How to Identify and Treat Anthracnose on Hydrangeas

How to Identify and Treat Anthracnose on Hydrangeas

Hydrangeas can be lush, carefree plants, but they are also vulnerable to a number of different fungal infections that can sully the leaves – and in some cases, the flowers.

A vertical picture showing a hydrangea plant suffering with a disease causing the leaves and flowers to turn brown. Toit eh center and bottom of the frame is green and white text.Symptoms

The first indication that your hydrangea is infected with will be brown spots on the leaves. Unlike those caused by other pathogens such as Botrytis (grey mold), these spots will be circular or slightly irregular. The center of the spot will later turn tan.

A close up of a leaf suffering from a fungal disease that causes dark brown lesions.Photo by Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org.

If the spots border a leaf vein, they can develop an angular shape. If conditions are favorable, whole leaves and flower petals can develop large, irregular dark brown spots that look like blotches.

A close up of a watermelon leaf with the fungal infection anthracnose causing dark brown lesions along the leaf veins.Anthracnose lesions on leaf. Photo via Alamy.

In addition, the stems can develop sunken areas with raised margins known as cankers. These can be serious, since they can encircle the branches and stems and kill them.

A close up of a canker on the stem of a plant caused by a fungal disease called anthracnose.Photo by Cesar Calderon, Cesar Calderon Pathology Collection, USDA APHIS PPQ, via Bugwood.org.

New growth may be crooked or deformed, making the branches look gnarled. A severe infection that has spread throughout the plant and caused deformed stems can kill the hydrangea.

How to Distinguish Anthracnose from Cercospora Leaf Spot

Anthracnose is often confused with Cercospora leaf spot, but there are key differences.

While Cercospora also forms circular spots on hydrangea leaves, the lesions start out purple.

As they grow larger, they develop lighter colored centers that look like frog eyes. When this infection gets established, whole leaves can turn purple. (This is not to be confused with a phosphorus deficiency.)

A close up of a hydrangea suffering from Cercospora leaf spot, pictured in bright sunshine on a soft focus background.Hydrangea suffering with Cercospora leaf spot, not to be confused with anthracnose.

Also, a key difference is that when your hydrangea is infected with Cercospora, you will see spots on the lower leaves first.

In the case of an anthracnose infection, the lesions can appear simultaneously throughout the top and bottom parts of the plant.

A final difference is that Cercospora will not infect the flowers.

Conditions That Favor Infection

Anthracnose is typically a disease seen in hot, wet conditions.

A close up of a large hydrangea growing behind a concrete retaining wall, with pink and blue flowers and bright green foliage.Prevention

There are some steps you can take to keep this infection from establishing itself in your prized plants.

This disease is spread by water, so avoid spraying the flowers and foliage of your plants when you water.

You are much better off watering at the base of the plant. Hydrangeas like a lot of water, but ideally you should water deeply with a soaker hose and then let the soil dry to the touch in between waterings.

Another tip is to not over fertilize the plants.

Dr. Fulya Baysal-Gurel, et al of Tennessee State University College of Agriculture, Human, and Natural Sciences report that hydrangeas that have been heavily fertilized may be more susceptible to anthracnose.

Management

Once this disease takes hold, it is very difficult to eradicate. However, you can take steps to control the infection.

Since anthracnose can be such a difficult disease to treat, when you see the first signs, you should take cuttings from healthy parts of your plant.

By taking cuttings, if the disease does end up killing up your hydrangea, you will at least have a replacement.

Learn how to propagate hydrangeas from cuttings here.

Pruning and Sanitation

If you see signs of infection, prune out as much of the diseased tissue as you can, to prevent the infection from spreading. The fungus can overwinter in plant debris, which can then serve as a source of infection the following spring.

Pick up any fallen leaves or other plant debris on the ground under your hydrangea and dispose of it away from your plants. And do not put it in your compost pile.

Make sure to disinfect your pruning shears afterwards, with a solution of 10 percent bleach or 70 percent rubbing alcohol.

Pick up any fallen leaves or other plant debris on the ground under your hydrangea and dispose of it away from your plants. And do not put it in your compost pile.

Fungicides

You may not be able to totally eradicate the infection, but you can prevent it from spreading by treating the plants with fungicides.

A close up of the packaging of a Copper Fungicide, to use for treating plant diseases.Bonide Copper Fungicide Dust

Spraying with a copper-based fungicide can be effective, such as Bonide Copper Fungicide Dust, available at Arbico Organics.

Copper fungicides are a popular choice, because they are organic.

However, one thing to keep in mind is that the copper can be toxic to plants.. For example, the use of acidic water that is below pH 6.0-7.0 can lead to excessive amounts of copper ions that can damage the foliage.

Do not spray with copper when the weather is cold and moist. The copper sprays take longer to dry under these conditions and are more likely to be toxic to your hydrangea.

Other options include chlorothalonil or mancozeb. Both of these compounds are non-selective and will treat a range of fungi.

A close up of the packaging for Fung-onil, a multi-purpose fungicide for treating plant diseases.Bonide Fung-onil Concentrate

You can find chlorothalonil as Bonide Fung-onil Concentrate available from Tractor Supply.

Mancozeb has an advantage in that it is relatively inexpensive and has had fewer reports of fungi becoming resistant.

Bonide Mancozeb Flowable with Zinc Concentrate

Bonide Mancozeb Flowable with Zinc Concentrate contains mancozeb and is available from Redwood Chemical Products via Amazon.

You should also consider spraying healthy hydrangea plants on your property to protect them from the infection.

If you have problems one year, you should treat your plants the following season, since the infection will remain in the area.

Spray your plants every 10-14 days throughout the summer. However, if the infection is severe, you can spray more frequently.

You might want to buy two types of fungicides and alternate them, so that the fungus does not develop resistance to the chemical and then be able to spread unchecked!

And be sure and give time for the fungicides to take effect. It might take a week or more to see a difference, so don’t lose hope if you don’t see any changes after a couple days.

A Wealth of Spores Can Cause Severe Infections

While anthracnose is typically a problem in large plantings in greenhouses or fields, this fungus can also plague home gardeners.

A close up of a hydrangea leaf suffering from a fungal infection called anthracnose causing dark brown lesions to appear on the foliage.growing hydrangeas, you’ll need these guides next:

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.

“How to Identify and Treat Anthracnose on Hydrangeas” was first posted here

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