Green Mold in Mushroom Grow Bag

What Is Trichoderma, Exactly?

Trichoderma is not just a mold — it is a mycoparasitic fungus, meaning it actively hunts and feeds on other fungi, including your mushroom mycelium. There are over 300 species in the Trichoderma genus, but T. harzianum and T. viride are the ones that most commonly devastate mushroom grows. In nature, Trichoderma plays a beneficial role by decomposing dead plant material, but in a controlled growing environment, it is an apex predator that will outcompete and consume your mycelium.

Why Trichoderma Is the #1 Enemy of Mushroom Growers

Trichoderma earns its reputation as the most feared contaminant because of its aggressive growth rate and prolific spore production. Under warm, humid conditions — exactly the conditions you create for mushroom cultivation — Trichoderma can colonize an entire substrate bag in 48 to 72 hours. Once it begins sporulating, a single green patch can release millions of spores into the air, contaminating every other grow in the same room. It is faster, more aggressive, and more resilient than almost any gourmet mushroom species you are trying to cultivate.

The Trichoderma Lifecycle: White to Green in 24-48 Hours

One of the most dangerous aspects of Trichoderma is that it starts out looking almost identical to healthy mycelium. In its early stage, it appears as a dense white patch that grows noticeably faster than the surrounding mushroom mycelium. Within 24 to 48 hours, the white growth begins turning green as the fungus starts producing spores. This is the sporulation phase, and by the time you see green, the contamination is already well established. The colour will deepen from light green to dark forest green as spore density increases.

How Trichoderma Gets In

There are four main entry points for Trichoderma contamination. The most common is inadequate sterilization of your substrate or grain — Trichoderma spores are heat-resistant and require a full 15 PSI for 90 minutes to kill reliably. The second is a break in sterile technique during inoculation, such as working without a flow hood or still air box. Third, contaminated spawn from your supplier can introduce Trichoderma before you even begin. Fourth, airborne spores in your grow space can settle on exposed substrate, especially during spawning or when you open bags for fruiting.

Environmental Conditions That Favour Trichoderma

Trichoderma thrives in warm temperatures between 25 and 30 degrees Celsius and high moisture environments — the same conditions many tropical mushroom species require. Poor air circulation, stagnant humidity, and excess moisture on the substrate surface all create ideal conditions. Trichoderma also prefers slightly acidic substrates, which is why adding hydrated lime (calcium hydroxide) to raise the pH of your substrate to 8-9 can help suppress it. Overly wet grain spawn is another major risk factor.

What to Do at Different Growth Stages

During colonization, if you spot a small patch of Trichoderma (smaller than a coin) before it turns green, you may be able to isolate the bag in a separate room and monitor it. Do not open it. If the mushroom mycelium is well established, it may outcompete the contaminant. However, once the patch turns green and begins sporulating, the bag must be removed from your grow space immediately. During fruiting, contamination is more common because the substrate is exposed to open air. Remove contaminated blocks and wipe down all surfaces with a 10% bleach solution or isopropyl alcohol.

Decontaminating Your Grow Space After an Outbreak

After a Trichoderma outbreak, simply removing the contaminated bags is not enough. Trichoderma spores can persist on surfaces, walls, shelving, and in HVAC systems for months. Wipe all hard surfaces with a 10% bleach solution. Consider running an ozone generator in the sealed room for several hours. Wash or replace any fabric materials (humidifier wicks, shelf liners). Wait at least 48 hours after decontamination before introducing new grows. Many experienced cultivators maintain a separate, sealed inoculation room specifically to prevent cross-contamination.

Prevention at Every Step

Prevention is always cheaper than losing a batch. Use properly sterilized grain and substrate every time — never cut corners on sterilization duration or pressure. Work in front of a laminar flow hood or inside a still air box for all transfers. Source spawn from reputable suppliers and inspect it for any off-colours before use. Maintain your grow space with HEPA filtration if possible. Keep temperatures below 24 degrees Celsius during colonization to slow Trichoderma growth while your mushroom mycelium establishes. And always, always wash your hands and arms with soap before entering your grow space.