A Beginner's Guide to Fighting Powdery Mildew Hydrangea

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1. Recognize Powdery Mildew

Symptoms

When hydrangea gets powdery mildew, it shows white or gray powder-like spots on its new buds, leaves, and stems. These spots are like sprinkling a layer of flour on a plant. If powdery mildew keeps spreading, it can cover the whole plant. At the same time, hydrangea leaves can turn yellow and twist, and plants may stop growing.

The difference from other problems

When judging powdery mildew on hydrangea, we must distinguish it from gray mold, leaf spot, and wilt disease.

Powdery mildew is a common fungal disease that makes white spots on the surface of plants like buds, leaves, and stems. Botrytis blight will show gray or brown mold on the buds and petals of plants. Leaf spots are black or reddish-brown spots with well-defined boundaries on the leaves. Although the leaves of plants with wilt disease also turn yellow, it is caused by root rot, and plants do not have white spots.

Powdery mildew (1), Botrytis blight (2), Leaf spot (3), and Wilt disease (4)

2. Cause

Powdery mildew usually happens when the air is not circulating well, and the weather is cool and dry. If we use too much nitrogen fertilizer, it can also cause powdery mildew.

Hydrangea, chrysanthemum, rose, and clematis are all prone to powdery mildew. The onset period of powdery mildew is in early summer and early fall.

We should take the initiative to prevent and treat powdery mildew except for January, February, March, and December.

3. Treating Powdery Mildew

Natural methods

First, removing sick leaves and stems as soon as possible prevents the spread. Second, choose natural treatment methods such as baking soda, milk, neem oil, etc.

baking soda

Baking soda spray is a great way to protect your plants from powdery mildew. It can also help to treat plant diseases in the early stages.

I will add one teaspoon of baking soda and one teaspoon of liquid soap to a gallon of water. The purpose of adding liquid soap is to make the spray stick to the plant better. Generally, I spray the whole plant evenly.

Mouthwash

Mouthwash can remove fungi on leaves. However, be sure to use mouthwash labeled kill germs or bacteria.

I will add 4-quart mouthwash to a gallon of water. Unlike baking soda, I will not spray the whole plant or use it on a large scale. It is worth noting that mouthwash can hurt new leaves and buds.

3. Milk

Adding 2 quarts of milk to a gallon of water can treat powdery mildew by using it once a week.

However, there seems to be no experiment to prove why milk can treat powdery mildew. Moreover, I have never used this method because I think this method is a waste of food.

Neem oil

Neem oil is very effective for many diseases, but for powdery mildew, neem oil plays a more preventive role. I will use neem oil every 14 days during the high incidence of powdery mildew, covering the whole plant each time.

Chemical fungicides

When hydrangea leaves have twisted or yellowing leaves, remove diseased parts as soon as possible since they can not no longer restore health. When plants show signs of powdery mildew, we can treat them using a 0.5% sodium bicarbonate solution. Or we can treat plants with chemicals labeled to control powdery mildew.

These products are easy to buy at the gardening center or online. Before we start with these products, we need to read the instructions on the label carefully. When using, we should wear gloves and masks to avoid using spray in windy weather.

4. Prevention Strategies

Prevention is always better than treatment. First, I plant hydrangea in a place with good air circulation. I will increase the spacing between each hydrangea plant and trim the inward-growing stems regularly. However, it is not enough to do so.

Second, I will not keep the plants in a shaded place for a long time. On the premise that the plants don't get sunburned, I will make sure that the plants get enough sunlight. Sunlight kills fungi.

Third, I will control the amount of nitrogen fertilizer and use slow-release fertilizer as much as possible. At the same time, supplement phosphate and potassium to plants to enhance their disease resistance.

Then, when I grow hydrangea, I choose varieties resistant to powdery mildew and other diseases. This will reduce the risk of plants getting disease and prevent it from spreading.

Finally, I will take prompt action once the hydrangea shows signs of powdery mildew. Remove any infected leaves and stems from the plant in time. If the whole plant is sick, I will remove the infected plant. Once I have gotten rid of the sick plants, I'll closely check the ones around them to ensure other plants are still healthy.

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Tips: Avoid using infected leaves, stems, or plants when composting. Instead, please place them in a bag and dispose of them properly. If possible, it’s better to burn them so the fungal spores don't spread around.

5. Conclusion

Knowing how to prevent and address powdery mildew is vital to keep your hydrangea healthy all year. With some easy tips and methods, you can do it. Remember, prevention and early detection are the key. Check your plants regularly and take timely action when you find a problem.

If you have a better way to treat powdery mildew, you are welcome to leave me a message. Have a nice gardening trip.

Bibliography:
PÉREZ-GARCÍA, A., ROMERO, D., FERNÁNDEZ-ORTUÑO, D., LÓPEZ-RUIZ, F., DE VICENTE, A. and TORÉS, J.A. (2009), The powdery mildew fungus Podosphaera fusca (synonym Podosphaera xanthii), a constant threat to cucurbits. Molecular Plant Pathology, 10: 153-160. https://doi.org/10.1111/j.1364-3703.2008.00527.x
Jennings C, Baysal-Gurel F, Alexander LW. Powdery Mildew of Bigleaf Hydrangea: Biology, Control, and Breeding Strategies for Resistance. Horticulturae. 2024; 10(3):216. https://doi.org/10.3390/horticulturae10030216
Casulli, F., Santomauro, A. and Faretra, F. (2000), Natural compounds in the control of powdery mildew on Cucurbitaceae†. EPPO Bulletin, 30: 209-212. https://doi.org/10.1111/j.1365-2338.2000.tb00881.x

My name is Arthur Mo. I am a gardener who loves gardening so much. I graduated from Ocean University of China. After I returned to Japan, I began to learn how to cultivate hydrangeas and daisies. Mr. Mitsuhiro (my master) taught me to sow, cut, fertilize, and manage diseases and pests. For nine years, I have been sharpening my planting skills. In 2022, my family and I moved to Florida. The warm climate of Florida gives me the impulse to continue cultivating hydrangeas and daisies. I began my gardening trip in my backyard and unswervingly contributed my efforts to these plants. I love these plants deeply, and at the same time, I also thank my family, my master, and friends for their understanding and support.