How to Identify Contamination on Agar Plates

A beginner’s guide to spotting trouble before it spreads

Agar work is the cornerstone of clean, successful mushroom cultivation. But as rewarding as it is to watch mycelium colonise a plate, it’s equally frustrating when contamination creeps in. Whether you're cloning a fruit, testing a spore print, or expanding a clean culture-learning to identify contamination is absolutely essential.

In this post, we'll break down what to look for, what’s normal, and when to throw a plate in the bin (and not in your grow).

 

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🧫 First: What Does Healthy Mycelium Look Like?

Before we dive into the nasties, let’s get clear on what you do want to see:

• Bright white
• Fluffy, rhizomorphic, or tomentose (stringy or cloudy/cottony, both are fine)
• Radiating evenly outward in a circle from the inoculation point
• Slow but steady growth (faster ≠ better, especially if it's not white)

If it’s white and behaving, it’s probably cubensis. If it’s any other colour or racing across the plate, pause and investigate.

 

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🚨 Common Types of Contamination

Here’s a breakdown of the most common invaders you'll see on agar plates:

 

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1. Trichoderma (Green Mould)

🟢 Public Enemy #1

• Starts white and fast-growing - often mistaken for mycelium
• Turns bright or dull green when it sporulates
• Powdery or fuzzy texture
• Usually takes over a plate rapidly

What to do: Chuck it. Don’t open the plate indoors once it turns green-those spores will travel.

 

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2. Bacteria (Wet Spot / Slime)

🟡 Slick, sticky, and usually yellowish

• Appears shiny, slimy, or greasy
• Often pools around the inoculation point
• Smells sour or “off” when opened
• Mycelium may grow over it but will look weak and wispy

What to do: Bin the plate. Sometimes worth isolating from clean edges, but usually not worth the risk.

 

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3. Cobweb Mould (Dactylium)

⚪ Looks like grey spiderwebs

• Light grey and thin-almost invisible at first
• Grows faster than mycelium and spreads loosely
• Often mistaken for healthy fuzz, but too airy
• May have a faint earthy or musty smell

What to do: Bin it. Can be confused with fluffy mycelium-look for speed and colour.

 

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4. Yeast

🟣 Specks, dots, or cloudy zones

• Creamy, tan, pink, or even purple specks
• May appear in isolated colonies
• Often grows in blobs rather than spreading across the whole plate
• Can give off a sour beer-like smell

What to do: If it’s far from clean growth, you might transfer away. Otherwise, bin.

 

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5. Black Moulds (Aspergillus / Penicillium)

⚫ Rare but dangerous

• Dark green, black, or grey spores
• Very fast-growing, often with intense sporulation
• Spreads aggressively and smells musty
• Can be a health hazard

What to do: Immediately discard, ideally outside or into a sealed biohazard bag.

 

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🧐 How to Inspect Your Plates

Use these signs to tell good from bad: