Zinnias are gorgeous, colorful flowering annuals that are easy to grow and cultivate. Zinnias also attract beneficial insects such as bees, butterflies, and hoverflies. Although some types of zinnias are resistant to diseases, infections can still be a problem. In this guide, we’ll look at nine common zinnia pests and diseases and how to tackle them.
How Do I Deal With Zinnia Pests and Diseases? Key Takeaways
Zinnias are easy to grow but can suffer problems. Common zinnia diseases include Alternaria blight, aster yellows, and powdery mildew. Pests like aphids, mites, and whiteflies can also attack Zinnias. To mitigate, use organic solutions to protect the beneficial insects that zinnias attract.
The Most Common Zinnia Pests and Diseases & How to Identify Them
Despite being low-maintenance annuals, zinnias can still experience problems with diseases and pests. These problems usually appear if your zinnias aren’t growing in ideal conditions. Diseases can also be spread on garden tools, so it’s important to sterilize your equipment before cutting zinnias (particularly for a vase or bouquet arrangement).
Here are the nine most common zinnia diseases and pests and how to identify them:
1. Alternaria Blight
Alternaria blight is one of the most commonly encountered zinnia diseases. It’s an infection caused by the Alternaria zinniae fungus and looks like a leaf spot disease. The disease even infects seeds and can afflict multiple generations of zinnia plants.
The main symptom of Alternaria blight is the appearance of large irregular red or purple spots on the leaves. Other symptoms include holes in the leaves and brown, shriveled leaves. You may also see unsightly lesions on the stems or brown patches on the flowers.
Organic copper fungicides are the best solution if your zinnias are infected with Alternaria blight. You can also try to prevent outbreaks of Alternaria blight by growing zinnia varieties that are resistant to the disease. Water your zinnias at the base of the stems and remove nearby debris or leaf litter to reduce the risk of infection.
2. Aster Yellows
Aster yellows affect several plants in the Asteraceae family, including zinnias. This disease is caused by phytoplasma – tiny parasites spread by leafhoppers as they feed on sap. A zinnia infected by aster yellows may have strange green growths emerging from the flower head. Other symptoms include stunted growth and reddening or yellowing leaves.
The best way to combat aster yellows is to keep leafhoppers away from your zinnias. Give your zinnias enough space away from other plants and keep the area clear of leaf litter. Alternatively, you can grow companion plants like mint or yarrow around your zinnias.
3. Bacterial Leaf Spot
Bacterial leaf spot is another disease that affects zinnias. This disease usually develops on zinnia leaves that are too wet. Infected leaves have small purple spots with yellow rings around them. The disease can also infect seeds, impacting new plants.
Organic copper fungicides are the best cure for bacterial leaf spot and should be sprayed on after rainfall. Try and keep the leaves of your zinnias from getting wet by watering at the base of the plants. Remove moist leaf litter from the area as well to reduce the spread of the disease.
4. Powdery Mildew
Powdery mildew is a fungal infection that covers infected leaves with white dust-like growths. This disease likes dry conditions during the day and humid conditions at night. It can be treated using horticultural oils or organic copper fungicides. Ensure your zinnias are growing in ideal conditions to reduce the risk of powdery mildew.
Most gardeners have dealt with aphids at some point. These tiny green or black sap-sucking insects can infest vulnerable zinnias, mainly when being farmed by ants. They usually hide in colonies on the undersides of leaves.
Aphids cause distorted leaf growth as they extract sap from their hosts. Other symptoms of aphid infestations include black mold and stunted growth. Biological controls, horticultural oils, and insecticides can deal with aphids. You can also evict them from your zinnias with a well-aimed blast from a hose.
6. Broad Mites
Broad mites are microscopic brown or translucent mites that extract sap from zinnias. They need warm temperatures and will die if it gets too cold. Plants that are infested with broad mites may have discolored or misshapen leaves that turn brown or purple. Broad mites can also cause weak or stunted growth.
You can get rid of broad mites using horticultural oils or insecticidal soaps. However, it’s difficult to confirm that the infestation is gone because these pests are so small. Spray all over the foliage to eliminate as many mites as possible.
Leafhoppers are the primary carriers of aster yellows and can really damage your zinnias. These insects feed on sap and can transfer phytoplasma to your zinnias in the process. If your plant looks like it has aster yellows, leafhoppers are active nearby.
Use insecticidal soap to combat the leafhoppers. Weed the soil around your zinnias regularly and remove leaf litter to make things more difficult for leafhoppers. You can also use plant companion plants like garlic, mint, and yarrow to lure these bugs away from your zinnias.
8. Spider Mites
Spider mites are another common pest that may target your zinnias. These tiny insects feed on chlorophyll and thrive in hot, dry conditions. Spider mites often coat the leaves of infected plants with webbing. Infected foliage may also have small brown or yellow spots.
You can eliminate spider mites by hosing them off your zinnias or using insecticidal soap. You can also use biological controls to introduce predatory organisms that feed on spider mites.
Whiteflies are another pest that you might find on your zinnias, usually underneath the leaves. Related to aphids, these insects feast on sap, causing yellowing leaves. While feeding, whiteflies produce sticky deposits of honeydew that can cause leaves to display sooty black mold.
You can dislodge whiteflies and their larvae by spraying your zinnias with water. You can also use horticultural oils and insecticidal soaps to eliminate them.
Pesticide and Insecticide Options and Solutions
If diseases or pests are attacking your zinnias, don’t despair. There are plenty of ways to tackle these problems, including oils, pesticides, and insecticidal soap. Whatever method you choose, always follow the instructions on the packet.
Using biological controls is an environmentally friendly way of tackling pests and diseases. These products come from predatory organisms such as bugs, nematodes, and worms. These organisms are the natural predators of pests like aphids and whiteflies.
Fungicides are types of pesticides that are specially designed to target fungal diseases such as Alternaria blight. Synthetic fungicides rely on chemicals, but you can also get organic copper fungicides. You can even make your own using ingredients like baking soda, cooking oils, or dishwasher soap.
Horticultural oils are one of the most effective ways to tackle zinnia pests. Things like canola and neem oil are widely available, and you might even find them in the grocery store. These products kill pests like aphids by disrupting their life cycles.
Insecticidal soap is an excellent way to eliminate pests attacking your zinnias. These soaps suffocate pests before breaking down their cellular structures. You can also make a substitute for insecticidal soap by mixing some dish soap with warm water.
If you’re using insecticidal soaps, apply them to your zinnias in the evening. This minimizes the risk of killing beneficial insects.
Pesticides usually rely on chemicals to kill pests, so they should only be used as a last resort. Although pesticides are highly effective at eliminating pests like aphids, they can also decimate beneficial insects. These chemicals can even contaminate your soil and damage your zinnias over time.
(Editors Note: Petal Republic participates in partnership programs with Amazon and other merchants to help connect readers with relevant products and services we may recommend).
Managing Pests and Diseases
Providing the right growing conditions for your zinnias is the easiest way to prevent pests and diseases. Zinnias are low-maintenance annuals that are easy to grow and will bloom throughout summer and into fall. Thankfully, it’s pretty easy to produce vigorous, healthy zinnia plants that can fight off diseases and pests.
Give your zinnias 6 to 8 hours of direct sunlight daily and plant them in fertile, well-draining soils. Water them once a week and promote vigorous plants by feeding them once or twice a month with fertilizer. Zinnias can reach up to 4 feet tall, so give your plants plenty of space. This promotes good airflow, which reduces the risk of disease outbreaks. Zinnia flowers typically bloom 40 to 60 days after planting from seed.
Prevent pests and diseases from gaining a foothold by regularly weeding around your zinnias. Keep the soil clear of leaf litter and other debris to combat pests like leafhoppers. Keep the foliage of your zinnias dry by watering at the base of the stems. Avoid overwatering, as waterlogged soil is a breeding ground for diseases.
Although zinnias are easy to grow, you must be vigilant against pests and diseases. Zinnias can suffer from diseases like Alternaria blight, powdery mildew, and pests like aphids or spider mites. If you spot any pests or diseases, use horticultural oils and insecticidal soaps to combat these issues. However, prevention is the best cure, so ensure that your zinnias receive ideal care to keep them strong and healthy.
For more, see our in-depth guide on how to grow zinnias in pots and containers.