Cosmos are beautiful, colorful annuals that are also extremely easy to grow throughout the flowering season. They make fantastic cut flowers and also have rich meaning and symbolism. However, all types of cosmos flowers can still be at risk from diseases and pests. This article will show you how to identify and deal with common cosmos pests, bugs, and diseases in your garden. 

10 Common Cosmos Pests, Bugs, and Diseases

Dealing with Common Cosmos Pests, Bugs, and Diseases – The Essentials

Cosmos can be afflicted by pests such as aphids, spider mites, and thrips. Cosmos flowers are also vulnerable to diseases like aster yellows, botrytis blight, and powdery mildew. To combat diseases and pests, use things like horticultural oils, insecticidal soap, or organic fungicides and pesticides.

The 10 Most Common Cosmos Pests, Bugs, and Diseases

1. Aphids


Aphids are one of the most common pests encountered by gardeners. They gather in large groups underneath leaves to extract sap and are usually black or green. Other symptoms of an aphid infestation include black mold, deformed leaves, and stunted growth. Aphids are also usually farmed by ants.

Aphids can be evicted by spraying them with a hose. You can also use horticultural oils, insecticidal soap, and biological control measures. Hoverflies, lacewings, and ladybirds are all natural aphid predators that are good to try and encourage in your garden. This will help naturally control aphid populations.

2. Plant Bugs

Plant Bugs

Various species of plant bugs may also target cosmos flowers. Like aphids, plant bugs feed on the sap of plants like cosmos and other members of the Asteraceae family. Plant bugs are small flying insects with colorful patterns on their backs.

If the leaves of your cosmos have holes or bite marks in them, plant bugs could be to blame. Spray your cosmos with a hose to dislodge plant bugs. Alternatively, you can use insecticidal soap or horticultural oils like neem oil to remove them.

3. Spider Mites

Spider Mites

Spider mites are a group of small insects that consume the chlorophyll produced by cosmos plants. There are several species, but all of them have the same symptoms. Spider mites can leave white webbing on cosmos foliage. Spider mites may also cause brown or yellow spots on the leaves or stems.

All types of spider mites can be dealt with using horticultural oils and insecticidal soap. Biological controls are also effective at tackling spider mite infestations.

4. Thrips


Thrips are well-known houseplant pests, but they can also cause problems for cosmos flowers. They are tiny, thin black or brown insects that feed on plant sap. Symptoms of thrips infestations include silver or stippled markings on the leaves of your cosmos.

If your cosmos is healthy, thrips infestations shouldn’t cause too many significant problems. However, weak plants will experience serious issues. Thrips can be eliminated using insecticidal soap, horticultural oils, and biological control measures.

5. Aster Yellows

Aster Yellows

Most species in the Asteraceae family are at risk from aster yellows, including cosmos. This disease is caused by microscopic parasites called phytoplasma. These parasites are spread by insects like leafhoppers, which resemble mini grasshoppers.

Cosmos flowers that are infected with aster yellows may have strange green growths coming out of the central florets. Other symptoms include stunted growth and yellowing leaves. Give your cosmos plenty of space from other plants, and try to deter leafhoppers by clearing fallen leaves and weeds.

6. Botrytis Blight

Botrytis Blight

Botrytis blight is also called gray mold due to the moldy growths that appear on infected plants. This fungal disease is a common problem for cosmos growing in excessively humid or wet conditions. Other symptoms include browning or dying leaves.

Use organic copper fungicides or other fungicides to get rid of botrytis blight. Maintaining good cosmos watering practices also helps avoid infection. Water your cosmos at the base of the stems. Watering from overhead risks water splashing onto flowers and leaves, leaving them vulnerable to blight.

7. Curly Top Virus

Curly Top Virus

Curly top virus is another disease that can be spread by leafhoppers. Although curly top virus usually attacks crops like beetroot or tomatoes, cosmos are also at risk. Infected cosmos may display curling leaves or yellowing foliage covered with purple lesions.

There’s no magic cure for curly top virus, so keeping leafhoppers away from cosmos flowers is your best bet. Make sure that the soil surrounding the cosmos is free from weeds or leaf debris. Giving your cosmos enough space between other plants helps prevent curly top virus.

8. Fusarium Wilt

Fusarium Wilt

Fusarium wilt is a fungal infection caused by the Fusarium oxysporum fungus, which is found in the soil. Cosmos flowers infected by fusarium wilt will appear limp and wilted during the day but will seem to perk up at night. Other symptoms include stunted growth and yellowing leaves towards the base of the plant.

To tackle fusarium wilt, remove infected cosmos flowers to stop the disease from spreading. Fusarium wilt thrives in waterlogged soil, so avoid overwatering your cosmos to reduce the risk of infection.

9. Powdery Mildew

Powdery Mildew

Powdery mildew is a common fungal disease affecting most plants, including cosmos flowers. This disease thrives in hot, dry conditions during the day and cold, wet or humid conditions at night.

The tell-tale symptom of powdery mildew is the appearance of dusty white growths on the leaves or stems. Stunted growth is another symptom of powdery mildew. Treat powdery mildew using organic copper fungicides or horticultural oils such as neem oil.

10. White Smut

White Smut

White smut is another fungal disease that can infect cosmos flowers. It’s caused by the Entyloma calendulae fungus and thrives in cold, wet conditions. Common symptoms of white smut include pale white spots on lower leaves. Dark brown spots may also be visible within these white dots.

Use organic copper fungicides to get rid of white smut. It’s also a good idea to ensure that your cosmos flowers receive good airflow. Give your cosmos plenty of space away from surrounding plants, and avoid leaving them in waterlogged soil.

Pesticide and Insecticide Options and Solutions

Pesticide and Insecticide Options and Solutions

Horticultural Oils

Horticultural oils work well against several cosmos pests and diseases. The most commonly-used horticultural oil is neem oil, but canola oil is another option. These oils control pests by causing problems during their life and breeding cycles.

Insecticidal Soap

Insecticidal soaps are highly effective against pests like aphids or plant bugs. These can dehydrate and suffocate offending pests. However, insecticidal soap may also kill beneficial insects. You can also make your own insecticidal soap using a mix of water and dish soap.

Biological Controls

Biological controls is an umbrella term for a range of predatory insects that feast on pests like aphids. Common types include nematodes and predatory mites or worms. This type of organic pesticide helps avoid the use of chemicals.


Fungicides are often the best way to treat any fungal diseases infecting your cosmos. Various organic and synthetic options are available, but use organic fungicides wherever possible. Organic copper fungicide is one of the most effective choices.


Organic or synthetic pesticides can make short work of annoying pests like aphids. However, synthetic pesticides can kill beneficial insects or negatively impact soil quality. Use organic pesticides wherever possible.

Managing Pests and Diseases

Managing Pests and Diseases

Protecting your cosmos from pests and diseases is often as simple as providing the right growing conditions. The main thing that cosmos flowers need is loose, well-draining soils. Waterlogged soil is a breeding ground for fungal diseases, so avoiding overwatering is key.

Cosmos also need 6 to 8 hours of full sun daily and should be watered approximately once per week. Always water the base of your cosmos to avoid getting water on flowers or foliage. Most cosmos won’t need extra fertilizer and can actually suffer if the soil contains too many nutrients.

Keep the soil around your cosmos free from fallen leaves or weeds to deter leafhoppers. These insects can carry aster yellows or curly top virus.

It’s also prudent for optimal blooming to pinch and deadhead cosmos flowers for optimal blooming throughout the flowering season.

Wrapping Up

Cosmos flowers are colorful, attractive, and beneficial annuals that add lovely shades of color to summer display. But even these low-maintenance plants can suffer from pests and diseases. Aphids, plant bugs, and spider mites all target cosmos. Common diseases include powdery mildew and white smut.

Most problems can be treated using horticultural oils, insecticidal soap, and organic copper fungicides. Always give cosmos flowers plenty of space to promote good airflow and avoid waterlogged soil.

Contributing Editor | Full Bio | + posts

Edd is a budding content writer and gardener living in the United Kingdom. He has a bachelor's degree in Creative and Professional Writing and has written for several gardening publications online. He is passionate about nature and sustainability with a focus on gardening and wildlife.

Author Edward Hodsdon

Edd is a budding content writer and gardener living in the United Kingdom. He has a bachelor's degree in Creative and Professional Writing and has written for several gardening publications online. He is passionate about nature and sustainability with a focus on gardening and wildlife.

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