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TRICHODERMA IN ORGANIC AGRICULTURE

Posted on | September 30, 2013 | No Comments

Biocontrol, or Biological Control, can be defined as the use of natural organisms, or genetically modified, genes or gene products, to reduce the effects of undesirable organisms to favour organisms useful to human, such as crops, trees, animals and beneficial microorganisms. This strategy of control is ecologicaly clean and compatible with different models of agriculture: organic, biological and integrated pest/pathogen management (IPM) programmes.

The main antagonist used in disease control in Agriculture is the fungus Trichoderma harzianum Rifai, a low cost biocontrol agent that can establish itself in different pathosystems, has moderate effects on soil balance and does not harm benefitial organims that contribute towards pathogen’scontrol. This biocontrol agent has not harmful effects on humans, wild life and other beneficialorganisms. T. harzianum is a safe and effective biocontrol agent in both natural and controlled environments that does not accumulate in the food chain and to which it has not been described resistance.

Trichoderma strains used as biocontrol agents can act:

a) Colonizing the soil and/or parts of the plant, occupying a physical space and avoiding the multiplication of the pathogens;

b) Producing cell wall degrading enzymes against the pathogens; c) producing antibiotics that can kill the pathogens;

d) Promoting the plant development and e) inducing the defensive mechanisms of the plant.

 Antifungal formulations based on Trichoderma strains and proteins require, as in the case of chemical fungicides, a costly and sound registration process previous to their commercialization. For

this reason, many of these biological products are being offered to the farmers under the category of fertilizers and other commercial products that are not tightly regulated, and, hence, they do not offer sufficient guarantee of quality and sanitary control.

INTRODUCTION

Trichoderma is a fungal genus that was described in 1794, including anamorphic fungi isolated primarily from soil and decomposing organic matter. Strains within this genus include a wide spectrum of evolutionary solutions that range from very effective soil colonizers with high biodegradation potential, to non-strict plant symbionts that colonize the rhizosphere. Species concepts within Trichoderma are very wide, which has resulted in the recognition of many infraspecific groups. Some groups of biotypes within this conglomerate are able to antagonize phytopathogenic fungi by using substrate colonization, antibiosis and/or mycoparasitism as the main mechanisms. This antagonistic potential is the base for effective applications of different Trichoderma strains as an alternative to the chemical control against a wide set of fungal plant pathogens. As a consequence of the variety of activities displayed by the

Trichoderma strain conglomerate, a large range of applications have been developed: the antagonistic potential is the basis for the effective control of a wide set of phytopathogenic fungi and the biodegradative capacity is a source of useful enzymes in different industrial.

MECHANISMS OF ACTION

The choice of active Trichoderma strains is important in designing effective and safe biocontrol strategies. Many species of Trichoderma have multiple strategies for fungal antagonism, and indirect effects on plant health (such as plant growth promotion effects and fertility improvements) also vary. Some strains are potent antibiotic producers, and their suitability for use in biocontrol systems must be carefully assessed. However, many other active strains have no antibiotic capacity,and these are likely to be more useful in food production systems. Trichoderma biocontrol strains

have evolved numerous mechanisms for both attacking other fungi and enhancing plant and root growth . The colonization of the root system by rhizosphere competent strains of Trichoderma results in increased development of root and/or aerial systems and crop. Other activities, like the induction of plant systemic resistance and antagonistic effects on plant pathogenic nematodes , have also been described. These facts strongly suggest that during the plant-Trichoderma interactions, the fungus participates actively in protecting and improving its ecological niche. The dual roles of antagonistic activity against plant pathogens and promotion of soil fertility make Trichoderma strains appealing alternatives to soil fumigation technologies such as methyl bromide.

Strains of Trichoderma may also be aggressive biodegraders and act as competitors to fungal pathogens in their saprofitic phases, especially when nutrients are a limiting factor .Strains have been reported as promoting activities of nonpathogenic bacteria and mycorrhizal fungi ,the ability of Trichoderma strains to synthesize substances inducing SAR-like responses in plants was shown. Molecules produced by Trichoderma and/or its metabolic activity also have potential for promoting plant growth. Application of the species T. harzianum to plants resulted in improved seed germination, increased plant size, and augment of leaf area and weight. The scenario of combined systemic biofungicides and plant growth promoters has great market potential if the molecular basis of the activities can be identified.

The strong biodegradation and substrate colonization performances of Trichoderma strains is the strong biodegradation and substrate colonization performances of Trichoderma strains is the result of an amazing metabolic versatility and a high secretory potential which leads to the production of a complex set of hydrolytic enzymes. Similarly, the mycoparasitic process is based on the secretion of a rich cocktail of cell wall degrading enzymes (CWDEs) able to hydrolize the cell wall of various hosts. Among others, chitinases, b-1,3- glucanases , b-1,6-glucanases,a-1,3-glucanases  and proteases have been described as important components of the multi-enzymatic system of  Trichoderma strains. Some of these proteins display strong antifungal activities when are applied  in vitro, alone and/or combined, against plant pathogens. Some lytic enzymes can be involved in both antagonistic and saprophytic processes providing an evolutionary advantage to strains with both biodegrading and antagonistic potential, for the efficient colonization of different ecological niches in soil. A principal role in mycoparasitism has been attributed to chitinases and glucanases . However, fungal proteases may also be significantly involved in cell wall degradation, since fungal cell walls contain chitin and glucan polymers embedded in and covalently linked to a protein matrix .

The production of secondary metabolites by  Trichoderma strains also shows great variety and application potential. Trichoderma strains seem to be an inexhaustible source of antibiotics, from the acetaldehydes gliotoxin and viridin, to alpha-pyrones, terpenes, polyketides, isocyanide derivatives, piperacines, and complex families of peptaibols . All these compounds produce synergistic effects in combination with CWDEs, with strong inhibitory activity on many fungal plant pathogens. The potential of genes involved in biosynthetic pathways of antibiotics and peptaibols with applications in human and veterinary medicine is not been explored yet.

Trichoderma is not only a good biocontrol agent, but also a general fertility promoter. In the absence of pathogens, application of appropriate Trichoderna formulations (following solarization and/or preceding fumigation with authorized and environmentally-friendly chemicals) can also serve to promote plant growth and crop precocity, increase fruit production and reduce chemical treatments.

SELECTION OF TRICHODERMA STRAINS

Once active strains have been identified with the in vitro assays, a further selection must be done by studying other factors such as:

 1) Activity in vivo using experimentally induced diseases on plants,

2) Tolerance of high or low temperatures (necessary to survive other IPM treatments),

3) Suitability for formulation as foliar sprays and/or soil enhancements (e.g. high sporulation levels,rapid growth in bulk conditions),

4) specificity (strains should be inactive against beneficial organisms and plant crops),

5) long-term survival in field conditions,

6) interactions with other Trichoderma  strains already present in the cropping systems,

7) compatibility with agrochemicals used in the crop, or

8) shelflife and inoculum efficacy under commercial conditions.

TRICHODERMA IN AVOCADO PROTECTION

We have developed a biocontrol formulation, based on Trichoderma conidia, that was tested with satisfactory results.against the main avocado root pathogens:  Phytophthora cinnamomi and Dematophora necatrix in plantations maintained in the ecological conditions of Motril (Granada,Spain). D. necatrix is more resistant than P. cinnamomi to the action of Trichoderma biocontrol strains. However, being more difficult, the control by Trichoderma of root diseases caused by D.necatrix is effective .

TRICHODERMA PROTEIN FORMULATIONS

Trichoderma protein extracts with high glucanase and chitinase activities, directly obtained fromwild type strains, have been demonstrated to be effective as biofungicides. They can also be combined with chemicals (carbendazim, iprodione) with synergistic effects, and are stable enough to be considered for commercial application. We have investigated the antifungal properties of the proteins produced by Trichoderma species in laboratory and field conditions, defining the concentration of protein necessary to produce fungicide effects. It is recommended that any protein formulations contains at least one enzyme from each of the following classes: endochitinase, exochitinase, endoglucanase, exoglucanase (ß-1,3 plus ß-1,6), proteases and cellulalase (endocellulase). More than two enzymes from each class did not provide additional anti-fungal effect. In the field trials carried out with Trichoderma protein extracts, increased average weight of both roots and fruit per plant was detected in plots treated with Trichoderma proteins. The protein filtrates increased the total useful fruit weight by increasing the number of fruits of commercial size. These tests showed that Trichoderma chitinases and glucanases have no effect on the plant even if relatively large quantities are injected into plant tissues. CWDEs are not harmful to humans and animals, as indicated by eco-toxicological tests for registration of strains of Trichoderma for use as biocontrol agents in USA and the EU, and degrade into environmentally friendly residues. CWDEs can be effectively combined with whole-organism Trichoderma control, with considerable opportunities for synergism. CWDEs are particularly suited to post-harvest control.

The genes coding for protein production can be introduced into suitable organisms to be used ascell factories for large-scale production of CWDEs.

TRICHODERMA GENES

Several methods for applying both biocontrol and plant growth promotion exerted by Trichoderma strains have recently been demonstrated and it is now clear that hundreds of separate genes and gene products are involved in the processes of mycoparasitism, antibiosis, competition for nutrients or space, tolerance to stress through enhanced root and plant development, stabilization and sequestration of inorganic nutrients, induced resistance and inactivation of enzymes produced by pathogens (Monte 2001). Some of these genes have been identified, cloned from Trichoderma spp. (that offer great promise as transgenes to produce crops that are resistant to plant diseasessince transgenic expression of high levels of chitinolytic and glucanolytic Trichoderma enzymes do

not affect plant morphology, development or yield, or infection by arbuscular mycorrhizal fungi),patented and used to transgenically increase plant disease resistance (Lorito et al. 1998), but most of them are still unexploited for developing new biotechnologies.

Acknowledgements

We want to thank the staff of Newbiotechnic S.A. (NBT), Seville (Spain), for their contribution to theproduction of Trichoderma formulations and generation of results necessary to write this manuscript.

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