Why Are Your Coreopsis Leaves Turning Yellow?

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Willie writes on small engine repair, landscaping, and lawn care. He owned a professional lawn care business in Alabama for almost 10 years.

5 Reasons Why Coreopsis Leaves Turn Yellow

Coreopsis is susceptible to plant diseases and pest infestations that can cause its leaves to become yellow. By regularly inspecting your plants and monitoring them closely, you can address these issues immediately and prevent them from spreading throughout your garden.

Keep an eye out for the diseases and pests mentioned below.

Aster Yellows

Aster yellows is a plant disease caused by bacteria transmitted by aster leafhopper insects, which can be brown, green, or yellow.

Besides yellowing leaves, other symptoms that your coreopsis flowers are being attacked by aster yellows include:

Stunted petal growth
Many flower heads
Stunted plant growth
Yellowing of new shoots

How To Stop It

Sadly, plants infected with aster yellows disease don’t recover (source). You’ll have to uproot the infected plants and throw them out so that they don’t cause other plants to get infected.

Make sure that you check plants before you bring them home, so you don’t accidentally plant infected coreopsis in your garden.

Rust Disease

Rust disease is a fungal infection that causes your coreopsis leaves to turn yellow, but you have to look underneath the leaves to spot the yellow-orange spores that appear in masses.

Pustules can also appear on the leaves, and these can be various colors, such as:

Yellow
Brown
White
Orange
Black

Rust infections primarily affect the leaves of plants, but they can also affect the flowers and fruits.

If the rust infection has progressed, the leaves will become yellow before falling off the plant (source).

How To Stop It

If your plants are showing signs of rust disease, take action immediately. Follow these tips:

Cut off any leaves that are affected. This will prevent the disease from spreading to other leaves.
Don’t feed your plant too much nitrogen fertilizer. Nitrogen boosts the growth of leaves that can become attacked by rust fungi.
Clear away dead plant material. Do this at the end of the growing season (source). Avoid adding dead plant material to your compost heap because if there is rust present, it will continue to grow.

Root Rot

Root rot is caused by a fungus called Rhizoctonia, but overwatering your plants is the main factor that contributes to it.

When infected with root rot, you’ll see that your plants become yellow before drooping and dying.

Other symptoms of root rot in plants include:

Slow or stunted growth
Wilting of leaves
Mushy stems
A rotten smell in soil
Reddish-brown roots

How To Stop It

You must act quickly when faced with root rot, as it can be fatal for your plants. It should be treated with a fungicide (source).

However, the best long-term solution is removing the plants and planting new ones in a different area of the garden (source).

Fungi that cause root rot can remain in the soil for a long period of time.

Powdery Mildew

Powdery mildew is another type of fungal infection that produces a white or grey powdery mass on leaves. As the disease progresses, the leaves will turn yellow before going brown and dying.

Over time, the disease can cause weaker or slower plant growth while also resulting in fewer flowers on the plant.

You can prevent powdery mildew from appearing on your coreopsis plants by following these tips:

Thin out the plants. Pruning helps to boost air circulation around your plants, which can prevent the growth of fungi that thrive in dark, cramped, moist areas.
Avoid using too much fertilizer. Fertilizers can cause too many leaves to appear on the plant, which can attract fungal infections.

How To Stop It

If your coreopsis plants are infected with powdery mildew, you can treat them in the following ways:

Spray potassium bicarbonate on the plant. Mix one tablespoon (15 grams) of potassium bicarbonate with half a teaspoon (2.5 ml) of liquid soap, then stir it into one gallon (3.78 liters) of water. Spray the mixture on all parts of the plant that have been affected by the mildew.
Spray neem oil on the plant. You can also treat mildew with neem oil. I’d recommend Neem Oil Plant Spray (link to Amazon). It comes in a spray bottle that’s ready to use, and it can be used on both indoor and outdoor plants.

Overwatering

When caring for your coreopsis plants, one of the worst things you can do is give them too much water. Overwatering can contribute to root rot and other diseases that can end its life.

Frequently watering your coreopsis plants from the top until their leaves get wet can intensify fungal infections.

How To Fix It

Make sure you water your coreopsis sufficiently and never too much. The frequency of watering your plant will depend on whether it’s a newly planted or established plant.

Newly-planted coreopsis plants want regular water so that their soil remains moist without being soggy.
For plants that are older than a year, water them deeply when the soil is dry to a depth of about one inch (2.54 cm).
Water your coreopsis plants early in the morning. This will ensure that the water dries out so that it won’t stay on the plant and cause infections.

Final Thoughts

Growing coreopsis doesn’t require much in terms of care, but this plant is susceptible to problems, such as fungal diseases, that can cause its leaves to turn yellow. These include:

Root rot
Aster yellows
Rust
Powdery mildew

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