How to Prevent Hydrangea Diseases

A bountiful display of hydrangeas against a bright blue sky.
What You'll Need
Well-balanced fertilizer
Pruning shears
Hydrogen peroxide

Hydrangeas delight the eyes with spectacular and showy blooms, rich foliage, and long-lasting beauty in the garden. Unfortunately, diseases like powdery mildew and leaf spot can afflict the plant and dim their glow. This guide will help you prevent and identify any ailments your flowers may have.

TIP: Our expert gardening advisor Susan Patterson suggests, "Grow disease resistant varieties of hydrangea for best results."

Powdery Mildew

Powdery mildew appears on the bottoms of lower leaves. It looks like small, circular white patches with feathery edges. The upper leaves may also show purple or yellow blotches. As the cottony mildew spreads and the stalks are destroyed, it can cause stunted and off-color foliage as well as smaller, discolored blooms.

TIP: Susan advises, "Always clean pruning tools in hydrogen peroxide before using."

Plants under stress, especially in conditions with high humidity, are prone to powdery mildew.

To prevent:

  • Regularly water your plants
  • Feed with a well-balanced fertilizer
  • Add mulch
  • Ensure the hydrangea has good air circulation and is not planted too close to other plants

If powdery mildew is already present, buy a good fungicide from the local nursery to combat the disease. Remove dead and diseased hydrangea leaves to control the spread of the disease and to give the plant a chance to regain its health.

Leaf Spot

Landscape and container-grown hydrangea plants can develop Cercospora leaf spot. This disease affects bigleaf, oakleaf, smooth and panicle hydrangea. It won’t kill the plant, but it does diminish the plant’s health and result in fewer buds and an unsightly appearance,

The first sign is brown or purple circular spots on leaves at the base of the plant. Heavily spotted leaves often turn a yellow-green color and fall to the ground. As the diseased leaves fall, they spread the infection to the rest of the plant. Remove the diseased leaves and apply nitrogen to help slow the development and spread of the leaf spot.

To control leaf spot when it first begins, apply a fungicide according to the instructions on the package. Do this especially with highly-valued plants that succumb to the disease every year.

Other Hydrangea Diseases

Hydrangea is susceptible to several other diseases. Here is a brief listing of what they are and how to treat them:

  • Anthracnose – Hot, wet weather conditions help foster development of this disease that normally affects heavily-fertilized hydrangeas. Brown spots appear, grow in size and spread over the plant. To prevent, apply a protective fungicide at 10 and 14-day intervals through the summer. Promptly remove diseased, fallen leaves and blighted flowers.
  • Botrytis Blight – A gray mold, this disease primarily affects the bigleaf hydrangea. It is promoted by cloudy, humid weather. It can appear overnight. Start fungicide treatments when plants begin to bloom and continue throughout summer.
  • Mushroom Root Rot – Common to landscape plantings, this disease most affects oakleaf hydrangea. To prevent, water thoroughly during summer drought and fertilize according to instructions. Fungicides will not prevent this disease, and affected plants must be completely destroyed.
  • Phytophthora Root Rot – Reduce risk of this disease by planting hydrangeas on raised beds amended with 3 to 4 inches of aged pine bark or compost. Fungicides can prevent, but will not cure, this disease.
Preventing diseases is half the battle. With this guide you'll be able to enjoy your hydrangea's beauty for years to come.