Trichoderma and Beauveria for gardeners: using bio-agents at home

Trichoderma protects roots from soil disease and goes into the potting mix at planting. Beauveria bassiana kills soft-bodied insects and is sprayed on leaves. Both are living fungi, so they need cool storage, evening application, and a gap of at least a week from any chemical fungicide.

These products behave nothing like a bottle of insecticide, and treating them as if they do is why so many gardeners try them once and give up. You are not applying a chemical. You are introducing a population of organisms and giving them conditions to establish. Get that right and they work for weeks. Get it wrong and nothing happens at all.

The split matters. A Trichoderma spray on leaves does very little. A Beauveria drench into soil is largely wasted. Match the organism to where its target lives.

Trichoderma is preventive. It colonises the root surface and occupies the space a pathogen would otherwise take. Once root rot has started, it slows the spread but will not revive dead roots. Apply it before you have a problem.

1. In potting mix

Mix a Trichoderma viride powder or Trichoderma harzianum powder into your potting medium at planting, roughly 5 g per kilogram of mix. Moisten the mix first. Dry powder sitting in dry medium does nothing.

2. As a drench

For plants already in pots, use a liquid Trichoderma harzianum at about 2 to 3 ml per litre and water it into the root zone. Repeat every four to six weeks during the growing season. Water the pot lightly first so the drench spreads rather than running down the sides.

3. As a seed treatment

Dust seed lightly with the powder before sowing, or soak seed briefly in a dilute suspension and dry it in shade. This is the cheapest disease protection available to a gardener and it prevents most damping off in seed trays.

These are sprays. Beauveria bassiana at 2 to 3 ml per litre, applied to cover leaf undersides thoroughly, is the standard rate for a home garden. Verticillium lecanii is used the same way and is stronger on aphids, whitefly and scale specifically.

For soil-dwelling larvae and grubs, Metarhizium anisopliae goes in as a drench rather than a spray. Where several pest types are present at once, a combined formulation carrying all three organisms saves separate applications.

Spray in the evening. Repeat after seven days. Expect the insect count to fall over one to two weeks rather than overnight; the fungus has to germinate on the insect and grow through its body before it kills.

Cornell's fact sheet on Beauveria describes the infection process and the humidity it needs, which explains why results vary so much between a humid monsoon and a dry summer. It is worth reading once.

Almost every reported failure comes from one of these.

1. Storing it in heat

These are live cultures. A bottle left in a hot cupboard or a car boot loses most of its viable spores. Store below about 25 degrees Celsius, away from sunlight, and check the manufacturing date rather than only the expiry.

2. Spraying in the middle of the day

Ultraviolet light kills spores on the leaf surface within hours. Evening application gives them the whole night to germinate in cooler, more humid conditions.

3. Tank mixing with a fungicide

A chemical fungicide kills Trichoderma and Beauveria as efficiently as it kills the pathogen you were targeting. Keep at least seven days between them. The same caution applies to neem oil, which coats spores and reduces germination.

4. Applying once and stopping

A single application establishes a small population that heavy rain or irrigation will wash out. Repeat every four to six weeks for soil applications and every seven days for foliar sprays during active pest pressure.

Mycorrhizal fungi are not pest controls. They form a partnership with roots, extending far beyond the root system to gather phosphorus and water in exchange for sugars. In a container, that translates to better drought tolerance and better phosphorus use. A VAM biofertilizer applied once at planting keeps working for the life of the plant, provided you do not apply a systemic fungicide afterwards.

Heavy phosphorus fertilisation suppresses mycorrhizal colonisation. If you are using mycorrhiza, ease back on high-phosphorus feeds.

1. At sowing: dust seed with Trichoderma powder.

2. At planting or repotting: mix Trichoderma and mycorrhiza into the medium.

3. Every 4 to 6 weeks: Trichoderma or Pseudomonas drench for root protection.

4. Weekly inspection: check leaf undersides for early pest signs.

5. At the first insects: Beauveria or Verticillium spray in the evening, repeated after 7 days.

6. After heavy rain or repotting: reapply the soil organisms, since much of the population will have been lost.

A Pseudomonas fluorescens drench is a good alternative to Trichoderma if you are rotating, and it establishes quickly on young roots.

Bio-agents reduce problems; they do not eliminate them. A garden running on biologicals will still see occasional pest outbreaks, and you will still lose the odd plant to disease. What changes is that the baseline pressure drops season by season as beneficial populations establish, rather than rising as it does under repeated broad-spectrum spraying.

Give it a full season before judging. The first month tells you very little.

Cornell maintains an index of the biocontrol organisms in commercial use and what each is suited to. It is a useful reference when you are choosing between products.