Trichoderma spp. are free-living fungi that are common in soil and root ecosystems. Recent discoveries show that they are opportunistic, avirulent plant symbionts, as well as being parasites of other fungi. At least some strains establish robust and long-lasting colonizations of root surfaces and penetrate into the epidermis and a few cells below this level. They produce or release a variety of compounds that induce localized or systemic resistance responses, and this explains their lack of pathogenicity to plants. These root-microorganism associations cause substantial changes to the plant proteome and metabolism. Plants are protected from numerous classes of plant pathogen by responses that are similar to systemic acquired resistance and rhizobacteria-induced systemic resistance. Root colonization by Trichoderma spp. also frequently enhances root growth and development, crop productivity, resistance to abiotic stresses and the uptake and use of nutrients.

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Trichoderma species--opportunistic, avirulent plant symbionts.

The aim of this review was to survey all fungal pathologists with an association with the journal Molecular Plant Pathology and ask them to nominate which fungal pathogens they would place in a 'Top 10' based on scientific/economic importance. The survey generated 495 votes from the international community, and resulted in the generation of a Top 10 fungal plant pathogen list for Molecular Plant Pathology. The Top 10 list includes, in rank order, (1) Magnaporthe oryzae; (2) Botrytis cinerea; (3) Puccinia spp.; (4) Fusarium graminearum; (5) Fusarium oxysporum; (6) Blumeria graminis; (7) Mycosphaerella graminicola; (8) Colletotrichum spp.; (9) Ustilago maydis; (10) Melampsora lini, with honourable mentions for fungi just missing out on the Top 10, including Phakopsora pachyrhizi and Rhizoctonia solani. This article presents a short resume of each fungus in the Top 10 list and its importance, with the intent of initiating discussion and debate amongst the plant mycology community, as well as laying down a bench-mark. It will be interesting to see in future years how perceptions change and what fungi will comprise any future Top 10.

/pdf/the-top-10-fungal-pathogens-in-molecular-plant-pathology-4nidxj1xqt.pdf

The Top 10 fungal pathogens in molecular plant pathology

Particular bacterial strains in certain natural environments prevent infectious diseases of plant roots. How these bacteria achieve this protection from pathogenic fungi has been analysed in detail in biocontrol strains of fluorescent pseudomonads. During root colonization, these bacteria produce antifungal antibiotics, elicit induced systemic resistance in the host plant or interfere specifically with fungal pathogenicity factors. Before engaging in these activities, biocontrol bacteria go through several regulatory processes at the transcriptional and post-transcriptional levels.

/pdf/biological-control-of-soil-borne-pathogens-by-fluorescent-4524lno10w.pdf

Biological control of soil-borne pathogens by fluorescent pseudomonads

Pathogenic microorganisms affecting plant health are a major and chronic threat to food production and ecosystem stability worldwide As agricultural production intensified over the past few decades, producers became more and more dependent on agrochemicals as a relatively reliable method of crop

Use of Plant Growth-Promoting Bacteria for Biocontrol of Plant Diseases: Principles, Mechanisms of Action, and Future Prospects

Beneficial microbes in the microbiome of plant roots improve plant health. Induced systemic resistance (ISR) emerged as an important mechanism by which selected plant growth–promoting bacteria and fungi in the rhizosphere prime the whole plant body for enhanced defense against a broad range of pathogens and insect herbivores. A wide variety of root-associated mutualists, including Pseudomonas, Bacillus, Trichoderma, and mycorrhiza species sensitize the plant immune system for enhanced defense without directly activating costly defenses. This review focuses on molecular processes at the interface between plant roots and ISR-eliciting mutualists, and on the progress in our understanding of ISR signaling and systemic defense priming. The central role of the root-specific transcription factor MYB72 in the onset of ISR and the role of phytohormones and defense regulatory proteins in the expression of ISR in aboveground plant parts are highlighted. Finally, the ecological function of ISR-inducing microbes in the root microbiome is discussed.

/pdf/induced-systemic-resistance-by-beneficial-microbes-9uemo31ql5.pdf

Induced systemic resistance by beneficial microbes

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.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1046/j.1469-8137.2003.00700.x

Fusarium oxysporum and its biocontrol

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.

Biological Control of Plant Diseases: The European Situation

Contents
 	Summary	 529	
I.	Biological control of plant diseases: state of the art	 530	
II.	Main modes of action of biological control agents	 530	
III.	The protective strains of F. oxysporum: an unexplored model	 532	
IV.	Future directions for the study of the protective capacity of strains of F. oxysporum	 539	
V.	How to make biological control successful in the field?	 540	
 	References	 541	


Summary
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.

https://nph.onlinelibrary.wiley.com/doi/epdf/10.1111/j.1469-8137.2009.03014.x

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

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.

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

A pathogenic strain of Fusarium oxysporum f. sp. lycopersici transformed with the glucuronidase (GUS) reporter gene was used to study the colonization process of tomato (Lycopersicon esculentum) roots in hydroponic culture. The plants were treated exactly as those used in a different study with a non-pathogenic strain of F. oxysporum in order to compare the two types of colonization process. The pathogenic strain rapidly colonized the root surface, forming a dense network of hyphae – shown by the GUS staining assay – as early as 48 h after inoculation. The first images of the pathogen penetrating into the epidermis of the root were observed 24 h after inoculation. The plant showed defence reactions mainly in the hypodermis, but also in the cortex. Generally these barriers failed to prevent the centripetal growth of the pathogen towards the stele. The GUS activity moved with the actively growing hyphae into the tissues; the stele appeared intensely stained 7 d after inoculation, whereas by this time the hyphae at the root surface did not show any more staining. The hyphae formed a dense network at the apex of the root, but direct contact between hyphae and living cells was prevented by several layers of sloughed cap cells. Although rarely observed, penetration of the pathogen into the apex was possible and led to a rapid destruction of apical cells. The only differences between this pattern of root colonization and that observed for a non-pathogenic strain of F. oxysporum appear to concern the frequency of dead apices and the intensity of fungal colonization in the cortex. When the pathogen passed around the barrier formed in the hypodermis it always reached the xylem, although this barrier and other defence reactions induced at different levels in the cortex always prevented the non-pathogenic strain from reaching the stele. These observations suggest that the main differences between the two types of interaction – between the plant and either the pathogenic or the non-pathogenic strain – are quantitative rather than qualitative.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1046/j.1469-8137.1999.00365.x

Chantal Olivain

Papers

Fusarium oxysporum and its biocontrol

Biological Control of Plant Diseases: The European Situation

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

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

Process of tomato root colonization by a pathogenic strain of Fusarium oxysporum f. sp. lycopersici in comparison with a non-pathogenic strain