Most things sold for root disease work by killing something. T22 does that too — but it also colonises your plant's roots, eats fungal pathogens, produces antifungal compounds that diffuse through the soil, and triggers the plant's own immune system to upregulate its defences. Then, separately from all of that, it improves root development and makes nutrients that were already in your media more available to the plant.
It's a lot. And it's well-documented — T22 is one of the most studied biocontrol strains in existence, developed at Cornell University and characterised across decades of controlled trials. We carry it because the research is unusually solid for this category, where marketing claims frequently outrun evidence.
Here's how it actually works.
What T22 does in brief
What T22 is and why the strain number matters
Trichoderma harzianum is a soil fungus that occurs naturally in agricultural soils all over the world. It's aggressive toward other fungi, colonises plant roots without causing disease, and grows fast. There are hundreds of Trichoderma species and dozens of commercially available strains — and they are not interchangeable. Performance, target pathogen range, and root colonisation ability vary significantly between strains. A product labelled "Trichoderma" without a strain designation is telling you very little.
T22 was developed at Cornell by selectively crossing two naturally occurring strains — one with strong root colonisation ability, one with broad pathogen suppression. The resulting strain establishes in the root zone rapidly, persists there, and performs across a wider range of soil conditions than most alternatives. The T22 designation on the label is a specific, meaningful thing — not a product name someone made up.
It's sold commercially as RootShield and is the active ingredient in our Trichoderma harzianum T22.
Mode of action 1: mycoparasitism — it eats the pathogen
T22 is a fungus that parasitises other fungi. When it encounters the hyphae of a pathogenic fungus in the soil, it doesn't just sit near it and compete for space. It hunts it.
T22 detects pathogens through the chemical compounds they release into the soil — essentially following the smell — and grows toward them. On contact, it coils around the pathogen's hyphae. Then it produces chitinases — enzymes that break down chitin, the structural polymer in fungal cell walls — along with glucanases, proteases, and other cell wall-degrading enzymes. These dissolve the pathogen's cell wall from the outside. T22 then penetrates and consumes it. What's left becomes organic matter in the soil.
It's genuinely strange to watch under a microscope. The coiling behaviour is characteristic enough that it's part of how the genus was originally identified as a mycoparasite. The pathogens it works against this way include Pythium, Phytophthora, Fusarium oxysporum, Rhizoctonia solani, Sclerotinia, and Botrytis — the major soil-borne and root disease pathogens that matter to both home growers and commercial operations.
Mode of action 2: competitive exclusion — it gets there first
T22 also suppresses pathogens through a mechanism that doesn't require finding them and eating them. It simply colonises roots faster than most pathogens can, uses up the resources they'd need to establish, and occupies the physical space before they arrive.
Root pathogens establish by following the chemical gradients that roots release, landing on the root surface, and colonising before the plant's defences kick in. T22 does the same thing faster. A root system that T22 has already colonised is a root system where the available attachment sites, nutrient pools, and root exudate chemistry have already been claimed. Pathogens arriving later find a surface that's already occupied and a niche that's already been exploited.
This is why applying T22 at potting works better than applying it later — not because it stops working once disease is present, but because competitive exclusion only functions when T22 is already there. It's also why a single application at potting provides ongoing protection: T22 grows alongside the expanding root system, continuously colonising new root tissue as it develops. You don't need to keep reapplying it.
Mode of action 3: antibiosis — chemical suppression
T22 produces antifungal compounds — peptaibols, harzianic acid, harzianopyridone, and others — that diffuse through soil water into the surrounding environment. Pathogens growing toward the root zone encounter inhibitory concentrations of these compounds before they make physical contact with T22 or reach root tissue. It's a chemical buffer zone around the colonies T22 has established.
What's interesting about this mechanism is that T22's metabolite production isn't fixed — it shifts in response to specific pathogens. The profile of compounds it produces in the presence of Rhizoctonia is different from what it produces in the presence of Pythium or Botrytis. It's not just flooding the root zone with antifungals at a standard rate. It's reading the environment and adjusting. That's the kind of detail that gets skipped in product descriptions but matters for understanding why T22 works against such a broad range of pathogens.
Mode of action 4: induced systemic resistance — changing the plant itself
This one is the most counterintuitive, and the most separation from how a conventional fungicide works.
When T22 colonises roots, fragments of its own cell wall — chitin oligosaccharides and β-glucan molecules — trigger the plant's pattern recognition receptors. The plant's immune signalling cascade fires. Defence genes upregulate throughout the plant — not just at the roots, systemically — increasing production of pathogenesis-related proteins, chitinases, and other compounds the plant uses against fungal attack. Proteomic analysis of T22-colonised maize identified upregulation of 27 endochitinase genes and 4 exochitinase genes. These changes persist as long as T22 is in the root zone.
The result is a plant with a higher baseline of immune readiness that responds faster when a pathogen makes contact with root tissue. It's not immune. But it's in a meaningfully different physiological state than an uncolonised plant — and that state is maintained continuously because T22 is a permanent root resident. The systemic priming also offers some degree of protection against foliar pathogens arriving after T22 establishes. Not a primary use case, but a real benefit.
What T22 does for growth — with no disease present
This is the part that surprises most people who buy T22 specifically for root rot prevention and then notice their plants just look better.
Root development
In controlled trials, seed treatment with T22 produced seedlings with nearly twice the root length of untreated controls. Both primary and secondary roots were longer, root surface area increased, root hair density improved. In containers — where the root zone is fixed and the plant has to work with what's in the pot — more root means better access to water and nutrients across that fixed volume of media. Faster establishment after repotting. Better performance between waterings. The difference is real and it shows up even with no pathogen present.
Nutrient solubilisation
A lot of the phosphorus, iron, manganese, and zinc in most growing media is chemically locked — bound to mineral compounds in forms plant roots can't directly access. T22 produces organic acids and chelating compounds that dissolve these bound forms, converting them to plant-available nutrients. This isn't a generic claim about soil fungi — it's been documented specifically for T22, including solubilisation of rock phosphate, iron oxides, and zinc complexes. The practical effect is that the nutrient density of your growing environment is higher with T22 present, without additional fertiliser.
This matters most in alkaline media, where micronutrient availability drops off as pH rises. T22 creates a more acidic microenvironment around the root surface regardless of the surrounding medium's pH — which partially compensates for conditions where nutrients would otherwise be fixed and unavailable.
Nitrogen use efficiency
T22 increases how much of the available nitrogen in the root zone actually ends up in plant tissue, rather than leaching out or being lost. The mechanism involves changes in amino acid transport and nitrogen allocation identified in proteomic studies of colonised plants. For container growers who are deliberate about fertiliser rates — because over-fertilisation in containers is a real problem — this means T22-colonised plants may genuinely need less supplemental nitrogen to achieve the same growth. That's not a trivial benefit.
Overall
Better roots, more available nutrients, faster immune response. The cumulative effect is a plant that's harder to stress and faster to recover when stressed. These effects are measurable in clean conditions with no disease pressure. For anyone growing high-value plants in containers — which is most of what we're talking about here — that's worth as much as the root rot protection.
Pathogens T22 is effective against
| Pathogen | Disease | Primary mechanism |
|---|---|---|
| Pythium ultimum, P. aphanidermatum | Damping off, root rot | Mycoparasitism, competition, antibiosis |
| Phytophthora spp. | Root and crown rot | Mycoparasitism, antibiosis |
| Fusarium oxysporum | Fusarium wilt, root rot | Competition, antibiosis, induced resistance |
| Rhizoctonia solani | Root rot, damping off, stem canker | Mycoparasitism, antibiosis |
| Sclerotinia sclerotiorum | White mould, stem rot | Mycoparasitism, competition |
| Botrytis cinerea | Grey mould | Antibiosis, induced resistance |
| Thielaviopsis spp. | Black root rot | Competition, mycoparasitism |
| Cylindrocladium / Calonectria spp. | Collar and root rot | Mycoparasitism, competition |
A note on oomycetes: Pythium and Phytophthora are technically not true fungi — they're oomycetes, more closely related to brown algae than to the fungal kingdom. Their cell walls contain cellulose rather than chitin as the primary structural component, which means some antifungal products that target chitin are less effective against them. T22's chitinases do still have activity against oomycete cell walls — oomycetes contain some chitin, particularly at hyphal tips — but the antibiosis and competition mechanisms are arguably more important against this group than pure mycoparasitism.
How to use it
The mechanisms make the instructions self-explanatory once you know what T22 is actually doing.
Apply early — at potting, not after
T22 works reactively too — mycoparasitism and antibiosis are active against established pathogens — but the competitive exclusion and induced resistance mechanisms both require establishment time. Apply at potting or transplant and T22 has weeks to colonise roots before the overwatering event, root damage, or temperature stress that typically opens a window for root rot. Apply after rot has set in and you're asking T22 to fight a pathogen that already has a foothold. It can help, but it's harder work and slower. Get it in early.
Moisture
T22 needs moisture to grow through soil and colonise roots. It won't die in dry media, but hyphal growth stops. Apply to pre-moistened media or water thoroughly immediately after, and keep moisture adequate for the first week or two of establishment.
Fungicides
T22 is a fungus. Fungicides kill fungi. Broad-spectrum synthetic fungicides and copper-based organic treatments will reduce or eliminate your T22 population — they don't distinguish between pathogenic and beneficial species. If you need to apply a fungicide, do it before introducing T22 and wait the full residue period before applying. Don't run both simultaneously.
Some systemic fungicides targeting oomycetes specifically — phosphonate-based products — are more compatible. If you're running a combined program, check compatibility for the specific products before committing.
pH
T22 works across a broad range but prefers slightly acidic conditions. Above pH 7.5, establishment is slower. T22 acidifies its local environment around roots somewhat regardless, which helps — but if your media is strongly alkaline, addressing that before or alongside T22 application will get you better results.
Storage
Refrigerated, not frozen, before the expiry date. Spores are more durable than nematode IJs — you don't need to use the entire packet the same day — but room-temperature storage degrades viable spore count over time. Keep it cold.
Using T22 when acclimating tissue culture plants
Tissue culture plants are about as vulnerable to root rot as a plant can be. They emerge from sterile agar with roots that have never encountered soil organisms, no established root microbiome, and a physiology calibrated to the stable, sterile conditions of a culture vessel. The transition to real growing media is the highest-risk window in a TC plant's life — humidity drops, bacteria and fungi arrive all at once, and roots that were never challenged suddenly are.
T22 is a good tool for this transition, for two reasons that compound each other.
First, the growth promotion effects. TC plants acclimating into media need to develop functional root systems fast — the sooner the root system establishes and starts absorbing water and nutrients from the media rather than relying on agar residue, the faster the plant stabilises. T22's root development effects — more root mass, better root hair density, improved nutrient uptake — are most valuable exactly here, in the early post-transfer window when the root system is small, undeveloped, and operating in an unfamiliar environment. Getting roots established faster shortens the acclimation window and reduces the time the plant spends at peak vulnerability.
Second, the disease suppression. TC plants have no established competition in the root zone — their roots are a blank slate that pathogens can colonise without resistance. Applying T22 at the point of transfer means T22 establishes alongside the developing root system from day one, occupying the niche before pathogens do. The induced systemic resistance effects build gradually as root colonisation increases — so the earlier T22 is introduced, the sooner the plant develops that primed immune state.
How to apply for TC acclimation
Mix T22 into the acclimation media before transferring the plant. For small pots and plug trays — the typical format for TC acclimation — this means incorporating the granules into the media at potting rather than applying as a drench after. Direct contact between T22 and the root zone from day one is the goal.
Keep media moisture consistent during the establishment window — both for the plant's acclimation and for T22's colonisation, which slows in dry media. If you're using a humidity dome or propagation chamber during acclimation, T22 will establish readily in those conditions.
One thing to be aware of: some TC protocols include a preventive fungicide drench at transfer — copper-based or otherwise — to address the pathogen load that sterile TC plants have no natural defence against. If you're using that protocol, apply the fungicide first and wait the full residue period before introducing T22. The two are incompatible simultaneously, and the fungicide will kill T22 before it can establish. Alternatively, skip the chemical drench and rely on T22 alone — its competitive exclusion and antibiosis mechanisms fill a similar role without the residue problem. That's a judgment call based on your pathogen pressure and growing environment, but it's a legitimate approach.
The short version
- T22 is a specific, well-characterised strain — not a generic Trichoderma product
- It works through four simultaneous mechanisms: mycoparasitism (attacking pathogens directly), competitive exclusion (occupying root space), antibiosis (producing antifungal compounds), and induced systemic resistance (changing plant biology)
- Target pathogens include Pythium, Phytophthora, Fusarium, Rhizoctonia, Sclerotinia, Botrytis, and others
- Growth promotion effects — expanded root mass, improved nutrient solubilisation, better nitrogen use efficiency — occur independently of disease pressure
- Preventive application is significantly more effective than reactive — apply at potting or transplant
- Incompatible with broad-spectrum fungicides — don't run both simultaneously
- Once established, T22 persists in the root zone and colonises new root tissue as it develops — a single application provides ongoing protection
Common questions
Frequently asked
-
Yes — and it's one of the better use cases for T22. TC plants transfer into media with no established root microbiome, no competition in the root zone, and roots that have never encountered soil organisms. That makes them unusually vulnerable to root rot in the first weeks after transfer. Mixing T22 into the acclimation media at potting gives it immediate contact with the developing root system and lets it establish before any pathogen does. The root development and nutrient uptake benefits also kick in during the exact window when TC plants need them most. If your TC protocol includes a preventive fungicide drench at transfer, apply that first and wait out the residue period before introducing T22 — they can't run simultaneously.
-
Yes — but know what you're asking it to do. Mycoparasitism and antibiosis are active against established pathogens, so it's not pointless. The competitive exclusion and induced resistance mechanisms are less relevant once rot has set in — those work by preventing colonisation, not reversing it. For a plant with active rot: remove affected root tissue where you can, fix whatever caused it (overwatering, poor drainage), and apply T22 to suppress what remains and prevent re-infection. Don't expect it to undo severe rot on its own. That's not what it does.
-
Yes. T22 colonises roots without damaging them — it benefits from root exudates, the plant benefits from T22's presence, and healthy root tissue is left alone. It has no activity against insects, vertebrates, or non-fungal soil organisms. It's EPA-exempt from pesticide registration in the US (40 CFR 152.25) with no re-entry intervals and no handling restrictions beyond basic biosafety practices for any biological product.
-
Once established, it persists for the life of the plant under most growing conditions. T22 grows continuously alongside the expanding root system, colonising new root tissue as it develops. One application at potting typically provides ongoing protection without retreatment — unless you apply a broad-spectrum fungicide that wipes out the established population, at which point you'd need to reintroduce.
-
Generally yes. T22 and mycorrhizal fungi occupy different root zone niches and coexist without significant antagonism in most conditions. Some research suggests T22 may slightly reduce mycorrhizal colonisation rates in certain conditions — its antifungal activity isn't perfectly specific to pathogens — but in commercial settings the two are run together routinely without issues. If using both, apply mycorrhizae at potting and T22 as a drench or mixed into media, and don't apply both at very high concentrations in direct contact.
-
T22 can establish on roots in semi-hydroponic systems including LECA, but it's optimised for soil and soil-like growing media and that's where it performs most reliably. The competition and persistence mechanisms work best with a substrate matrix. In fully recirculating hydroponic systems, T22 may not colonise effectively and can clog emitters and filters — not ideal. For LECA and similar inert media, apply as a root soak cautiously, know you're outside the primary use case, and manage expectations accordingly.
-
Yes — they address completely different problems and don't interfere with each other. T22 targets fungal root pathogens. Steinernema feltiae nematodes target fungus gnat larvae and other soil insect pests. Different mechanisms, different ecological niches, no meaningful antagonism between them. Running both gives you protection against root disease and soil insect pests simultaneously, which in container growing is a common combination worth having.
Trichoderma harzianum T22 — apply at potting, not after the fact.
Wettable granules. Ships Monday–Thursday.
References
- Harman, G. E., Howell, C. R., Viterbo, A., Chet, I., & Lorito, M. (2004). Trichoderma species — opportunistic, avirulent plant symbionts. Nature Reviews Microbiology, 2(1), 43–56. https://doi.org/10.1038/nrmicro797
- Harman, G. E. (2006). Overview of mechanisms and uses of Trichoderma spp. Phytopathology, 96(2), 190–194. https://doi.org/10.1094/PHYTO-96-0190
- Shoresh, M., & Harman, G. E. (2010). Genome-wide analysis of Trichoderma harzianum T22 interactions with maize: gene expression and protein profiling. Molecular Plant-Microbe Interactions, 23(7), 872–885. https://doi.org/10.1094/MPMI-23-7-0872
- Howell, C. R. (2003). Mechanisms employed by Trichoderma species in the biological control of plant diseases: the history and evolution of current concepts. Plant Disease, 87(1), 4–10. https://doi.org/10.1094/PDIS.2003.87.1.4
- Altomare, C., Norvell, W. A., Björkman, T., & Harman, G. E. (1999). Solubilization of phosphates and micronutrients by the plant-growth–promoting and biocontrol fungus Trichoderma harzianum Rifai 1295-22. Applied and Environmental Microbiology, 65(7), 2926–2933. https://doi.org/10.1128/AEM.65.7.2926-2933.1999
- Vitale, A., Cirvilleri, G., Castello, I., & Polizzi, G. (2012). Evaluation of Trichoderma harzianum strain T22 as biological control agent of Calonectria pauciramosa. BioControl, 57, 687–696. https://doi.org/10.1007/s10526-011-9423-1
