(Editor's Note: This article was originally published on February 27, 2008. Your comments are welcome, but please be aware that authors of previously published articles may not be able to respond to your questions.)

..this bodyguard can have the spare room!

If any fungus could ever be said to be a multitasker, Trichoderma would be one of the leading candidates. Researchers have found various strains to be useful for production of enzymes, as biocontrol agents, in plant growth promotion, and as a source of transgenes. In this article, I'm going to focus on the biocontrol aspect of one particular strain, the T-22, or Trichoderma harzianum because what this fungus is best at is of great interest to us as gardeners.

Trichoderma is common in nearly all soils and their activity is favored by the presence of plant roots. Some strains are better at colonizing the root zone, or rhizosphere, than others. T-22, known as RootShield, is a hybrid strain developed to incorporate many qualities of the better strains into one fungus. This workhorse colonizes the rhizosphere readily, and even grows along as the host roots grow. The more abundant and healthy the plant roots are, the better the fungus proliferates. This relationship is mutually beneficial. The plant roots provide the rhizosphere as a hospitable spare room and board for the fungus while the fungus attacks any pathogenic fungi attempting to invade the root zone.

The thumbnail picture above, right, shows RootShield hyphae ( shown in green) associating with a plant root (shown in brown).

Trichoderma is neighborly and tolerant as well, getting along with mycorrhizal fungi and able to function well even in the presence of many chemical fungicides. In fact, some work I've done demonstrates that different biocontrol organisms are most effective when working together as a group rather than as "lone rangers" on solo patrol. A normal or healthy soil biota will have a quantity of different types and strains of fungi and bacteria, so using several biocontrol agents together is more like the state of normal soil than a single overpowering strain would be.

T-22 attacking Rhizoctonia fungus

The ability to stay on duty when chemical fungicides are present is a boon because the biocontrol fungus cannot, by itself, help a plant that is already infected with a pathogenic fungus. First, the unwelcome invader must be evicted. Then Trichoderma can move in and provide preventive protection for the plant. So you see that the best time to utilize this biocontrol agent is before an infection strikes, not after.

The picture at left shows a RootShield hypha attacking and invading Rhizoctonia, a pathogenic fungus.

Best to keep this one on the payroll!

RootShield is most effective when multiple applications are used. You want the entire root system to be protected and a plant in vigorous growth can outstrip the ability of the fungus to keep up. This can leave some roots unprotected, so the solution is to make new applications as your plants grow. The commercially available formulation includes recommendations and instructions for the proper timing of these applications.

One need not worry where the payoff is for Trichoderma; natural root exudates from the host provide sustenance for the fungus as it grows. So in this manner, the plant being protected is taking care of the fungus payroll! The post assumed by Trichoderma confers upon it a sort of "squatter's rights", enabling it to fend off the pathogens more effectively. This is also the reason why applying this bicontrol after pathogen infection is not effective. In that case, the pathogen has already exercised the squatter's rights, feeding off the same root exudates but damaging the roots and excluding the beneficial fungus in the process. In the photo below, right, Rootshield hyphae can be seen coiling around plant roots, making themselves at home in preparation for their role as defenders against disease-causing fungi.

T-22 coiling around plant root

My plants say, "We want the right fungus among-us!"

Home gardeners and plant fanciers can now obtain their own Trichoderma harzianum T-22 in the formulation known as RootShield. Some sources are Gardener's Supply Company, Johnny's Selected Seeds, and Territorial Seed Company. Each of these sources offer the home and garden version of RootShield. Commercial agricultural and horticultural interests can obtain professional formulations from BioWorks, Inc.. Much more information about RootShield is available on the BioWorks website.

Picture credit: All photographs courtesy of Dr. Randy Martin, BioWorks, Inc.
Also, additional information can be found at:Weeden, C.R., A. M. Shelton, and M. P. Hoffman. Biological Control: A Guide to Natural Enemies in North America.