Look out for these leaf spots on your hydrangea plants.
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Hydrangea Leaf Spots
Credit: Steve Bender

Call the paramedics! Your hydrangea's leaves have spots on them! What, oh what can prevent your beloved plant's imminent demise?

Don't worry. Nothing dire is going on. Something very common is. Your hydrangea has leaf spots caused by a fungus (Cercospora or Phyllosticta hydrangea). To demonstrate just how common this is, that's Grumpy's hydrangea in the picture above. I know what you're thinking: "Well, if it could happen to Grumpy, it could happen to anybody." You are correct.

It may be different where you live, but here in Alabama, it's been oppressively hot and humid with almost daily thunderstorms. While these muggy weather conditions make people miserable, the leaf spot fungus kicks up its heels to make the hydrangeas miserable as well. Humidity, heat, and splashing water provide the perfect conditions for the fungal spores to hop from leaf to leaf. Pretty soon your hydrangea looks like it contracted chickenpox.

How To Get Rid of Fungus on Hydrangeas

Fear not. This is an easy enough malady to cure. Just spray your hydrangea's foliage according to label directions with a fungicide such as liquid copper, Serenade Disease Control, Daconil, or Immunox. This won't do anything to get rid of the spots already there, but it will prevent any more of them from forming. Pick up any spotty leaves that have fallen and throw them out with the trash. Do not water your hydrangea with overhead sprinklers. This spreads the spores just like rainfall does. When all the hydrangea's leaves drop in autumn, rake them up and toss them out with the trash again.

More Hydrangea Diseases To Look Out For

Unfortunately, there are even more fungal, viral, and even bacterial diseases that can affect your hydrangea's appearance, health, and longevity. Here are a few of them.

Botrytis Blight Fungus (Botrytis cinerea)

How it happens: Cool, damp conditions and cramped space.

How to spot it: Botrytis Blight attacks flower buds and is even able to kill them off before they begin to open. When the infected flower parts fall onto the leaves below, the fungal infection is able to spread to the leaves as well. Usually, the first sign of this fungus is water-soaked spots on the flowers. Then, reddish/brown-looking lesions.

How to treat or prevent it: Since Botrytis Blight thrives in cool, damp conditions, do your best to keep your hydrangeas in low humidity, keep good airflow around them, and space them out properly. Furthermore, watering earlier in the day and concentrating the water on the root rather than the flowers and leaves will help. When you prune, make sure your shears have been treated with bleach in order to prevent the further spread of any present disease. Also, be sure to remove any dead, fallen, or damaged flowers and leaves. If the fungus persists, treat it with fungicides.

Anthracnose Fungus (Colletotrichum gloeosporioides)

How it happens: Heavily fertilized plants under heavy fog or regular rainy weather.

How to spot it: Large brown circular or slightly irregular spots, lighter colored at their center, begin to appear on the flowers and leaves of the hydrangeas.

How to treat or prevent it: Water at the base of the plant instead of spraying the flowers and leaves. You should trim any diseased leaves or flowers as soon as you spot them. Treat the diseased plant with copper-based fungicide. Liquid kelp, hydrogen peroxide, and garlic oil are also useful.

Bacterial Leaf Spot (Xanthomonas campestris)

How to spot it: The leaves and flowers developed water-soaked spots that darken and become angular. When these spots grow, they become more like lesions and begin to kill off the leaves.

How to treat or prevent it: Bacteria is able to enter the plant through openings and wounds. If you notice these are your hydrangeas, you can take the initiative to protect them from bacteria by treating them with copper hydroxide (Kocide).

Tomato Ringspot Virus

How to spot it: The leaves of the hydrangeas begin to yellow and the growth of the plant is below average.

How to treat or prevent it: This virus is spread by nematodes. So, make sure you are using soil or a mixture that is nematode-free. If your hydrangeas already live in soil with nematodes, you can treat is with a nematode repellent.