How Do I Deal with Contamination in My Mushroom Grow?

Contamination is the most common problem every mushroom grower faces. It happens when competing organisms, primarily moulds, bacteria, or other fungi, establish themselves in your substrate before or alongside your chosen mushroom mycelium. Learning to identify contamination early, understand why it occurred, and prevent it from recurring is one of the most valuable skills you can develop.

How to Identify Contamination

Healthy mushroom mycelium is consistently white or off-white, with a fluffy or rope-like texture spreading evenly through the substrate. Any colour other than white is a warning sign. Green, black, orange, or pink patches are almost always contaminating moulds. Yellow or wet patches with a sour smell indicate bacterial contamination. A thin crust or ring of discolouration at inoculation points can sometimes be normal metabolite production, known as mycelium overlay or secretion, but if it spreads, discard the bag. Do not smell contaminated bags directly as mould spores are harmful to breathe in quantity.

The Most Common Causes

Most contamination in home grows falls into three causes. First, inadequate sterilisation or pasteurisation of the substrate allows surviving competitor spores to outcompete slow-colonising mycelium. Second, contamination during inoculation, introduced when working with spawn in an unclean environment or with dirty hands and tools. Third, substrate that is too wet, which creates anaerobic conditions bacteria thrive in. Water content between 60 and 65 percent, the correct range, feels moist when squeezed but releases only a few drops, not a stream.

What to Do with a Contaminated Bag

Isolate the contaminated bag immediately to prevent spores spreading to other blocks. Seal it in a plastic bag before removing it from your growing area. Do not attempt to cut away the contaminated section and continue growing; mould sends invisible threads through the entire substrate long before surface growth is visible. Discard contaminated substrate in an outdoor bin or compost away from your growing area. Sterilise any surface that contacted the bag with isopropyl alcohol. Then review your process to find the likely cause before starting the next batch.

Prevention Strategies That Work

Work in the cleanest environment possible when handling spawn, ideally next to a still air box made from a plastic tote. Wipe surfaces with isopropyl alcohol before starting. Wash hands thoroughly and wear gloves. Ensure substrate moisture is correct and never inoculate warm substrate, which promotes bacterial growth before the mycelium can establish. Increase your spawn rate, the amount of spawn relative to substrate, to help the mycelium colonise faster and outcompete any organisms that do get in.