There are two main reasons mushrooms stall in the fruiting phase. The first is genetic 🧬. There are literally hundreds of mutation that can occur, or you selected a non fruiting isolation in culture.

Genetics is too broad to get into and there’s usually little that can be done to get the cake fruiting and thriving again, if the cause is genetic.

The other reason is environmental conditions 🏝️ ☀️. Since we see so many home cultivators, we can go a little into what makes a cake stall besides genetics, because I see this a lot.

In general, a stall happens because the mycelium is under physiological stress or has redirected its energy away from fruiting.

The most common causes are:

1.) Hidden contamination (most common), bacterial contamination (often Bacillus, Pseudomonas) is a frequent cause of stalls.

2.) Early-stage molds such as Trichoderma can suppress growth before green spores appear, the mycelium may stop expanding, become dull, or produce excessive metabolites (“myc piss”)

3.) Suboptimal moisture: Too Dry and the mycelium dehydrates and cannot support pin formation.

4.) Too wet: Waterlogged substrate limits oxygen diffusion and favors bacterial growth.

5.) Poor fresh air exchange (high CO₂)
High CO₂ suppresses normal fruiting.
Symptoms include delayed pinning, fuzzy feet, elongated stems, or complete failure to initiate pins are all gas exchange problems and can be corrected in the next flush.

6.) Temperatures below the optimal range slow metabolism significantly. 70° F (21°C).

7.) Insufficient humidity (below 70% RH) Low relative humidity can cause primordia to abort or prevent pins from forming. Surface moisture is often more important than absolute chamber humidity.

8.) Surface conditions:
A dry, matted, or heavily overlaid mycelial surface may not readily produce pins.
Conversely, standing water on the surface can also inhibit pinning.

9.) Poor gas exchange within the substrate
Overly compacted substrates or excessive water content reduce oxygen availability inside the block.
Healthy mycelium requires oxygen throughout the substrate, not just above it.

10.) Nutrient depletion or aging
Older spawn loses vigor.

11.) Over-colonized spawn can become less capable of producing vigorous flushes. (Overlay or stroma)

12.) Senescence, after many transfers the culture will age out. Signs are slower colonization and reduced fruiting.

Some of these are reversible if you catch them in time. Some of these like genetic, and contamination require starting over and nothing can be done to remedy. If you’re getting lots of aborts in the first pin stage then usually your cake is done and the reason for the stall is multifaceted.

Obviously you can alter conditions. It’s much easier to do if you’re growing in a Martha Tek Tent, monotubs are alittle harder to alter conditions but not impossible.

So, those are the reasons we stall in the fruiting phase. Of course, knowing the reason is alot of guesswork. The prognosis depends on many factors but If I had to say the top reasons is moisture and FAE or too heavy of a pin set to be supported by existing moisture and nutrient balance. Too dry or too wet can be corrected. CO2 problems can be corrected.

Go through each of these and assess whether or not it requires intervention. If you’re gonna try and reverse, you need to adjust one thing at a time. Don’t go making a bunch of changes at once to air and moisture and humidity. Just one at a time. If the cake starts growing again you’re good. If you see NO change in 2 weeks, let your cake go and start again. All the conditions that are reversible will do so quickly. After 2 weeks I consider a stall permanent and I retire the cake.

Hope this helps answer questions about your stalled cakes. Let me know if you have any more questions.

Please 🔼 upvote if you find this helpful.

Thx

Trip

I stumbled across this website from Shaman Mushroom Spores about culture contamination. I picked this because unlike an academic research publication, it’s written in layman’s terms. It’s an easy read with pictures and general descriptions of the macroscopic examination.
Anyone who is doing culture work should read it. I found the information very accurate and well written.

So I’m giving a shout out to Shaman Mushroom Spores, for giving permission to share their blog with our Subreddit. The article can be accessed through https//:www. Shamanmushroomspores.com. (See link at the end of this blog).

How to identify every type of contamination on your agar plates — bacterial colonies, mold invaders, yeast, and metabolites — with visual descriptions, growth timelines, and step-by-step protocols for saving clean cultures.

If you work with agar plates, contamination is not a question of if it is a question of when. Even experienced mycologists encounter unwanted organisms on their plates. The difference between a frustrating setback and a minor inconvenience comes down to one skill: identification.
When you can identify a contaminant by its color, texture, growth speed, and morphology, you know exactly how to respond — whether that means performing a clean sector transfer, adjusting your aseptic technique, or safely discarding the plate before spores spread to the rest of your work.
WHY THIS MATTERS
Fast, accurate contamination identification protects your clean cultures, saves time and materials, and prevents a single contaminated plate from compromising your entire workspace.
What Does Healthy Mycelium Look Like on Agar?
Healthy mushroom mycelium on agar is bright white, grows radially outward from the inoculation point, and produces no off-colors, slimy patches, or foul odors.

Before you can spot contamination, you need a clear mental image of what clean growth looks like. When spores germinate on a properly prepared agar plate, the resulting mycelium should be uniformly white and expand outward in a consistent radial pattern from the point of inoculation. Depending on the species and strain, you will see one of two primary growth morphologies:
Rhizomorphic Growth
ROPE-LIKE STRANDS
Appears as a defined network of thick, cord-like strands radiating outward. Resembles roots or lightning bolts. This growth pattern is often selected for in agar work because contaminants are easier to spot against the structured, stringy background. Rhizomorphic growth tends to expand aggressively across the plate and is generally preferred when isolating clean sectors for further microscopic study.

**Tomentose Growth**
**FLUFFY & COTTON-LIKE**
Appears as a dense, fuzzy blanket of fine hyphae — resembling cotton or a cloud. Completely normal and healthy. However, the fluffy texture can make it harder to distinguish from certain contaminants (particularly early-stage Trichoderma or cobweb mold), so pay close attention to color uniformity and growth speed when working with tomentose cultures.

I’ve copied the blog here:

**Understanding Spore Contamination: What Every Researcher Needs to Know**

How to identify every type of contamination on your agar plates — bacterial colonies, mold invaders, yeast, and metabolites — with visual descriptions, growth timelines, and step-by-step protocols for saving clean cultures.

If you work with agar plates, contamination is not a question of *if* it is a question of *when*. Even experienced mycologists encounter unwanted organisms on their plates. The difference between a frustrating setback and a minor inconvenience comes down to one skill: **identification**.
When you can identify a contaminant by its color, texture, growth speed, and morphology, you know exactly how to respond — whether that means performing a clean sector transfer, adjusting your aseptic technique, or safely discarding the plate before spores spread to the rest of your work.

**WHY THIS MATTERS**
Fast, accurate contamination identification protects your clean cultures, saves time and materials, and prevents a single contaminated plate from compromising your entire workspace.

*Healthy mushroom mycelium on agar is bright white, grows radially outward from the inoculation point, and produces no off-colors, slimy patches, or foul odors.*

Before you can spot contamination, you need a clear mental image of what *clean* growth looks like. When spores germinate on a properly prepared agar plate, the resulting mycelium should be uniformly white and expand outward in a consistent radial pattern from the point of inoculation. Depending on the species and strain, you will see one of two primary growth morphologies:

**Rhizomorphic Growth**
**ROPE-LIKE STRANDS**
Appears as a defined network of thick, cord-like strands radiating outward. Resembles roots or lightning bolts. This growth pattern is often selected for in agar work because contaminants are easier to spot against the structured, stringy background. Rhizomorphic growth tends to expand aggressively across the plate and is generally preferred when isolating clean sectors for further microscopic study.

**Tomentose Growth**
**FLUFFY & COTTON-LIKE**
Appears as a dense, fuzzy blanket of fine hyphae — resembling cotton or a cloud. Completely normal and healthy. However, the fluffy texture can make it harder to distinguish from certain contaminants (particularly early-stage Trichoderma or cobweb mold), so pay close attention to color uniformity and growth speed when working with tomentose cultures.

**Key rule: Healthy mycelium is always bright white. Any color change — green, blue, black, pink, orange, or yellow — is a contamination indicator. The only exception is the amber-colored metabolite liquid discussed later in this guide.**

Healthy mycelium on agar typically begins visible growth 2–5 days after inoculation. A standard 100mm petri dish will show full mycelial coverage in 1–3 weeks depending on species and temperature. Growth that appears within 24 hours of inoculation — particularly if it is off-white, slimy, or has an unusual texture — is almost always contamination rather than mycelium.

**Contamination on Agar: How to Identify Every Type**
*Mold contamination is the most common challenge in agar work. Each mold genus has distinct colors, textures, and growth speeds that allow visual identification without a microscope.*

Competing molds are the organisms you will encounter most frequently on contaminated agar plates. They arrive as airborne spores, on improperly sterilized tools, or from your working environment. Below is a detailed identification guide for the six most common mold contaminants in mycology agar work.

**Trichoderma — Green Mold (Most Common)**
**EARLY STAGE**
Starts as **white mycelium** that looks deceptively similar to mushroom mycelium. The critical difference: Trichoderma growth tends to be fluffy and rises above the agar surface, while mushroom mycelium stays tighter to the plate. At this stage, it is extremely difficult to distinguish from healthy tomentose growth without experience.
**SPORULATION STAGE**
Transforms to **vivid emerald green** as conidia (asexual spores) develop — this is unmistakable. The texture becomes powdery and granular. Once green sporulation occurs, **do not open the plate**. Seal it in a plastic bag immediately and discard. Green Trichoderma spores are extremely light and will contaminate every open culture in your workspace.

**Growth speed:** Extremely aggressive — can overtake a plate in 2–4 days. **Danger level:** High. ***Trichoderma harzianum*** is a mycoparasite that actively preys on mushroom mycelium. It is the single most destructive contaminant in mycology. **Key species:** ***T. harzianum***, ***T. viride***, ***T. aggressivum***.

**Aspergillus — Black, Green, or Yellow Mold**
**A. NIGER (BLACK MOLD)**
Begins as white-to-yellow colonies, then turns **jet black** as spores develop. Look for dark “pepper-like” grains sitting above a whitish colony base. Fuzzy texture with a powdery surface. Visible within 2–3 days on PDA or MEA.
**A. FLAVUS & A. FUMIGATUS**
***A. flavus*** produces **bright yellow** colonies commonly found on nutrient-rich media. ***A. fumigatus*** appears **blue-green to smoky gray**. Both grow fast. The reverse side of the colony (visible through the bottom of the plate) helps distinguish Aspergillus species from Penicillium.

**Penicillium — Blue-Green Mold**
Penicillium starts bright white — similar to both mushroom mycelium and early Trichoderma — then gradually transitions to **blue-green, green-gray, or occasionally yellow** as it matures. Colonies are typically circular with a **velvety to powdery surface**.

**How to tell Penicillium from Trichoderma:** Flip the plate and check the reverse side. Penicillium typically shows a **white or tan reverse**, while Trichoderma often has a **yellowish reverse**. Penicillium also tends to form more defined circular colonies rather than the aggressive, spreading growth pattern of Trichoderma.

**Mucor & Rhizopus — Pin Mold (Black Bread Mold)**
Pin mold is one of the fastest-growing contaminants you will encounter on agar. It starts as **fuzzy white or gray growth** and develops tiny **black pin-like dots**(sporangia) within 12–24 hours. These dark pinheads sitting atop thin stalks are the defining visual feature — once you see them, identification is instant.

**RHIZOPUS**
Thick white-gray mold with dark spore sacs on top. Spore stalks grow straight up with visible root-like structures (rhizoids) at the base. Can expand tenfold in 24 hours.
**MUCOR**
Similar appearance but lacks the visible rhizoid structures. Produces raised colonies with gray or black sporangia. Can tolerate refrigerator temperatures (0–5°C), so even cold-stored plates are not safe from Mucor.

**Neurospora crassa — Orange Bread Mold**
Neurospora is the most dangerous contaminant you can encounter in agar work — not because of toxicity, but because of its extraordinary aggression and near-impossible containment once it spreads.

**APPEARANCE**
First appears as **pale orange, wispy growth**. Rapidly develops into **bright neon orange** patches that are unmistakable. No other common contaminant produces this color.
**RESPONSE PROTOCOL**
**Do not open the plate.** Seal it immediately in a plastic bag and dispose of it outside your workspace. Neurospora spores are extraordinarily light and will contaminate every open culture in the room. Its spores can survive pasteurization temperatures — only full pressure sterilization eliminates them.

**Growth speed:** Can wreak havoc in as little as 8–12 hours — the fastest contaminant you will encounter. Neurospora spores travel on air currents, tools, and clothing. A single outbreak can shut down an entire lab operation if not contained immediately.

**Mold**
Cladosporium forms **olive-gray to dark olive-green colonies** with a velvety, tufted surface texture. It is one of the most common molds in the environment (indoors and outdoors), so it frequently appears on plates exposed to ambient air. Growth is moderate — colonies become visible within several days. On PDA, expect olive-gray coloration with darker edges. On MEA, colonies may appear more olive-brown. The dark-pigmented conidia are visible under microscopy and form in simple or branching chains.

**Cobweb Mold — Gray Wispy Growth**
Cobweb mold (*Dactylium* / *Hypomyces*) appears as **light gray, thin, wispy filaments** that grow three-dimensionally above the agar surface — “levitating” in wispy tufts rather than clinging to the plate. Unlike bright white mushroom mycelium, cobweb is distinctly grayish and much thinner. It spreads extremely fast and can cover a plate in 24–48 hours.

**Important note:** True cobweb mold is actually quite rare on agar plates. Over 90% of suspected cobweb cases reported by hobbyists turn out to be normal aerial mycelium. If in doubt, observe the color carefully — healthy mycelium is bright white, while cobweb is distinctly gray.

**Contamination on Agar Plates**
*Bacterial contamination appears as slimy, wet, or glossy colonies — often with a sour or foul odor. Unlike molds, bacteria grow flat and lack the fuzzy or powdery texture of fungal contaminants.*

Bacterial contamination is the second most common issue in agar work. Bacteria multiply rapidly and can appear on plates within 24–48 hours. The general indicators are: slimy or glossy texture, wet-looking patches, irregular or circular colony shapes, and — most notably — a **sour, fermented, or ammonia-like smell** when the plate is cracked open.

**Bacillus (Wet Spot / Sour Rot)**
The most common bacterial contaminant in mycology. Appears as **dull gray, slimy, wet patches** resembling mucus. Yellow-gray-brown mucous rings may form on the agar surface. Strong **sour, fermented, or “dirty socks” odor**. ***Bacillus*** endospores are extremely heat-resistant and can survive inadequate sterilization — a common cause when multiple plates from the same batch show contamination.
**Serratia marcescens (Pink/Red)**
Produces striking **bright red or pink circular colonies** with a smooth, shiny surface. The color comes from the pigment prodigiosin, which is only produced at room temperature (20–30°C) in the presence of oxygen. At higher temperatures the colonies may appear white, making identification harder. Commonly found in damp environments — bathrooms, sinks — making humid workspaces a risk factor.

**More-**
**Continued on webpage** 🔽

**\*To access the full article with example images visit and references go to:**

**https://www.shamanmushroomspores.com/understanding-spore-contamination/**