Know Your Friends

Gliocladium virens

Gliocladium virens is a naturally occurring, ubiquitous soil saprophyte found throughout the United States in various soil types. This common soil fungus has been shown to suppress a variety of soilborne plant pathogens, including Pythium spp., Rhizoctonia solani, and Sclerotium rolfsii that cause damping-off, root rots, and various other seedling diseases on a wide variety of host plants.

G. virens is a hypomycete with no confirmed sexual stage (possibly Hypocrea gelatinosa). It reproduces asexually, producing conidia that are held in masses of moist spores. It survives as thick-walled vegetative segments of mycelium, termed chlamydospores, usually embedded in organic matter. The spores are dispersed only in water or carried in soil or organic matter, and are not airborne.

This fungus is one of the first to be registered for biological control of plant diseases. The strain GL-21 was first registered as a biological pesticide in 1990 by W.R. Grace & Co. (Columbia, MD) for controlling damping-off diseases, particularly those caused by Pythium and Rhizoctonia, of greenhouse ornamental and food crops. Commercial preparations of formulations of the fungus have appeared on the market as GlioGardTM, an alginate prill formulation that is no longer available, and more recently as SoilGardTM. Developed in collaboration with Grace Biopesticides and the Biocontrol of Plant Diseases Laboratory of USDA, ARS in Beltsville, Maryland, SoilGardTM consists of spores of the fungal strain GL-21 in a granular formulation. It should be blended with soil or soilless plant growing media at the rate of 1 - 1 1/4 oz./ft3 of media at least 3 days prior to planting, or incorporated into the media surface in plant beds at the rate of 3/4 -1 oz./ft2 prior to or at planting. The fungus is not persistent at the inoculated high concentrations and declines to pre-application levels over a period of a few weeks.

When incorporated into potting media, SoilGardTM controls plant pathogens through a variety of mechanisms including parasitism, antibiosis, competition and exclusion. G. virens is known to parasitize some soil pathogens such as R. solani. The Gliocladium will actually wrap itself around the pathogen and release enzymes that destroy the pathogen's cuticle, leaving the pathogens susceptible to attack. GL-21 also produces a broad spectrum antibiotic called gliotoxin which kills many soil pathogens. Gliotoxin is not found in the SoilGard™ formulations, but when the spores of GL-21 begin to grow in the soil or soilless mix, the antibiotic is produced. Temperature, and possibly organic matter, nutrient status, and other chemical and physical factors affect the ability and capacity of G. virens to produce gliotoxin.

Even though gliotoxin has moderate mammalian toxicity, the formulated material contains essentially no toxin, so it would not be harmful if ingested. SoilGardTM is exempt from tolerance for use on all food crops. It is an alternative to currently registered chemical seed treatments and in some situations may be useful as a replacement for methyl bromide for control of soilborne disease problems.

G. virens has performed well in many greenhouse and field tests for controlling root diseases on many different plant species, including damping-off of snapbeans, zinnia and cabbage; southern blight of tomato; and cotton seedling diseases. Coating seeds with G. virens GL-3 was even better than treating seeds with the fungicide captan in reducing corn damping-off in greenhouse tests.

G. virens has considerable potential as a biological control agent for many soil diseases.

- Susan Mahr, University of Wisconsin - Madison

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