There are many natural antagonists that keep disease organisms under suppression. Members of the genus Trichoderma are filamentous fungi that can be isolated from many soil types. They are part of a healthy soil environment with numerous species found worldwide. A few select strains of T. harzianum have been shown to suppress plant pathogens. However, they are limited in the scope of plants they protect and the pathogens they control. For example, one strain can control Pythium and grow in cooler soils, while another can control Rhizoctonia and colonizes the root system.
To overcome these limitations, researchers at Cornell University produced a hybrid strain that had enhanced attributes of the parents. The strain, T-22, protects the root system against Fusarium, Pythium and Rhizoctonia on a number of crops including corn (field, sweet, silage), soybeans, potatoes, tomatoes, beans (green and dry), cabbage, cucumbers, cotton, peanuts, turf, trees, shrubs, and other transplants and ornamental crops. T-22 is able to grow in a range of soil types at temperatures above 50°F. Because of its superior attributes, T-22 has been commercially developed as one of the first biofungicides.
Several mechanisms allow T-22 to control various plant pathogens. The biofungicide protects the plant by establishing itself on the outside of the root system in the rhizosphere (root zone). Because it is a living organism, T-22 can grow along the entire length of the root system where it establishes a barrier against pathogen attack. As long as the root system remains active in its growth and development, T-22 will continue to grow along with it by feeding on the waste products released. These waste products can also serve as a meal for pathogens. Early applications of T-22 protects plant roots by removing secreted nutrients that pathogens might use. Occupation of the roots by T-22 does not seem to interfere with the activity of mycorrhizae or nitrogen-fixing Rhizobium.
A second line of defense by T-22 against root-rotting fungi is the release of hydrolytic enzymes. Many plant pathogens contain chitin as a component of their cell wall. T-22 releases chitinases that have been shown to disrupt the cell wall of these pathogens. Electron micrographs illustrate how these enzymes dissolve the cell wall and create holes in the pathogen. Once damaged, the pathogen itself becomes the prey of other soil microflora.
These enzymes work optimally in an acidic environment. Insects also contain chitin as a constituent of their exoskeleton, but their pH is typically in the alkaline range. Thus, the T-22 chitinases will not work on these nontarget organisms; plants, birds, fish, humans and other organisms are also not affected by T-22.
T-22 can control some of the most common fungal pathogens. Because of squatter's rights, it can also suppress other pathogens by displacing them from the roots. This maintenance of the root system allows for a larger root biomass to develop. Increased plant yields begin with a well-established root system. Once the plant is harvested, T-22 levels in the soil drop. The levels remaining after harvest will not be high enough for efficacious fungal control on new plantings.
T-22 has been granted an exemption from the need for a tolerance by the US-EPA and is approved for use in certified organic production in several states. Its effectiveness against a range of pathogens on a diversity of plants allows it to be a part of many integrated pest management programs. T-22 can protect plant parts (roots) not previously protected by chemical fungicides. It can be used at the time of planting as a seed treatment. The T-22 planter box formulation is compatible with many standard chemical seed treatments (e.g. Captan) and is applied directly over chemically treated seeds at the time of planting. The chemical allows for stand establishment, but that's all. T-22 then kicks in to protect the root system from fungal attack. In addition, T-22 is available in a granular formulation for incorporation in soilless potting mixes and as a wettable powder for drenching greenhouse or agricultural plants.
- Dr. Chris Hayes, BioWorks, Inc., Geneva, NY 14456
|Return to Contents Menu Vol. V No. 4|
Go To Index