Zinnia leaves can develop black spots if they are infected by fungal leaf spot, powdery mildew or bacterial leaf spot. Avoid overwatering zinnias because this can cause fungal root rot and the leaves to turn black.
This article will explore the top reasons why zinnia leaves get black spots, what you can do to solve this problem and how to prevent it in the future.
Why zinnia leaves get back spots
Here are the top reasons why zinnia plants get black leaf spots and what to do about each one.
1. Alternaria Leaf Spot
Zinnias can develop fungal leaf spot when grown in part shade or if water is splashing up from the soil onto the leaves. They start small and dark red then spread to turn black with white centers.
Copper fungicides can be used to treat this problem however it if it is affecting a single plant it is easier just to remove it. Put the whole plant in the bin with the seeds as well. The seeds can continue to carry the disease so don’t save them for next year.
I like to pinch off any affected leaves to reduce the spread. The plant can recover without using fungicides if the problem is caught early.
2. Powderly mildew
A fungal disease that can look black or grey on the leaves. It can feel soft and can rub off with your fingers. It can also have a fluffy texture, rough on the leaves and spread down to the stems. Zinnias are hardy and can survive small amounts of powdery mildew growth.
It is best to move potted zinnias to a sunnier position with more air movement and to make sure that they are being watered only on the soil and not on the leaves. Water in the morning so the leaves will dry quickly if any water splashes end up on them.
Horticultural oils (ie. Neem oil) can be used to treat powdery mildew if the problem continues to get worse.
3. Bacteria leaf spot
Bacteria leaf spot (compared to fungal) is caused by a bacteria, Xanthomonas campestris. They will cause the leaves to develop dark black or purple spots. Copper fungicides can be used to treat leaf spot like the fungal varieties.
Overwatering can cause zinnias to develop black spots on their leaves. They start small but can affect the whole leaf over time, causing it to wilt and die back. As the problem gets worse, the stems can turn black as well killing the plant over time.
Leaves can become black on zinnias due to overwatering if the roots develop fungal rot, if the soil is compacted, the oxygen is reduced and the roots can’t absorb the nutrients they need.
Zinnias are surprisingly drought hardy once they establish so only need watering once every few days over summer. Check the soil and if it feels dry 2 inches down then water the plants. Allow a day at least between watering to allow the soil to dry out slightly before watering again.
How to prevent black spots on Zinnia Leaves
Here are my top tips to prevent black spots on zinnia leaves from fungal or bacterial leaf spot and overwatering.
1. Water around the root zone
Always water your zinnia plants around the root zone and keep the water off the leaves. Any water on the leaves can cause powder mildew. Soil that is splashed on the leaves can carry bacteria or fungus from the soil which can infect the leaves.
Keep the leaves dry to reduce the chance of black spots from developing on the leaves.
2. Water in the morning
Water zinnias in the morning to allow the leaves to dry out quickly and to avoid them staying wet overnight. I like to water with my hose with a shower sprayer fitting so I can direct the water.
Small watering cans also work well and you can add some extra liquid seaweed to strengthen the roots at the same time.
3. Mulch with bark
Mulch around zinnias with bark mulch to help to prevent water and soil from splashing on the leaves. This will help to avoid bacterial or fungal leaf spot and can even help to absorb extra water from the rain. This helps to prevent overwatering and root rot which can cause the leaves to turn black.
4. Avoid overwatering
The top way to avoid black leaves is to avoid overwatering zinnias. They are actually drought hardy plants, originating from Mexico so like to dry out between watering. They like hot summer days and only need to be watered every 2-3 days in the heat.
Check the soil with your finger to see if it is damp 2 inches below the surface. If the soil is still moist wait for another day or so and deep water the plant when it feels dry.
5. Plant zinnia plants in full sun
Planting zinnias in full sun will give you more flowers and help to avoid black leaves. Sunny positions will dry leaves quicker and avoid bacterial and fungal leaf growth. Zinnias look great when grown densely and can make a great, bee friendly backyard meadow.
If you are having problems with black leaves, spread the seeds and seedlings further apart to allow more air movement, more sunlight and to reduce the amount of water sitting on the leaves.
6. Plant zinnia seeds in spring
Plant zinnia seeds in spring directly into your garden soil or into seed trays. Plant in spring when the weather has started to warm which reduces the chance of bacterial and fungal disease. Cooler weather can make it easier for the plants to establish quickly and will avoid their root staying in wet soil.
7. Rotate your zinnia crops
Rotate the location that you are planting zinnia plants in your garden to avoid spreading disease from the plants from year to year. Bacteria and fungi that cause black leaf spots can stay in the soil and infect your next year’s crop.
If you have run out of space in your garden like me, plant zinnias in pots for a year while you let your soil have a break. This will stop diseases from spreading to your new zinnia crops year to year.
8. Remove damaged plants quickly
If you notice black spots on your zinnia plants that look like bacteria or fungal leaf spot it might be best just to remove the affected plant. Removing the plant before the problem can spread to the other plants. You can treat them with copper fungicides but if you don’t want to use fungicides, just remove the plant.
9. Put diseased plants in the bin not your compost
Zinnia plants that have been removed because they have fungal spots, put them in the bin and not in your compost to avoid spreading the disease to future crops.
Avoid saving seed from affected plants because the seeds can carry the fungal or bacteria disease. This can affect next years plants so throw them out and start with fresh seeds in a different patch.
Why Zinnia Leaves Get Black Spots | Summary
Zinnia plants can develop black spots on their leaves if they get wet, are affected by bacteria or fungi or if they are overwatered. Zinnia are hardy plants so can recover quickly if they watering is reduced. Plant them in spring for an amazing summer display to show off in your front or backyard.
I am an accredited practicing dietitian, experienced gardener and a dedicated cook. I love writing and sharing my experience so you can learn from my successes and mistakes.