Pocket Botanist
Check these first
- Inspect the underside of the leaves for orange dust.
- Check if the spots have a yellow halo around them.
- Look for signs of high humidity or poor airflow.
Likely causes
Fungal Rust
Most likelyHow to tell: You will see small orange or reddish brown bumps on the underside of the leaves. These bumps often release a fine, rusty dust when touched.
What to do: Remove and destroy the most heavily infected leaves to stop the spread. Improve air circulation around the plant by thinning out dense growth and avoid overhead watering.
Leaf Spot Disease
PossibleHow to tell: These appear as dark, irregular patches that may have a yellow halo around them. They usually develop during periods of high humidity or frequent rain.
What to do: Keep the area around the base of the plant clear of old debris. Apply a copper based fungicide if the infection continues to spread across the plant.
Find out what's wrong with your johnsongrass
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
Worry if the spots cover more than half of the leaf surface or if the entire plant begins to yellow. It is normal to see a few tiny spots on older leaves as the season progresses.
Stopping it coming back
Space your plants out to ensure good airflow and always water at the base to keep the foliage dry.
Johnsongrass: spots? Check yours with a photo
Every plant is a little different. Take a photo of your johnsongrass 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.
Scan with your phone's camera to get the app
Or go to pocketbotanist.app/get on your phone
Get the app · Or check it from a photo now, free
Questions people ask
- Will these spots kill my plant?
- Usually not, but they can weaken it significantly. If left untreated, the plant may lose vigor and produce fewer seeds.
- Can I use a fungicide?
- Yes, a copper based fungicide is effective for most fungal issues. Follow the label instructions carefully for the best results.
Guides to these causes
More problems with Johnsongrass
Back to the Johnsongrass 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.