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Wild Blue Phlox · Phlox divaricata

Why are there white spots on my Wild Blue Phlox?

Those white spots are almost certainly powdery mildew, a common fungal issue for phlox. It happens when the air around your plant is too still or humid. Check your plant's airflow first.

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spots on a plant
Photo: Scot Nelson, CC0

Check these first

  1. Inspect the undersides of leaves for white dust.
  2. Check if the plant is crowded by other shrubs.
  3. Look for signs of moisture sitting on the leaves.

Likely causes

  1. Powdery Mildew

    Most likely

    How to tell: You will see a white, dusty coating on the leaves that looks like flour. It usually starts on the lower foliage and spreads upward.

    What to do: Remove the most heavily infected leaves to stop the spread. Apply a fungicide or a mixture of one tablespoon of baking soda in a gallon of water to the leaves. Ensure you provide better spacing between plants for air circulation.

  2. Leaf Spot Disease

    Possible

    How to tell: These appear as brown or black circles with yellow halos on the leaves. They are caused by fungal spores splashing up from the soil during rain.

    What to do: Clean up all fallen debris around the base of the plant to remove spores. Water only at the base of the plant to keep the foliage dry. Apply a copper based fungicide if the spots continue to spread.

Leaf spots on other plants

Find out what's wrong with your wild blue phlox

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 serious if the leaves turn brown and drop off entirely. A few spots are normal in late summer as the plant prepares for dormancy.

Stopping it coming back

Space your plants at least 12 to 18 inches apart to allow for good airflow. Always water at the soil level rather than overhead.

Wild Blue Phlox: spots? Check yours with a photo

Every plant is a little different. Take a photo of your wild blue phlox 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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Questions people ask

Will these spots kill my phlox?
Usually, no. It is mostly an aesthetic issue, but it can weaken the plant if left untreated for a long time.
Should I cut the plant back?
If the infection is severe, cutting the plant back by half can encourage fresh, healthy growth. Just make sure to dispose of the clippings in the trash, not your compost.

Guides to these causes

More problems with Wild Blue Phlox

Back to the Wild Blue Phlox 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.