Pocket Botanist
Check these first
- Inspect the undersides of the leaves for orange pustules.
- Check for dark spots with yellow halos.
- Remove any dead or dying foliage from the ground.
Likely causes
Rust fungus
Most likelyHow to tell: Small orange or reddish brown pustules appear on the undersides of the leaves. The leaves may eventually yellow and drop off.
What to do: Remove and destroy any heavily infected leaves to stop the spread. Improve air circulation around the plant by thinning out nearby vegetation.
Leaf spot disease
PossibleHow to tell: Dark brown or black spots with yellow halos appear on the leaves. This is common when the foliage stays wet for too long.
What to do: Water at the base of the plant rather than overhead to keep the leaves dry. Remove fallen debris from around the plant to reduce fungal spores.
When to worry
Small amounts of spotting are manageable, but if the entire plant is covered, it will struggle to photosynthesize. Remove infected parts promptly to save the plant.
Stopping it coming back
Water at the soil level and ensure the plant has plenty of space for air to move through its leaves.
Not sure which one it is? Photograph your marsh thistle.
This page lists the usual causes. Pocket Botanist looks at your actual leaf and tells you the likely cause, how serious it is, and what to try first. Identification and a health check are free.
Questions people ask
- Is rust fungus contagious to other plants?
- Yes, rust spores can spread to other susceptible plants in your garden. Keep infected plants isolated if possible.
- Can I use a fungicide?
- You can use a copper based fungicide if the infection is severe. Always follow the label instructions carefully.
More problems with Marsh thistle
Back to the Marsh thistle guide
This article was generated with AI from the plant’s care profile and is general information, not horticultural advice. Read about how we build these guides.