What is agar?

Mushroom growers use agar to clone a strain, isolate or test a sample of colonized grain, cut healthy mycelium away from contamination, and store mycelium. Because agar is colorless, contamination is easy to spot and cut out (GroCycle).

At home, agar is often used to test liquid culture: a drop on a plate at 68–77°F (20–25°C) shows growth in 4–10 days, and clean growth can go on to inoculate grain spawn (Mycoboutique).

Make agar plates by sterilizing a malt-and-agar mix, then pouring it into sterile dishes in clean air. GroCycle's light malt extract agar (GroCycle):

1. Mix about 500 mL of water, 10 g of agar powder and 10 g of light malt extract.
2. Sterilize for 30 minutes at 15 PSI, following your cooker's instructions and timing from when it reaches pressure.
3. Cool. Don't open the cooker until the pressure is gone, or the medium may boil over (UF/IFAS). Let the mix cool to around 115–125°F (46–52°C), still liquid.
4. Pour in front of a laminar flow hood if you have one; a still air box "will also help reduce contamination".
5. Store unused plates in the fridge at 35–40°F (2–4°C) and use them within two to four weeks.

The Potter Valley Tribe's mushroom lab guide, hosted by SARE, waits 1 to 3 weeks for a tissue clone to grow out on a plate (SARE).

Transfer from a plate by cutting a small piece from clean, typical growth and moving it to fresh agar or grain. Heat the scalpel in a flame and let it cool before cutting (UF/IFAS), and never use an open flame inside a still air box (North Spore). Penn State's culture guide, written for button mushrooms, keeps the piece to 3 × 3 mm or less and doesn't take it from the original piece at the center. It checks new transfers weekly for "uniformity of growth, sectoring, contamination, or abnormal appearance" (Penn State).

If a plate shows contamination, cut a piece of healthy mycelium away from it onto a fresh plate (GroCycle). Our contamination ID guide shows what to look for.

Agar cultures keep for months in the fridge, but sources differ on how many. GroCycle says a fully colonized plate, sealed in a bag in the fridge, stays viable for up to 12 months (GroCycle). North Spore rates plate cultures at up to 6 months, with a viability check after 4, and slants at up to a year. It says pink oyster cultures can't be refrigerated: keep them at room temperature and use them within a month (North Spore help center).

• Transferring from a sector. Penn State defines sectoring as growth that differs in appearance, growth rate and color from the strain's typical growth, and says to avoid propagating it (Penn State).
• Subculturing too many times. A 2020 study of oyster mushrooms says "successive subcultures usually lead to strain degeneration", and notes that degenerated mycelium looks the same as healthy mycelium (Molecules, via PMC). Keep a clean mother plate and take fresh working cultures from it, as our grow log guide explains.
• Losing count of transfers. In Hypha, create each plate and jar as a transfer from its parent, and you can trace every culture back to its source.