Pocket Botanist
Check these first
- Inspect the undersides of the leaves for orange or brown spots.
- Look for a white powdery film on the top of the leaves.
- Check if the plant is crowded by other nearby shrubs.
Likely causes
Rust fungus
Most likelyHow to tell: Small orange or reddish brown pustules appear on the undersides of leaves.
What to do: Remove and destroy the affected leaves immediately to stop the spread. Improve air circulation around the plant by thinning out crowded stems.
Powdery mildew
Most likelyHow to tell: A white, dusty coating covers the leaves, often looking like flour.
What to do: Water only at the base of the plant to keep the foliage dry. If the infection is severe, apply a neem oil spray in the early morning or late evening.
Find out what's wrong with your smooth blue aster
The list above covers the usual causes. Pocket Botanist looks at a photo of your actual plant and tells you the likely cause, how serious it is, and what to try first. Identification and a health check are free.
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When to worry
It is time to worry if the spots cover more than half of the foliage and the plant begins to drop leaves. Minor spotting is common in humid late summers.
Stopping it coming back
Provide plenty of space between plants to encourage airflow and always water at the soil level rather than overhead.
Smooth Blue Aster: spots? Check yours with a photo
Every plant is a little different. Take a photo of your smooth blue aster and Pocket Botanist tells you which cause is most likely, how serious it is, and what to try first. A few checks a day are free. Save the plant and you get care reminders timed for it.
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Or go to pocketbotanist.app/get on your phone
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Questions people ask
- Will these spots kill my aster?
- Usually no, but they can weaken the plant and make it look unsightly. Treating it early helps the plant stay strong for next year.
- Should I use a fungicide?
- Fungicides are rarely needed if you manage your spacing and watering. Try cultural changes like pruning first.
Guides to these causes
More problems with Smooth Blue Aster
Back to the Smooth Blue Aster guide
This article was generated with AI on from the plant’s care profile and is general information, not horticultural advice. Read about how we build these guides.