All the Tek

Tek library / Culture

Agar

Petri plates of nutrient gel for cloning a mushroom and keeping a strain clean.

tissue wedge from inside the stem sealed, clean growth fresh transfer
Plate 2 of 16Agar, drawn for All the Tek.

What it is

Agar is a nutrient gel poured into petri plates. It is very rich and very easy to contaminate, so it is used in small amounts for storing a strain or tissue cloning, in lab-like conditions1.

UF/IFAS uses potato dextrose agar (PDA), sterilised in an autoclave or pressure cooker at 15 psi for about 30 minutes and cooled to about 50 °C (122 °F) before pouring2. Tissue from the inside of a fresh mushroom goes onto the plate with flame-sterilised tweezers, and the plate is sealed2. Clean plates are then used to inoculate sterilised grain2.

The point of agar is that you can see what is growing. Mushroom mycelium grows out as a clean white mat; anything else shows up as a different colour or texture, and you can cut away from it or throw the plate out.

What it suits

Every cultivated species. Growers use agar to store a strain or to clone one from a piece of fresh mushroom1.

Equipment

  • Pressure cooker that holds 15 psi
  • Petri dishes (glass or pre-sterilised plastic)
  • Agar powder and a nutrient such as potato dextrose or malt extract
  • Still air box or flow hood
  • Scalpel or fine tweezers, flame or alcohol lamp, gloves
  • Parafilm or tape, marker

Time to first harvest

Days on the plate, then 10–20 days on grain. UF/IFAS compares cultures at five days of growth on the plate. The grain spawn made from a clean plate then takes about 10 to 20 days to fully colonise2.

Contamination and other risks

  • Bacteria and unwanted fungi. UF/IFAS checks plates for any sign of them and discards contaminated plates to keep cultures clean2.
  • Boil-over and burns from opening the cooker while it still holds pressure2.
  • Gelatin as a cheap substitute softens or liquefies within weeks, so UF/IFAS does not recommend it for long-term storage2.

Steps, in overview

  1. Mix the medium. Combine agar and nutrient with water in a heat-safe bottle or jar.
  2. Sterilise. Run it in the pressure cooker at 15 psi for about 30 minutes2. Let the cooker depressurise on its own.
  3. Pour. When the medium has cooled to about 50 °C, pour plates inside the still air box or in front of the hood2.
  4. Inoculate. Place a small piece of clean tissue, or a wedge from an older plate, in the middle of each plate with a flamed tool2.
  5. Seal and watch. Seal, label and keep the plates somewhere clean. Throw out any plate with growth that is not your white mycelium.
  6. Move it on. Cut a wedge from the leading edge of a clean plate and drop it into sterilised grain2.

Common mistakes

  • Opening the pressure cooker before it has fully depressurised2.
  • Keeping contaminated plates in the same room as clean ones.
  • Taking tissue from the outside of a mushroom, where it has touched the air, instead of from the inside.
  • Skipping gloves and the still air box because one plate worked without them.

Safety

Pressure cookers: vent before pressurising, never force-cool, and wait about 10 minutes after the pressure reads zero before opening the lid away from your face3. Follow your cooker maker’s instructions2.

New tek guides by email

We add teks and update these pages as sources change. Get a short note when there is something new.

Sign up on the join page

Sources

  1. Methods of Commercial Mushroom Cultivation in the Northeastern United States. Willie Crosby (Fungi Ally) and Steve Gabriel, Cornell Small Farms Program, 2019 (Northeast SARE). Extension publication, checked 26 October 2026.
  2. Isolation of Mother Cultures and Preparation of Spawn for Oyster Mushroom Cultivation. Liao, Hsu, Hameed and Cotter, UF/IFAS Extension (EDIS SS663). Extension publication, checked 26 October 2026.
  3. Using Pressure Canners. National Center for Home Food Preservation (University of Georgia). Extension publication, checked 26 October 2026.