Mushrooms
Senior 2 · Agriculture
Mushrooms
Lesson 2 of 3
20 min
Let's begin
Most failures in mushroom growing happen before a single mushroom appears. If the growing material is too wet, too dry, poorly heated or handled with dirty hands, green or black moulds take over the bag and the spawn never has a chance.
Oyster mushrooms in Rwanda are usually grown in tubes: clear polythene bags packed with a treated growing material and mixed with spawn. Preparing good tubes is the single most important skill for a grower.
Oyster mushrooms can break down cellulose and lignin, so many farm residues work: maize cobs and stalks, wheat, rice or sorghum straw, bean straw, sugar-cane bagasse, coffee husks and pulp from washing stations, and sawdust. The material must be fresh, dry and free of mould when collected. A little bran (wheat or rice) may be added as a supplement, and a small amount of agricultural lime helps keep the pH suitable for the mushroom and unsuitable for many competitors.
| Factor | What goes wrong |
|---|---|
| Moisture content | Too wet: bacteria and rot, water collects at the bottom. Too dry: the mycelium grows slowly or stops. |
| Pasteurisation temperature and time | Too little: moulds survive. Too much or too long: the substrate "cooks" and loses nutrients, and useful microbes die. |
| Hygiene | Dirty hands, tools or rooms bring in green mould spores. |
| Spawn quality and rate | Old or contaminated spawn (green, black or pink spots, sour smell) fails; too little spawn lets competitors win. |
| Temperature at spawning | Substrate above about 30 °C kills the spawn. |
| Substrate quality | Mouldy or rotten material carries contamination from the start. |
Question: Ishimwe soaks 10 kg of dry maize cobs (assume they hold almost no water). After draining they weigh 25 kg. (a) What is the moisture content? (b) How much spawn does he need at 5% of the wet mass? (c) If each tube holds 2.5 kg of wet substrate, how many tubes will he fill and how much spawn goes into each?
(a) Water absorbed = 25 − 10 = 15 kg. Moisture content = water ÷ total mass × 100 = 15 ÷ 25 × 100 = 60%. This is within the ideal range.
(b) Spawn = 5% × 25 kg = 0.05 × 25 = 1.25 kg.
(c) Tubes = 25 ÷ 2.5 = 10 tubes. Spawn per tube = 1,250 g ÷ 10 = 125 g.
Tip
Memory hook: "Chop, Soak, Drain, Heat, Cool, Spawn, Bag."
Exam technique: moisture content is always water ÷ total wet mass, not water ÷ dry mass. Write the formula before substituting.
Distinguish between pasteurisation and sterilisation of a mushroom substrate.
Solution: Pasteurisation heats to about 60–80 °C, killing most competitors but leaving some harmless microbes; it is used for straw and cobs. Sterilisation heats to about 121 °C under pressure to kill almost all organisms; it is needed for sawdust and grain.
Why must the substrate be cooled before spawning?
Solution: Spawn is living mycelium. Substrate above about 30 °C would kill it.
A handful of squeezed substrate releases a steady stream of water. What is wrong and what should the grower do?
Solution: It is too wet (above about 65% moisture). Drain it longer, or spread it out to lose some water, before pasteurising and bagging.
How much spawn is needed for 50 kg of wet substrate at a rate of 4%?
Solution: 0.04 × 50 = 2 kg of spawn.
Give three hygiene measures during spawning.
Solution: Any three: wash hands; wipe tables and tools with 70% alcohol; work in a clean, closed room with no draughts; use clean new bags; keep the spawn bag closed until needed.
Summary
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
