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item Howell, Charles - Charlie

Submitted to: American Phytopathological Society Annual Meeting
Publication Type: Proceedings
Publication Acceptance Date: 8/29/2004
Publication Date: 12/20/2004
Citation: Howell, C.R. 2004. Understanding the mechanisms employed by Trichoderma virens to effect biological control [abstract]. Phytopathology. 94:S-138.

Interpretive Summary: This paper describes the characteristics of Trichoderma virens, and reports the progress that has been made in understanding what mechanisms are involved in the biocontrol of cotton seedling diseases by strains of the fungus. It outlines why some mechanisms are important to the biocontrol process and others are not. In addition, it presents information on the application strategies best suited to obtain the optimum biocontrol activity from this fungus. It also presents new data describing why some strains of T. virens are pathogenic to seedlings of susceptible cotton cultivars while others are not.

Technical Abstract: Trichoderma virens appears to be one of the most versatile and effective biocontrol agents we have studied. The species contains an arsenal of mechanisms and antimicrobial metabolites that give it a wide spectrum of a activity against different pathogens and the capacity to control a number of diseases. One of the salient features of Trichoderma virens is that it is an aggressive mycoparasite of other fungi, many of which are plant pathogens. It not only parasitizes the hyphae of many fungal species, but it also can penetrate and destroy some of the resting structures, thereby reducing their inoculum potential in soil. Mycoparasitism in T. virens has been associated with its capacity to synthesize extracellular chitinase. In addition to its mycoparasitic character, T. virens is also a vigorous saphrophyte that can be cultured on any one of a number of substrates and can maintain itself in the absence of a host. T. virens also produces several potent antibiotics. The antibiotic gliotoxic that is produced by 'Q' Strains of T. virens, has a wide spectrum of activity against bacteria, actinomycetes and fungi. The antibiotic gliovirin produced by 'P' strains of this fungus, although closely related to gliotoxin, has a much more restricted spectrum of activity, and it is not effective against bacteria, actinomycetes or most fungi. It is, however, a potent inhibitor or Oomycetes, such as Pythium and Phytophtora species. 'Q' strains of T. virens do not produce gliovirin, and 'P' strains do not produce gliotoxin. When either 'P' or 'Q' strains of T. virens are grown on substrates with high C/N ratios, they both produce the steroid-like compound viridiol. Viridiol has little or no antibiotic activity, but it is a potent phytotoxin when placed in contact with the roots of plants. With proper placement, i.e., mixed into surface soil, it can act as an effective herbicide for weeds, without harm to the crop plant that is planted at greater depth. It has recently been discovered that strains of T. virens produce a mixture of peptaibols, linear peptide antibiotics, but their role in biocontrol is as yet unknown.