Marigolds face a wide range of pests and diseases in the garden each season. In my experience, common marigold pests include aphids, caterpillars, slugs, and snails. These flowers can also suffer from diseases such as Botrytis blight and powdery mildew. I find that horticultural oils, insecticidal soaps, or fungicides are some of the best ways to combat these problems.
Aphids are a common and troublesome garden pest. These small black or green insects feed on sap and collect in large numbers on the undersides of leaves. Aphid-infested plants may also suffer from black mold and stunted or misshapen leaf growth.
A well-aimed spray from a hose should dislodge aphids, but biological controls, insecticidal soap, and horticultural oils are also effective solutions. Try and encourage natural aphid predators such as hoverflies, lacewings, or ladybirds.
Marigold foliage presents a tempting target for many species of caterpillars. These include cabbage looper, orange tortrix, and sunflower moth caterpillars. If you spot caterpillars on your marigolds, protect your hands with gloves and pick them off.
Regularly weed the area around your marigolds to deter caterpillars. Most butterflies or moths lay eggs on weeds because they are major caterpillar food plants. Removing weeds reduced the risk of caterpillar eggs being laid near your marigolds.
Earwigs are omnivorous pests that can target your marigold flowers. In addition, earwigs have reddish-brown bodies with prominent pincers on their abdomens. They’re usually active at night rather than during the day. Remove hiding places like fallen leaves and use horticultural oils, insecticidal soap, or homemade traps to eradicate earwigs.
4. Leaf Miners
Leaf miners sometimes use marigolds as nursery plants for their larvae. Once the larvae emerge, they “mine” or eat their way through the marigold leaves. Infested leaves often have small, meandering lines visible on the surface.
Leaf miners are small flying insects with black and yellow bodies. Remove infested leaves and use insecticidal soap or horticultural oils to kill any remaining invaders. Water your marigolds well and keep them healthy to deter leaf miners.
5. Slugs and Snails
Slugs and snails are ever-present foes for most gardeners. These mollusks devour young marigold plants or seedlings. Slugs and snails are more active in wet conditions.
If you see slugs or snails attacking your marigolds, pick them off and crush them. To prevent slugs and snails from reaching your marigolds, set out fruit or beer traps to kill them. You can also encourage natural slug and snail predators such as birds, hedgehogs, and frogs.
6. Spider Mites
Spider mites are tiny insects that can infest marigolds to feed on chlorophyll. If the leaves are covered with white webbing, spider mites are probably to blame. The leaves may also have brown or yellow spots. Use biological controls, horticultural oils, insecticidal soap, or a well-aimed burst from a hose to remove spider mites.
Like aphids, thrips can feed on the sap from your marigolds. These tiny black or brown insects can cause the leaves to appear stippled or silver. Although healthy marigolds can survive a thrip infestation, weak plants will suffer badly. Get rid of thrips using horticultural oils, insecticidal soaps, or biological control measures.
Whiteflies are another type of sap-sucking insect that can attack marigolds. These small white or silver flies congregate underneath the leaves. Symptoms of a whitefly infestation include honeydew deposits, sooty black mold, or yellowing leaves.
Horticultural oils and insecticidal soap can get rid of whiteflies fairly quickly. You can also dislodge them using a burst from a hose. Biological control measures such as predatory mites also work well.
9. Alternaria Leaf Spot
Alternaria leaf spot is a common fungal disease that can infect marigolds. It’s caused by various species of fungus from the Alternaria group. If your marigolds suffer from Alternaria leaf spot, you may see purple spots covering the stems and foliage.
The best cure for Alternaria leaf spot is to use a fungicide treatment. Organic copper fungicides are the best option. Good watering practices can reduce the risk of Alternaria leaf spot, so water at the base of the stems.
10. Aster Yellows
Aster yellows commonly infect members of the Asteraceae family, such as marigolds. This is caused by microscopic parasites called phytoplasma, which can spread to plants by hitching a ride on leafhoppers.
Common symptoms of aster yellows include strange green growths emerging from the flower heads, stunted growth, and yellowing leaves. Keep the area around your marigolds clear of leaf debris and weeds. Leafhoppers use these to access your marigolds, which unleashes the phytoplasma.
11. Botrytis Blight
Botrytis blight, also known as gray mold, is another common fungal disease affecting marigolds. This disease thrives in wet, humid conditions. Marigolds infected with botrytis blight may have brown, dead foliage and visible deposits of gray mold.
Botrytis blight can be treated using organic copper fungicides or similar products. Water your marigolds at the base of the stems rather than from overhead to reduce the risk of blight. Wet flowers and leaves are at greater risk of developing blight.
12. Powdery Mildew
Most gardeners will be familiar with powdery mildew. This fungal disease thrives in dry conditions during the day and wet conditions at night. Marigolds suffering from powdery mildew will have dusty white patches on the leaves. Use horticultural oils or organic copper fungicides to treat powdery mildew.
13. Root or Stem Rot
Root or stem rot is a common marigold disease caused by fungal infections. It can also be caused by overwatering or waterlogged soil. Symptoms include black or brown lesions on the stems and brown, mushy, smelly roots.
Use organic copper fungicides to combat stem rot. For root rot, trim away smelly or mushy roots and avoid watering your marigold until the soil has dried out a bit more.
Pesticide and Insecticide Options and Solutions
You can easily find a solution no matter which pest or disease attacks your marigolds. If you’re using fungicides, pesticides, or biological control measures, always follow the instructions on the packaging.
Horticultural oils such as canola or neem oil are the most effective treatments for most marigold pests and diseases. You can find canola oil in grocery stores, while neem oil is available online or in garden centers—horticultural oils interrupt pests’ life cycles like aphids.
Insecticidal soaps can suffocate and destroy unwanted pests like aphids or leaf miners. However, they can pose a risk to beneficial insects, so try and apply these soaps in the evening. If you can’t find insecticidal soap, dish soap and water should do the trick.
Biological controls are an organic and natural way of controlling pests using natural predators. Nematodes, predatory mites, and worms are common types of biological controls. These measures work well against pests like aphids.
Fungicides are used to treat fungal diseases that may be ravaging your marigolds. Organic fungicides, such as organic copper fungicides, are preferable to synthetic products. Synthetic fungicides may contain aggressive chemicals that can kill insects or deteriorate soil health. You can sometimes substitute fungicides for homemade remedies using baking soda, cooking oil, or dish soap.
Pesticide treatments can quickly eliminate pests, but many of these products rely on chemicals. While organic pesticides are available, synthetic ones can kill beneficial insects and pollute your soil. Try to use pesticides only as a last resort.
Managing Pests and Diseases
Providing the right growing conditions is the best way to protect your marigolds from pests and diseases. Marigolds are low-maintenance annuals that are pretty easy to grow.
Marigolds need full sun – at least 6 to 8 hours daily – in a sheltered spot. Water your marigolds once a week by aiming your watering can at the base of the stems. This avoids getting water on the leaves or flowers, which can lead to fungal infections.
Fertilizer can help your marigolds grow strong enough to fight diseases and pests. Give your marigolds a couple of doses of diluted fertilizer during the growing season. Provide fertile, well-draining soil to minimize the risk of waterlogged soil and provide plenty of nutrition.
Weed the area around your marigolds regularly. This removes hiding places for pests such as earwigs, slugs, and snails. It also provides good airflow around your marigolds by giving them plenty of space.
Marigolds make excellent companion plants and are prized for their attractive flowers. However, this also makes marigolds attractive to pests such as aphids, caterpillars, slugs, and snails. Marigolds can also suffer from diseases such as aster yellows and powdery mildew. Provide good growing conditions to protect your marigolds. Use horticultural oils, insecticidal soap, and biological control measures to eradicate pests and diseases.
For more, see our in-depth guide on how to grow marigolds in your garden.