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.
You know that old saying, “There’s a fungus among us?” There is some truth to that. A fungus called anthracnose can infect a large number of tropical and temperate plants, and it is widespread throughout the world.
Unfortunately, hydrangeas can be one of its victims, and bigleaf hydrangeas – Hydrangea macrophylla – are particularly susceptible.
In this article, we’re going to help you to prevent, diagnose, and treat anthracnose in your hydrangeas.
I’ll cover the following topics:
Anthracnose in Hydrangeas
The first indication that your hydrangea is infected with Colletotrichum gloeosporioides will be brown spots on the leaves. Unlike those caused by other pathogens such as Botrytis (aka gray mold), these spots will be circular or slightly irregular. The center of each spot will later turn tan.
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.
In addition, the stems can develop sunken areas with raised margins known as cankers. These can be serious, since they can encircle or girdle the branches and stems and eventually kill them.
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 that you may observe.
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 becomes established, whole leaves can turn purple. Keep in mind that this is not to be confused with a phosphorus deficiency either, which may also turn foliage purple.
Another 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 notable difference is that Cercospora will not infect the flowers.
Conditions That Favor Infection
Anthracnose is a disease typically seen in hot, wet conditions.
The increased moisture facilitates the spread of the spores, which then infect other areas of the plant. It can also spread to other hydrangeas. The ideal temperatures for infection range from 75 to 90°F.
Several days of wet weather and high temperatures dramatically increase the chances of infection, since the spores spread more quickly.
If there are infected leaves on the plant or leaf debris on the ground beneath it, fruiting bodies will form masses of spores.
All it takes is a bit of rain or overhead watering to spread these spores to other leaves – and even flowers. Prolonged periods of dew and heavy fog also facilitate the spread of this pathogen.
In addition to the masses of spores found in infected tissue on your hydrangeas, this pathogen is widespread in the surrounding environment. Therefore, it is likely to be present in your area and may pose a risk to your hydrangeas whenever the weather is favorable.
The good news is that there are some steps you can take to keep this infection from becoming established 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 become dry to the touch in between waterings.
Another tip is avoid over-fertilizing 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.
Once this disease takes hold, it is very difficult to eradicate. But don’t fret! You can take steps to control an infection, or start new plants if you need to.
Since anthracnose can be a difficult disease to treat, when you see the first signs of infection, I recommend taking cuttings from healthy parts of your plant right away.
By taking cuttings that you may root and transplant into the garden, if the disease does end up killing up your hydrangea, you will at least have a few replacements ready to go.
Learn how to propagate hydrangeas from cuttings here.
Next up, let’s take a look at the available options to manage an anthracnose infection.
Pruning and Sanitation
If you see signs of infection, prune out as much of the diseased plant tissue as you can, to prevent the infection from spreading. Trim away any diseased foliage and branches, and gather them for disposal.
Be aware that although hydrangeas can generally tolerate quite a hard pruning, if you have to remove more than 1/3 of the plant, it may not recover.
The fungus can overwinter in plant debris, which can then serve as a source of infection the following spring. Do whatever you can to prevent this as well.
Pick up any fallen leaves or other plant debris on the ground under your hydrangea and dispose of it away from your plants. Preferably, you should place it in a tied bag in the garbage. Do not put infected plant material in your compost pile.
Make sure to disinfect your pruning shears afterwards, with a solution of 10 percent bleach or 70 percent rubbing alcohol. You may also want to consider cleaning your tools between cuts or between plants while you are pruning, to avoid additional unwanted spread.
You may not be able to totally eradicate the infection, but you can prevent it from spreading further by treating the plants with fungicides after pruning.
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. Simply spray or dust your plants according to package instructions.
Other options include chlorothalonil or mancozeb. Both of these compounds are non-selective and will treat a range of fungi.
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 contains mancozeb and is available from Redwood Chemical Products via Amazon.
Spray your plants with your choice of fungicide every 10-14 days throughout the summer.
If the infection is severe, you can spray more frequently. But be sure to allow enough time for the fungicide 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 of days.
You should also consider spraying any healthy hydrangea plants on your property to protect them from the infection.
If you have problems one year, you should also treat your plants the following season, since the fungi will likely remain in the area.
You might want to buy two types of fungicides and alternate them, so the fungus does not develop resistance to the chemical and gains the ability to spread unchecked!
A Wealth of Spores Can Cause Severe Infections
While anthracnose is typically a problem found in large plantings of hydrangeas in greenhouses or fields, this fungus can also plague home gardeners.
Once it takes hold on a hydrangea plant, the infected leaves or old debris from the previous year can be a source of unending spores in hot, wet weather.
You will have to take aggressive action to save your plants, including pruning out infected tissue and treating the plants with fungicides.
However, with quick action, you have a good shot at limiting the infection.
Have you won a battle with anthracnose? Let us know how you fared in the comments below!
And for more information about growing hydrangeas, you’ll need these guides next:
- How to Identify and Treat Hydrangea Diseases
- The 25 Best Hydrangea Varieties for Landscaping Large and Small
- How to Grow Hydrangeas for Big Blossomed Beauty
© Ask the Experts, LLC. ALL RIGHTS RESERVED. See our TOS for more details. Product photos via Arbico Organics, Bonide, Tractor Supply, and Redwood Chemical Products. Uncredited photos: Shutterstock.
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 childhood discovery 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.