Agar Petri Dish Preparation

The starting point for mushroom mycelium is the spore. As spores germinate into hyphae, they’re at their weakest and most vulnerable stage. Any contamination they encounter in our artificial environment at this point is likely fatal to them. Therefore, when preparing our Petri dishes we must work under the most sterile conditions we’re able to achieve.

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There seems to be a mindset among amateur mushroom growers that using agar is hard. That’s a shame. It’s not hard. It’s a lot of work, and it involves smart work. If you’re lazy or careless, you’ll certainly fail. Careful attention to sterile procedure is a must. You’ll want to at least start out using a glovebox, as you saw demonstrated in Part 2 of the BRF Tek.

The secret to how a glovebox works is still air. Many growers waste endless time and energy trying to Lysol and bleach the inside of their gloveboxes, when a simple washing with soap and water would suffice.

The reality is that wind currents are what drives contaminants into our sterile media. If we can stop the air from moving, we can safely open a Petri dish for a short period of time with reduced chances of contaminant spores entering. Of course, no sterile procedure is going to be 100% successful, so it’s important to inspect your dishes daily so you can immediately transfer your healthy mycelium away from any contaminants that may appear.

If you’re going to transfer mushroom mycelium away from contaminants on a Petri dish, open the dish very slowly so as not to disturb the air and stir up contaminant spores. Always transfer a tiny piece of mushroom mycelium and move it away from the contamination to a new dish. The smaller the piece of mycelium you cut, the fewer contaminant spores are going to be on it, and therefore the greater your chance of success. Simply continue making transfers as necessary until you have clean, pure mushroom mycelium. Never try to transfer contamination away from your mushroom mycelium. You’ll only stir up and release more contaminant spores into the dish, ruining your culture.

This chapter of Let's Grow Mushrooms demonstrates sterile procedure and one method for preparing the agar and pouring Petri dishes. The work is demonstrated in front of a laminar flow hood to make filming easier. If you have $500 to spare, a flowhood is a great tool. If not, you can use a glovebox and still have success, although it’s a bit more work.

Try to never have a Petri dish open for more than a few seconds. The longer the dish is open, the greater the exposure to contamination. Remember, just about anything will grow on agar. If you want your mushroom mycelium to remain uncontaminated, you’ll have to keep everything else away. Read and study everything you can find about sterile procedure. Once understood, your success is guaranteed.

Enjoy the clip.

Marc R Keith