What is rhizoctonia and how to fix it
Rhizoctonia is a soil-borne fungal disease that attacks seedlings at the base, often causing them to collapse suddenly. It’s one of the main causes behind what people call “damping off.”
It works fast — but it’s also preventable.
Quick answer: what is rhizoctonia
Rhizoctonia is a fungal disease that damages stems and roots at soil level, causing seedlings to collapse. Control it by improving airflow, reducing moisture, and using clean compost and containers.
5 step guide to fixing rhizoctonia
Step 1: Remove affected seedlings
- Pull out any collapsed or infected plants immediately
- Dispose of them — don’t compost
- This limits spread
Step 2: Reduce moisture
- Let the surface dry slightly between watering
- Avoid constantly wet soil
- Water from below if possible
Step 3: Improve airflow
- Remove lids or ventilate propagators daily
- Space seedlings out
- Stagnant air encourages fungal growth
Step 4: Switch to clean materials
- Use fresh, sterile compost
- Clean trays and tools before reuse
- Avoid reusing infected soil
Step 5: Stabilise conditions
- Keep temperatures steady
- Avoid sudden changes
- Maintain balanced light and moisture
Best varieties & why (less prone seedlings)
- Zinnia
- Fast-growing and less sensitive
- Marigold
- More tolerant of variable conditions
- Nasturtium
- Beans and peas
- Larger seeds, stronger early growth
Very fine or slow-growing seedlings are more vulnerable.
Common issues, pests & diseases
Seedlings collapsing at soil level
- Classic rhizoctonia symptom
- Stems look pinched or rotted
Patchy losses in trays
- Disease spreading through compost
White or grey fungal growth
Recurring seedling failure
Best fertiliser, soil, lighting & growing conditions
- Light
- Bright light helps strengthen seedlings
- Soil
- Free-draining, sterile seed compost
- Water
- Light, controlled watering
- Avoid waterlogging
- Feeding
Strong, well-grown seedlings are less vulnerable.
Time of year
- Most common in early spring
- When temperatures are warm but light is still limited
- Indoor sowing increases risk if conditions aren’t balanced
Climate and regional notes
UK & Ireland
- High risk due to damp conditions
- Airflow and drainage are critical
Europe
- Northern regions similar to UK
- Southern regions less prone if well ventilated
USA
- Indoor setups often the main risk
- Overwatering is the biggest trigger
Pro tips for prevention
- Don’t overwater — this is the biggest factor
- Ventilate propagators daily
- Sow seeds thinly
- Use fresh compost every time
- Act fast at the first sign of trouble
10 frequently asked questions about rhizoctonia
What is rhizoctonia
A fungal disease that causes seedlings to rot at soil level.
Is it the same as damping off
It’s one of the main causes of damping off.
Can infected seedlings recover
No — remove them immediately.
Why are my seedlings collapsing
Usually due to excess moisture and poor airflow.
Can I reuse soil after infection
Not recommended — use fresh compost.
How do I prevent rhizoctonia
Control moisture, improve airflow, and maintain hygiene.
Does light help prevent it
Yes, stronger seedlings are more resistant.
Can it spread to other trays
Yes, especially in humid conditions.
Is it common
Yes, especially in indoor seed starting.
What’s the best defence
Good setup — clean compost, airflow, and careful watering.
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