectomycorrhizal fungi, and plant growth-promoting bacteria and fungi. This review focuses on the population dynamics and activity of soilborne pathogens and beneficial microorganisms. Specific attention is given to mechanisms involved in the tripartite interactions between beneficial microorganisms, pathogens and the plant. We also discuss how agricultural practices affect pathogen and antagonist populations and how these practices can be adopted to promote plant growth and health.

Use of sewage sludge compost and Trichoderma asperellum isolates to suppress Fusarium wilt of tomato

It has been reported that plant growth media amended with composted bark suppress Fusarium wilts whereas media amended with composted municipal sludge aggravate this disease. However, in this study, a compost prepared from vegetable and animal market wastes, sewage sludge and yard wastes showed a high ability to suppress Fusarium wilt of tomato caused by Fusarium oxysporum f. sp. lycopersici race 1. The ability of this compost to suppress Fusarium wilt of tomato was compared with that of a peat mix (peat:vermiculite, 1:1 v/v) and a naturally suppressive soil from Chateaurenard, France. The compost and the soil from Chateaurenard were highly suppressive, whereas the peat mix was highly conducive. Amendment with this compost significantly (P<0.05) increased the suppressiveness of the peat mix. Biotic and abiotic properties were compared among these substrates. The peat mix was acidic, and had a low EC, whereas the compost was basic and a high EC. The compost–peat mix had a similar pH to the compost, however EC was approximately half that of the compost. The bacterial populations and microbial activity were highest in the compost and the compost–peat mix. Compost (10%; v/v), Trichoderma asperellum isolates isolated from natural compost–peat mix, and the nonpathogenic biocontrol agent F. oxysporum Fo47 isolated from Chateaurenard soil were inoculated into sterilized compost–peat mix and Chateaurenard soil to assess their ability to restore suppressiveness in the sterilized substrates. Both the natural compost and the T. asperellum isolates significantly (P<0.05) increased the suppressive ability of sterilized compost–peat mix and Chateaurenard soil. Fo47 was relatively the most effective biocontrol agent. The incidence of Fusarium wilt was lowest in tomato plants grown in either sterilized compost–peat mix or Chateaurenard soil inoculated with this strain. Our results show that the use of some composted sewage sludge in the plant growth medium is effective for suppression of Fusarium wilt at the early stage of plant growth. In addition, the T. asperellum isolates isolated from the suppressive compost–peat mix appear to have the potential to be a new alternative of biocontrol of Fusarium wilt.

Biological control of fusarium diseases by fluorescent Pseudomonas and non-pathogenic Fusarium

Abstract In soil-less culture of vegetables and flowers in greenhouses, fusarium diseases may induce severe damage. Under these growing conditions, biological control could be achieved by application of selected strains of fluorescent Pseudomonas or non-pathogenic Fusarium oxysporum . Seventy-four strains of fluorescent Pseudomonas were tested for their ability to reduce the incidence of fusarium wilt of flax when applied either alone or in association with one preselected non-pathogenic strain of Fusarium oxysporum (Fo47). Four classes were established, based on the effect of bacteria on disease severity, on their own or in association with Fo47. Most of the strains did not modify the percentage of wilted plants. However 10.8% of them, although having no effect on their own, significantly improved the control attributable to Fo47. One of these bacterial strains (C7) was selected for further experiments. Two trials conducted under commercial-type conditions demonstrated the effectiveness of the association of the bacterial strain C7 with the non-pathogenic Fusarium strain Fo47 to control fusarium crown and root rot of tomato, even when each antagonistic micro-organism was not efficient by itself. The yields were not significantly different in the protected plots in comparison with the healthy control.

Biological Control of Plant Diseases: The European Situation

The most common approach to biological control consists of selecting antagonistic microorganisms, studying their modes of action and developing a biological control product. Despite progress made in the knowledge of the modes of action of these biological control agents (BCAs), practical application often fails to control disease in the fields. One of the reasons explaining this failure is that the bio-control product is used the same way as a chemical product. Being biological these products have to be applied in accordance with their ecological requirements. Another approach consists of induction of plant defence reactions. This can be done by application of natural substances produced by or extracted from microorganisms, plants, or algae. Since they do not aim at killing the pathogens, these methods of disease control are totally different from chemical control. Although promising, these methods have not been sufficiently implemented under field conditions. A third approach consists of choosing cultural practices that might decrease the incidence or severity of diseases. These methods include the choice of an appropriate crop rotation with management of the crop residues, application of organic amendments and the use of new technology such as the biological disinfestation of soils. Biological control practices need an integrative approach, and more knowledge than chemical control.

Implication of systemic induced resistance in the suppression of Fusarium wilt of tomato by Pseudomonas fluorescens WCS417r and by nonpathogenic Fusarium oxysporum Fo47.

Fluorescent pseudomonads and nonpathogenic Fusarium oxysporum have been shown to suppress fusarium wilts. This suppression has been related to both microbial antagonism and induced resistance.The aim of the present study was to assess the relative importance of systemic induced resistance in the suppression of fusarium wilt of tomato in commercial-like conditions by a reference strain of each type of microorganism (P. fluorescens WCS417r and nonpathogenic F. oxysporum Fo47). The spatial separation of the pathogen and the biocontrol strains excluded any possible microbial antagonism and implicated the involvement of the systemic induced resistance; whereas the absence of any separation between these microorganisms allowed the expression of both mechanisms. Since systemic induced resistance has often been associated with the synthesis of PR-proteins, their accumulation in tomato plants inoculated with WCS417r or with Fo47 was determined.The analysis of the results indicates that the suppression of fusarium wilt by P. fluorescens WCS417r was ascribed to systemic induced resistance without any detection of the PR-proteins tested (PR-1 and chitinases). In contrast, the suppression achieved by nonpathogenic F. oxysporum Fo47 appeared to be mainly ascribed to microbial antagonism but also to a lesser extent to systemic induced resistance. This induced resistance could be related to the accumulation of PR-1 and chitinases.The possible relationship between the ability of Fo47 to suppress fusarium wilt more efficiently than WCS417r and its ability to show both mechanisms is discussed.

Microbiological control of soil‐borne phytopathogenic fungi with special emphasis on wilt‐inducing Fusarium oxysporum

Plant diseases induced by soil-borne plant pathogens are among the most difficult to control. In the absence of effective chemical control methods, there is renewed interest in biological control based on application of populations of antagonistic micro-organisms. In addition to Pseudomonas spp. and Trichoderma spp., which are the two most widely studied groups of biological control agents, the protective strains of Fusarium oxysporum represent an original model. These protective strains of F. oxysporum can be used to control wilt induced by pathogenic strains of the same species. Exploring the mechanisms involved in the protective capability of these strains is not only necessary for their development as commercial biocontrol agents but raises many basic questions related to the determinism of pathogenicity versus biocontrol capacity in the F. oxysporum species complex. In this paper, current knowledge regarding the interaction between the plant and the protective strains is reviewed in comparison with interactions between the plant and pathogenic strains. The success of biological control depends not only on plant-microbial interactions but also on the ecological fitness of the biological control agents.

The microbiology of Lascaux Cave

Lascaux Cave (Montignac, France) contains paintings from the Upper Paleolithic period. Shortly after its discovery in 1940, the cave was seriously disturbed by major destructive interventions. In 1963, the cave was closed due to algal growth on the walls. In 2001, the ceiling, walls and sediments were colonized by the fungus Fusarium solani. Later, black stains, probably of fungal origin, appeared on the walls. Biocide treatments, including quaternary ammonium derivatives, were extensively applied for a few years, and have been in use again since January 2008. The microbial communities in Lascaux Cave were shown to be composed of human-pathogenic bacteria and entomopathogenic fungi, the former as a result of the biocide selection. The data show that fungi play an important role in the cave, and arthropods contribute to the dispersion of conidia. A careful study on the fungal ecology is needed in order to complete the cave food web and to control the black stains threatening the Paleolithic paintings.

Increased soil suppressiveness to Fusarium wilt of flax after addition of municipal solid waste compost

Abstract The suppressiveness of a loamy soil amended with municipal solid waste compost to Fusarium wilt of flax (caused by Fusarium oxysporum f. sp. lini ) was studied. The soil was moderately conducive to the disease, with an estimated half life time (HLT) of the flax population of 41 days. Heat-treatment made the soil highly conducive (HLT of 28 days). Compost addition increased the suppressiveness of the control soil, proportionally to the application rate (10, 20 and 30%). Heat-treated compost was as effective as control compost in inducing the suppressiveness in the control soil (HLT of 103 and 97 days, respectively). Heat-treated soil amended with control compost and control soil amended with heat-treated compost were incubated before infestation with the pathogen, in order to allow the microflora of the control component to colonize the heat-treated component. Incubation had no marked effect on the suppressiveness of the mixtures. Both microflora of soil and compost were involved in the suppressiveness and mainly acted through nutrient and space competition towards the population of the pathogen.

Colonization of tomato root by pathogenic and nonpathogenic Fusarium oxysporum strains inoculated together and separately into the soil

ABSTRACT In soil, fungal colonization of plant roots has been traditionally studied by indirect methods such as microbial isolation that do not enable direct observation of infection sites or of interactions between fungal pathogens and their antagonists. Confocal laser scanning microscopy was used to visualize the colonization of tomato roots in heat-treated soil and to observe the interactions between a nonpathogenic strain, Fo47, and a pathogenic strain, Fol8, inoculated onto tomato roots in soil. When inoculated separately, both fungi colonized the entire root surface, with the exception of the apical zone. When both strains were introduced together, they both colonized the root surface and were observed at the same locations. When Fo47 was introduced at a higher concentration than Fol8, it colonized much of the root surface, but hyphae of Fol8 could still be observed at the same location on the root. There was no exclusion of the pathogenic strain by the presence of the nonpathogenic strain. These results are not consistent with the hypothesis that specific infection sites exist on the root for Fusarium oxysporum and instead support the hypothesis that competition occurs for nutrients rather than for infection sites.

Defense Responses of Fusarium oxysporum to 2,4-Diacetylphloroglucinol, a Broad-Spectrum Antibiotic Produced by Pseudomonas fluorescens

A collection of 76 plant-pathogenic and 41 saprophytic Fusarium oxysporum strains was screened for sensitivity to 2,4-diacetylphloroglucinol (2,4-DAPG), a broad-spectrum antibiotic produced by multiple strains of antagonistic Pseudomonas fluorescens. Approximately 17% of the F. oxysporum strains were relatively tolerant to high 2,4-DAPG concentrations. Tolerance to 2,4-DAPG did not correlate with the geographic origin of the strains, formae speciales, intergenic spacer (IGS) group, or fusaric acid production levels. Biochemical analysis showed that 18 of 20 tolerant F. oxysporum strains were capable of metabolizing 2,4-DAPG. For two tolerant strains, analysis by mass spectrometry indicated that deacetylation of 2,4-DAPG to the less fungitoxic derivatives monoacetylphloroglucinol and phloroglucinol is among the initial mechanisms of 2,4-DAPG degradation. Production of fusaric acid, a known inhibitor of 2,4-DAPG biosynthesis in P. fluorescens, differed considerably among both 2,4-DAPG-sensitive and -tolerant F. oxysporum strains, indicating that fusaric acid production may be as important for 2,4-DAPG-sensitive as for -tolerant F. oxysporum strains. Whether 2,4-DAPG triggers fusaric acid production was studied for six F. oxysporum strains; 2,4-DAPG had no significant effect on fusaric acid production in four strains. In two strains, however, sublethal concentrations of 2,4-DAPG either enhanced or significantly decreased fusaric acid production. The implications of 2,4-DAPG degradation, the distribution of this trait within F. oxysporum and other plant-pathogenic fungi, and the consequences for the efficacy of biological control are discussed.