Pocket Botanist
Check these first
- Rub a spot with your finger to see if it leaves a powdery residue.
- Check if the spots are appearing mostly after a rainy spell.
- Look at the density of your plants to see if they are too crowded.
Likely causes
Rust fungus
Most likelyHow to tell: You will see reddish brown or orange pustules on the leaf surfaces. These spots often rub off as a fine powder on your fingers.
What to do: Increase air circulation around the plants by thinning them out if they are crowded. Avoid overhead watering in the evening to keep the foliage dry overnight.
Leaf spot disease
PossibleHow to tell: Dark brown or black spots appear on the leaves, sometimes with a yellow halo. These spots may grow larger over time.
What to do: Remove and dispose of the most heavily infected leaves to stop the spread. Keep the area clean of debris where fungi can overwinter.
Find out what's wrong with your large crabgrass
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
Small, isolated spots are rarely fatal, but widespread coverage can weaken the plant. Treat it if you see the infection spreading to more than half of the leaves.
Stopping it coming back
Water at the base of the plant rather than over the leaves to keep them dry.
Large crabgrass: spots? Check yours with a photo
Every plant is a little different. Take a photo of your large crabgrass 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
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Questions people ask
- Can I use a fungicide?
- You can, but it is usually not necessary for crabgrass. Improving airflow and keeping leaves dry is much more effective.
- Is rust fungus harmful to other plants?
- Some rust fungi are specific to certain grasses, but it is best to keep infected material away from your garden beds.
Guides to these causes
More problems with Large crabgrass
Back to the Large crabgrass 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.