In the world of beneficial fungi, the genus Trichoderma has yielded some of the most reliable workhorse strains of our modern age.
Trichoderma are everywhere. They are found in most soils and climates, with dozens of species discovered in aquatic environments and insect bodies, among many other places. It was 1794 when South African mycologist Christiaan Hendrik Persoon first named it; two centuries later, a handful of its members have become superstars of industry, their enzymes useful in the making of biofuels, textiles and paper.
As if all that weren’t enough, the genus offers huge benefits to plant production too. Trichoderma are considered opportunistic avirulent plant symbionts: they invade root tissue and extract certain compounds, while giving back a host of plant nutrients and metabolites the extent of which researchers are still discovering.
In addition, Trichoderma are highly competitive, parasitizing and killing other fungi. It’s this trait that makes them useful as biofungicides, proven effective in controlling Fusarium, Pythium and Rhizoctonia, among other pathogens. It’s also this trait, however – Trichoderma’s mycoparasitism – that can pose problems.
For many years, research has shown that Trichoderma fungi can work in conjunction with their mycorrhizal counterparts, the two groups often boosting one another’s efficacy. But more recently, there’s been mounting evidence that that’s not always how it works. Some examples: certain Trichoderma can block mycorrhizal fungi from transferring phosphorus to their host plants; others can grow throughout the spores and mycelia of neighboring fungi, eventually bursting out through the surface like some alien from the movies.
None of which is to say Trichoderma are inherently bad. In the rich subterranean world that we’re always working toward, there will likely be many Trichoderma, living as part of a healthy microbial community without need for any further inoculum.
Moreover, Trichoderma applications can prove useful in certain situations. In converting to biological health, we often start with compacted soils that allow limited oxygen penetration, and therefore harbor fungal pathogens. In that situation, Trichoderma can be a big help – the more quickly those pathogens disappear, the more quickly our conversion efforts can gain momentum. Other inoculum products can help in additional ways, but once those immediate problems are addressed, it’s time to shift focus toward diversity.
Those studies showing aggressive behavior in Trichoderma were performed using species from a small group of culturable microbes. This raises the obvious question: what danger might Trichoderma pose to all the lesser-known fungi that our soils need?
The thing to remember about microorganisms is that the vast majority remain mysterious to science. Of the thousands of species present in a handful of healthy soil, only a few dozen are found among the array of soil-inoculum products on the market. Researchers are always investigating new prospects to address various challenges to agriculture, but there are logistical realities: a species must be agreeable to lab culturing and tough enough to survive packaging, transportation and field application, while offering cost-effective benefits. At present, they have found only a relative handful that fit the bill.
In this context, the case of Trichoderma illustrates the best way to think about microbial-inoculum products: as useful tools in certain circumstances, but problematic if relied upon too heavily. It all comes down to the current industry approach to soil-biology management: it still focuses on products to treat symptoms, rather than on microbial diversity. It’s been proven time and again that truly healthy crops can only grow from soils containing rich microbial life. Inoculum products containing a handful of species can only get us partway there.
Among culturable Trichoderma strains, there is an exception to the above: T. reesei, a species known to be minimally mycoparasitic, and therefore of limited danger to other fungi. The species is also a strong biomass decomposer (and so a strong producer of plant nutrients) and effective plant symbiont, and is found in a number of soil-inoculum products.
A note regarding Trichoderma and mycorrhizal fungi: One study linked above found antagonism between T. harzianum and various species of Glomus, a genus responsible for most of the endomycorrhizal strains found in popular inoculum products. This research suggests an obvious problem involving products that contain both.