How to Treat a Black Spot on Hydrangeas

When trying to determine how to treat black spots on hydrangeas, it is important to first understand the cause for the black spot. Hydrangeas are susceptible to certain varieties of fungus; the fungus travel in spores most often through rainfall that spreads disease from fallen leaves and other hosts like trees and plant debris. An infected hydrangea develops dark spots as a symptom of the fungus. To treat black spots on hydrangeas, follow some simple steps to reestablish the health of your plant.

Things You'll Need

• Flower scissors
• Fungicide
• Gardening gloves
• Compost

Instructions

1. Remove traces of disease on and around your hydrangeas. Trim away leaves and blooms that display black spots and collect fallen diseased leaves from around your plant.

2. Water and add nitrogen to your plants correctly. To treat black spots on hydrangeas, Aces.edu suggests surface watering and making sure your soil has enough nitrogen to provide your hydrangeas with a stable growth rate. To maintain balanced soil and nitrogen levels, consider adding compost around your diseased hydrangea to enrich the soil; according to RubyGlen.com, adding fertilizer to a sick hydrangea is too stressful to an already weakened plant.

3. Determine what type of fungus is infecting your hydrangea. The two main fungal spores that infect hydrangeas and cause spots are "cercospora" and "anthracnose." Cercospora leaf spots are scattered, small, circular spots that appear black, brown, or purple; spots are first visible on the leaves near the base of your hydrangea. On bigger hydrangeas, the centers of the spots lighten and create a halo effect. Cercospora spots are one eighth to one quarter of an inch in diameter. Anthracnose leaf spots are sunken into the leaf, circular (sometimes irregular) in shape, and appear black or brown. The contrast between dark and lighter rings of dead leaf areas can cause the spots to resemble a bull's-eye. The center of the spots reach a diameter of approximately 1 inch.

4. Use a fungicide appropriate to your hydrangea's particular fungal infection. A fungicide may help treat the disease so your plant can return to health. Aces.edu suggests applying a fungicide the moment you notice spots on your hydrangeas and continuing applications as needed.

Tips & Warnings

• Always wear gloves when dealing with diseased plants or chemicals, or when handling sharp tools to prevent injury.

References

• Aces.edu: Diseases of Hydrangea
• RubyGlen: Hydrangeas