Grey mould (botrytis) and flower rot

A fungus that rots dense flowers from the inside in humid, still air. Cut it out well past the damage and drop the humidity straight away.

Symptoms

• A single leaf yellowing and wilting out of a flower is often the first clue.
• Brown, mushy or soft spots inside dense flowers.
• Grey, fuzzy mould that releases a cloud of spores when disturbed.
• A damp, musty smell.

Quick checks

• Check humidity at night as well as during the day.
• Gently open the densest flowers and look inside.
• Look for caterpillar droppings near the damage.
• Feel for condensation on the tent walls after lights-off.

How to fix it

1. 1
   Physical

   Cut it out carefully

   Cut out affected parts well beyond the visible damage, bag them inside the tent without shaking, and remove them. Clean your snips afterwards.

2. 2
   Environment

   Drop humidity and keep air moving

   Lower humidity to the chart's late-flower range (40–50%), increase extraction and keep air moving through the canopy, including at night. Avoid big temperature drops at lights-off.

3. 3
   Prevention

   Thin and tidy

   Remove dead leaves and debris, thin crowded areas and keep plants spaced.

4. 4
   Safe treatment

   No sprays on flowers

   RHS notes there are no fungicides approved for gardeners against grey mould, so rely on removal, hygiene and climate.

Likely causes

• Humidity too high for late flower, especially at night.
• Still air inside a dense canopy.
• Big temperature drops at lights-off, causing condensation.
• Damage from caterpillars or broken stems letting the fungus in.
• Dead leaves and debris left in the tent.

What to avoid

• Don't spray anything onto flowers, including foliar feeds like Monkey Mist.
• Don't shake infected material around the tent.
• Don't leave damaged material in place 'to see what happens'.
• Don't let humidity creep up overnight.

Prevention

• Follow the chart's falling humidity in flower (down to 40–45% by week 8).
• Good airflow and spacing.
• Keep moths out with intake mesh.
• Clear debris and dead leaves regularly.

Monkey products that can help

Stress

Silicon is known to strengthen cell walls and plays a role in the plant's defence responses; it can help, but won't save rotting flowers.

2 ml per 10 L every week. Always add it to the water first and mix thoroughly before adding anything else.

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Products support a healthy plant. They don't replace fixing the cause.

When to get help

If rot appears in several flowers or keeps coming back after you've corrected humidity, email info@monkeynutrients.co.uk with photos.

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