Fungi in the genus Trichoderma have been known since at least the 1920s for their ability to act as biocontrol agents against plant pathogens. Until recently, the principal mechanisms for control have been assumed to be those primarily acting upon the pathogens and included mycoparasitism, antibiosis, and competition for resources and space. Recent advances demonstrate that the effects of Trichoderma on plants, including induced systemic or localized resistance, are also very important. These fungi colonize the root epidermis and outer cortical layers and release bioactive molecules that cause walling off of the Trichoderma thallus. At the same time, the transcriptome and the proteome of plants are substantially altered. As a consequence, in addition to induction of pathways for resistance in plants, increased plant growth and nutrient uptake occur. However, at least in maize, the increased growth response is genotype specific, and some maize inbreds respond negatively to some strains. Trichoderma spp. are beginning to be used in reasonably large quantities in plant agriculture, both for disease control and yield increases. The studies of mycoparasitism also have demonstrated that these fungi produce a rich mixture of antifungal enzymes, including chitinases and beta-1,3 glucanases. These enzymes are synergistic with each other, with other antifungal enzymes, and with other materials. The genes encoding the enzymes appear useful for producing transgenic plants resistant to diseases and the enzymes themselves are beneficial for biological control and other processes.

https://apsjournals.apsnet.org/doi/pdf/10.1094/PHYTO-96-0190

Overview of Mechanisms and Uses of Trichoderma spp.

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Myths and Dogmas of Biocontrol Changes in Perceptions Derived from Research on Trichoderma harzinum T-22.

Heterokaryon fonnation between different fungal individuals is an important component of many fungal life cycles and may serve as the first step in the parasexual cycle and the transmission of hypovirulent factors such as dsRNAs. Heterokaryosis also is a means by which nonnally haploid fungi may enjoy the benefits of functional diploidy, such as complementation or heterosis. In plant pathogenic fungi, the entity that emerges following heterokaryosis may differ from its constituents in aggressiveness or host range; some of these aspects have been reviewed in previous volumes in this series (9, 28, 45, 62, 72, 112, 114, 117, 143, 154, 158, 160). In most heterothallic fungi, the fonnation of a heterokaryon between two genetically different haploid strains is an essential part of the life cycle. Such heterokaryons may be quite stable and persist vegetatively for an indefinite period of time or may last only long enough for the component haploid nuclei to fuse and then immediately undergo meiosis. In many fungi, sexual and vegetative heterokaryons are quite distinct from one another. Strains capable of fonning a successful sexual heterokaryon may be unable to fonn a successful vegetative heterokaryon and vice versa. Strains that are capable of fonning these types of heterokaryons are referred to as "sexually" or "vegetatively" compatible, respectively. Strains that are vege­ tatively compatible with one another are frequently described as members of the same vegetative compatibility group, or VCG. Sexual compatibility is usually governed by one or more mating-type loci that may have two or more alleles (58, 62). Vegetative compatibility may be governed by the mating-type loci in some fungi, e.g. many basidiomycetes, but there also are examples in which a separate set of genes controls the fonnation and stability of these vegetative heterokaryons.

Fungal vegetative compatibility

The controlled environment of greenhouses, the high value of the crops, and the limited number of registered fungicides offer a unique niche for the biological control of plant diseases. During the past ten years, over 80 biocontrol products have been marketed worldwide. A large percentage of these have been developed for greenhouse crops. Products to control soilborne pathogens such as Sclerotinia, Pythium, Rhizoctonia and Fusarium include Coniothyrium minitans, species of Gliocladium, Trichoderma, Streptomyces, and Bacillus, and nonpathogenic Fusarium. Products containing Trichoderma, Ampelomyces quisqualis, Bacillus, and Ulocladium are being developed to control the primary foliar diseases, Botrytis and powdery mildew. The development of Pseudomonas for the control of Pythium diseases in hydroponics and Pseudozyma flocculosa for the control of powdery mildew by two Canadian research programs is presented. In the future, biological control of diseases in greenhouses could predominate over chemical pesticides, in the same way that biological control of greenhouse insects predominates in the United Kingdom. The limitations in formulation, registration, and commercialization are discussed, along with suggested future research priorities.

Biological control in greenhouse systems

Fungal infections, especially those caused by opportunistic species, have become substantially more common in recent decades. Numerous species cause human infections, and several new human pathogens are discovered yearly. This situation has created an increasing interest in fungal taxonomy and has led to the development of new methods and approaches to fungal biosystematics which have promoted important practical advances in identification procedures. However, the significance of some data provided by the new approaches is still unclear, and results drawn from such studies may even increase nomenclatural confusion. Analyses of rRNA and rDNA sequences constitute an important complement of the morphological criteria needed to allow clinical fungi to be more easily identified and placed on a single phylogenetic tree. Most of the pathogenic fungi so far described belong to the kingdom Fungi; two belong to the kingdom Chromista. Within the Fungi, they are distributed in three phyla and in 15 orders (Pneumocystidales, Saccharomycetales, Dothideales, Sordariales, Onygenales, Eurotiales, Hypocreales, Ophiostomatales, Microascales, Tremellales, Poriales, Stereales, Agaricales, Schizophyllales, and Ustilaginales).

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Developments in Fungal Taxonomy

In all crop-pathogen combinations, Trichoderma strains increased stands relative to the untreated control and were as effective as the chemical fungicides, even in the absence of solid matrix priming. Priming of seeds treated with Trichoderma strains increased plant stands in solid infested with F. graminearum and P. ultimum but not in soils infested with R. solani or S. rolfsii. In soils infested with P. ultimum, strain T95 generally gave the poorest results, T12 was intermediate, and the progeny strains (22 and 106) gave improved stands

Combining effective strains of Trichoderma harzianum and solid matrix priming to improve biological seed treatments

High yields (approx. 108/ml) of protoplasts of high purity (>99%) can be obtained from young thalli of strains T12 and T95 of Trichoderma harzianum by digestion of cell walls with NovoZym 234. Prot...

Protoplast Preparation and Fusion in Two Biocontrol Strains of Trichoderma Harzianum

Taxonomy in Trichoderma is problematic because of the lack of reliable morphological characters. This study evaluates five morphological species using methods of isozyme analysis that allow determination of allozyme polymorphism. One hundred nine alleles were resolved at 16 putative enzyme loci among 71 strains tested. Morphological species were not characterized either by specific alleles at single loci or by specific patterns of alleles at multiple loci. Rather, most alleles were distributed among most species, and each species included several different alleles at most loci. Allele patterns were analysed by phylogenetic analysis using parsimony (PAUP). A strict consensus tree was derived from 20 equally parsimonious, shortest cladograms using Gliocladium virens as an outgroup. In the resulting cladogram, T harzianum included a main group of strains adjacent to the outgroup and other strains distributed throughout the cladogram; this species appears to be paraphyletic. A core group of T. pseudokoningii strains appears to be monophyletic and derived directly from a T. harzianum-like ancestral group. Trichoderma hamatum included three isozymically distinct strains with close affinity to Gliocladium virens and a main group which appears to have been derived from a T. harzianum-like ancestral group independently of the core group of T. pseudokoningii. Trichoderma koningii and T. viride are not well distinguished from each other, but T. viride appears to be more derived and may have arisen from a more ancestral T koningii type. Phylogenetic analysis of allozyme data identified isozymically similar core groups of strains within morphological species, clusters of strains from various species, and iso? zymically distinct strains for further study. Wide distribution of numerous alleles among morphological species suggests that extensive genetic exchange occurs among these taxa.

Thomas E. Stasz

Papers

Combining effective strains of Trichoderma harzianum and solid matrix priming to improve biological seed treatments

Protoplast Preparation and Fusion in Two Biocontrol Strains of Trichoderma Harzianum

Evaluation of phenetic species and phylogenetic relationships in the genus trichoderma by cladistic analysis of isozyme polymorphism

Nonparental progeny resulting from protoplast fusion inTrichoderma in the absence of parasexuality

Limited vegetative compatibility following intra- and interspecific protoplast fusion inTrichoderma