There are few things more beautiful than lush hydrangea bushes. Unfortunately, their beauty can be sullied by a number of fungal and viral diseases (and two bacterial diseases).
However, there are steps you can take to keep your beauteous shrubs from falling victim to one of these diseases.
We will introduce you to the major hydrangea diseases, so you know what to look for and how to prevent and treat them.
What You Will Learn
- Fungal Diseases
- Bacterial Diseases
- Viral Diseases
Botrytis Blight (Botrytis cinerea)
This fungus can severely affect the flower buds and even kill them before they open. In addition, infected flower parts can fall on the leaves and infect them.
The first symptoms are water-soaked spots on the flowers. However, these grow into reddish brown lesions.
Botrytis is more likely to be a problem under cool and damp conditions, such as several days of cloudy, humid, and rainy weather.
You can take steps to try and prevent this infection. Keep the humidity low. Don’t water late in the day, and only water at the roots, so you don’t get the flowers and leaves wet.
If you can, keep good airflow around your plants. Space them properly, and prune branches that are closely spaced. Treat your pruning shears with bleach as you prune, so you don’t accidentally spread any disease.
Also remove dead or damaged flowers and leaves to prevent the fungus from gaining egress into the plant. Clean up debris around the plant, so that Botrytis can’t live on the dead tissue.
If you have a persistent problem, you may need to use fungicides. Options include mancozeb, iprodione, or thiophanate-methyl.
Leaf Spots (Cercospora species and Phyllosticta hydrangea)
Cercospora manifests as circular purple or brown spots on the bottom of the plant. As the lesions get larger, the leaves can turn yellow and fall off the plant.
Watering without getting the leaves wet will help to prevent these diseases. If your hydrangeas do get infected, you have several options, including compost tea, hydrogen peroxide, garlic oil, or liquid kelp.
You can also apply the fungicides chlorothalonil or thiophanate-methyl.
Anthracnose (Colletotrichum gloeosporioides)
Plants that have been heavily fertilized are more likely to contract this common fungal pathogen.
Continued rainy weather or heavy fog produces the conditions that favor infection.
The fungus produces large brown spots on the leaves or flowers that will become more lightly colored in the centers. One distinctive symptom is that spots by the veins develop at an angle.
You can also treat this disease with liquid kelp, garlic oil, hydrogen peroxide, or compost tea.
Powdery Mildew (Erysiphe polygoni)
Powdery mildew manifests as a white powdery substance on the surface of the leaves. You can see white, cottony growth on the bottom of the leaves.
Left unchecked, the fungus can infect the newly developing buds and stunt their growth.
Powdery mildew is most likely to be a problem on hydrangeas when the days are warm and the nights cool.
You can prevent the disease by reducing humidity and increasing air circulation.
One way to control this disease is to apply a fungicide as soon as you discover it. Another option is to use neem.
Rust (Pucciniastrum hydrangea)
Like other rusts, hydrangea rust needs two hosts to survive and does not kill either of them. This rust only infects the smooth hydrangea, H. arborescens, and hemlock as its alternate host.
The first symptoms are orange pustules on the bottoms of the hydrangea leaves and yellow spots on top.
This disease is difficult to control, but you can manage it by cleaning up infected leaves and debris that has fallen to the ground around both hosts. Thin inside the hydrangea making sure to disinfect your pruning shears.
If you know that rust is likely to be a problem, you can grow the cultivar ‘Frosty,’ which is resistant to this disease.
Bacterial Wilt (Ralstonia solanacearum)
This important bacterial disease first manifests as blight in the leaves and flower clusters. However, more severe infestations can cause both wilting and root rot.
Bacterial wilt occurs mainly in hot weather and heavy rains. There are no chemical options to control this disease.
Bacterial Leaf Spot (Xanthomonas campestris)
The bacteria that cause this disease can enter the plant through natural openings like stomata or through wounds.
The first symptoms are water-soaked spots. The spots darken and become angular in shape. These spots become larger lesions and can kill the leaves.
If you have a susceptible plant, you can protect it with copper hydroxide (Kocide).
Fifteen different viruses afflict hydrangeas! Hydrangea macrophylla is the most susceptible.
Transmission can occur by knives, leaf contact, and insects like aphids. In some cases, plant parasitic nematodes can transmit the viruses.
Prevention is the key in these cases. Quickly remove infected plants and their parts. Sterilize your pruning shears before cutting the plants, and plant clean stock in soilless media to avoid the viruses that are transmitted by nematodes in the soil.
These three are the most common viral infections that you are likely to encounter:
Hydrangea Ringspot Virus
If your hydrangea has brown spots or rings on its leaves, there is a good chance that it is infected with hydrangea ringspot virus. Then the leaves of the plant will start to be distorted and rolled, and the growth of the plant will be stunted.
Aphids do not spread this disease. However, it spreads mechanically, so tools can transmit this virus. Sanitizing your pruning tools will help to prevent the spread of this disease.
Unfortunately, if your hydrangea contracts this disease, you will have to purge it. Varieties that are tolerant to this virus are available.
Hydrangea Mosaic Virus
Hydrangeas infected with this virus will have a pattern of yellow mosaics on their leaves.
This is another virus that is not transmitted by aphids. However, once again, you can spread the virus with your tools. So be sure and disinfect your pruning shears to avoid inadvertently spreading this virus.
Tomato Ringspot Virus
This virus causes the leaves to turn yellow and become distorted, and the growth of the plant will be stunted.
Nematodes, not pruning tools, spread this virus.
If you are growing your hydrangeas in containers, you can avoid tomato ringspot virus by using a soil mix that is free of nematodes.
Such Beautiful Plants and So Many Diseases
There are a number of different organisms that can infect hydrangea plants and sully their beauty.
However, you can take steps to keep your plants from becoming infected:
- Prune your plants, so that the insides are open and will not accumulate moisture. (And disinfect your pruning shears or flower-cutting knives whenever you use them!)
- Pick up dead flowers and leaves, since they can harbor fungi.
- Water your plants at the bottom, so the tops will not get wet.
- Control insects, since they spread many of these diseases.
Have you encountered a disease on your hydrangea? If so, let us know in the comments.
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About Helga George, PhD
One of Helga George’s greatest childhood joys was reading about rare and greenhouse plants that would not grow in Delaware. Now that she lives near Santa Barbara, California, she is delighted that many of these grow right outside! Fascinated by the knowledge that plants make chemicals to defend themselves, Helga embarked on further academic study and obtained two degrees, studying plant diseases as a plant pathology major. She holds a BS in agriculture from Cornell University, and an MS from the University of Massachusetts Amherst. Helga then returned to Cornell to obtain a PhD, studying one of the model systems of plant defense. She transitioned to full-time writing in 2009.