Common Contamination in Mushroom Cultivation

Sterile technique, or ‘tek’ refers to the process of maintaining a clean environment during the full length of the cultivation process. Use sterilized equipment, grow mushroom spawn only on sterilized substrates, and maintain a clean and controlled environment at all times.

Fungi reproduce through spores, which are microscopic cells released from the mushroom and most often dispersed through the air. They can remain viable for long periods of time, in some cases indefinitely. The best way to reduce undesired spores and other airborne contamination in your workspace is through the use of a laminar flow hood, which is a workbench that includes a filter pad, fan and a HEPA (High Efficiency Particulates Air) filter built into it. The system is costly but worth the investment for medium to large-scale culture plate work and liquid culture inoculations. If a flow hood is not in your budget, the next best option is a still air box like our NocBox Still Air Box which works well for small scale projects and it is a popular choice for home cultivators.

Mushroom substrates are the perfect breeding ground for unwanted microbes. If not properly treated through pasteurization or sterilization, contamination will lead to wasted crops. Pasteurization simply reduces microscopic competition in a substrate while sterilization eliminates it entirely. That's why purchasing or making your own sterile substrate is imperative to your operation. Different species prefer different treatments, but some options include pasteurization using our tek for growing mushrooms in buckets and containers, and sterilization using a pressure sterilizer such as the All American.

As the vegetative part of fungi, mycelium consists of a network of thin, branching filaments called ‘hyphae.’ In nature, mycelium is unseen as it exists below ground or inside decaying wood. In home cultivation, it's the white-ish growth we see on substrate blocks, plates, and jars. Mycelium can appear transparent or opaque, bumpy, fluffy, crystalline, webby, hairy, wispy, filamentous or clumpy and will often excrete an odorless, yellow liquid called exudate. Also called mushroom urea or mushroom pee, its presence is a natural part of growth and metabolism but can at times mean the mycelium is under attack or working hard to thwart a rival for its food supply. Spend time getting to know what your mycelium is supposed to look like so you can better identify unhealthy or contaminated patches.

While mycelium is typically white and filamentous, contamination generally takes the form of green, blue, gray, or black patches or discolorations in your substrate. Keep in mind that blue stains on certain kinds of mycelium may just be natural bruising and not contamination, especially if the substrate is pressing the mycelium against glass or plastic. One simple trick for determining whether your discoloration is contamination or not is to gently wipe a Q-tip over it, as contamination will transfer to the swab while bruising will not. Aside from discoloration, contamination can take the form of slimy patches on your substrate and can be the result of excess moisture which can foster unwanted bacterial growth.

Trichoderma is a group of green mold fungi that is present in all soils, with several species having the ability to form symbiotic relationships with plants and plant roots. One species in particular, Trichoderma harzianum is one of the most common forms of contamination seen in mushroom cultivation. This mold produces white mycelium that will rapidly cover substrates before producing emerald-green spore-bearing structures. In its mycelial stage, it can be difficult to detect as it appears similar to mushroom mycelium. Unlike mushroom mycelium, which appears more rope-like and remains tight on the substrate, Trichoderma has a fluffy appearance that rises from the substrate.

Orange bread mold, red bread mold, or pink mold are alternative names for Neurospora crassa, another common and extremely fast-growing fungal contaminant. While used for decades in genomics research, this neon orange mold can wreak havoc on your operation in as little as 8-12 hours. It first appears as an orange-white wisp but quickly develops into a bright orange powdery patch. If allowed to grow, these patches produce round lumpy formations, but it is best to catch the fungus prior to this stage. Once found, contain the source by sealing it in a plastic bag and disposing of it immediately, being careful not to disturb the mold as it will release a fine cloud of orange spores into the air which can quickly spread. After disposal, sanitize the affected area and inspect your workspace for the source, which could include discarded coffee grounds, compost, or damp, untreated wood.

The most common form of bacterial contamination in mushroom cultivation is known as “wet spot” or “sour rot”. It is characterized by a dull gray slime with a sour-smelling odor that appears excessively wet or mucus-like and often forms in uncolonized patches along the bottom of grain jars. Their reproductive structures are called endospores, which are heat resistant, meaning they will survive the sterilization process. To prevent it, grains can be soaked for 12 to 24 hours at room temperature prior to sterilization, allowing the endospores within the grain to germinate and be eliminated during sterilization. If the contamination is encountered, it is possible to separate it from the rest of the myceliated grains (so long as the patch is small and confined to one area) by physically scooping out the patch using an alcohol sanitized or flame sterilized spoon. This can also take place in a monotub with an isolated patch of contamination where mushroom exudate has turned the affected area yellow, indicating an attempt by the mushroom to fight off the rival bacteria.

Cobweb mold (Hypomyces rosellus) is another species of harmful fungi that can be encountered in mushroom cultivation, especially in environments with still air, very little oxygen, and high humidity such as grain jars and monotubs. It can be difficult to detect due to its gray, white, and fluffy appearance which resembles mycelium but its growth pattern is more three-dimensional, appearing to hover above substrate in wispy, white tufts. Additionally, this mold often forms in the last days of incubation, when substrates are fully colonized just prior to fruiting. Cobweb mold spreads fast: a small patch the size of a penny can grow to cover an entire jar or monotub in 24 to 48 hours. All fungi in the Hypomyces group live as parasites on other fungi, so if left untreated the infection can result in pinning mushrooms aborting their growth and mature mushrooms to become rotten.

Also known as pinhead mold, black bread mold (Rhizopus stolonifer) is a mushroom contaminant that is common in soil and air and is one of the first molds to appear on stale bread. This fast growing fungi has a dense mycelium that is white at first and becomes gray then takes on a black appearance. Similar to cobweb mold, black bread mold is wispy but develops tiny black dots at the ends of its mycelium where its spores are produced. Animal pests such as fungus gnats and fruit flies are the carriers of this mold’s spores. Hydrogen peroxide spray can help contain the early spread of most molds, including this one, but once the mold advances to the stage that it is easily identifiable, it may be too late for the grow. At this point, the best way to deal with it is to abandon it and start over.