Treating Black Spots on Roses

Overview

Black spot is a fungus that only infests roses. The disease occurs in places that get a lot of rain in the summertime and have high humidity levels. Black spot first appears as dark circles on new rose leaves. The circles, which are irregularly shaped, may have yellow rings around them. The fungus kills the leaves, which then fall off the rose. A badly-infested plant may lose all its leaves, which can kill it.

Step 1

Treat your infested roses with a mixture of 2 tsp. baking soda and an equal amount of summer oil, combined with 1 gallon of water. Mix the ingredients together, put the liquid into a spray bottle and spray the roses well. Apply this mixture once a week for best results.

Step 2

Treat your infested roses with homemade soap spray. Mix ½ tsp. of dish soap, 1 tsp. of cooking oil and 1 qt. of water. Put the combination into a spray bottle and apply it to the roses. After the initial treatment, spray the roses again once a week until the infestation is gone.

Step 3

Treat black spot with calcium sulphide or calcium polysulphide, usually sold as a concentrate and labeled "lime sulfur." Shake the container of concentrate well, mix it with water according to the directions on the packaging and spray it on your roses. Reapply according to the manufacturer's directions.

Step 4

Apply a stronger chemical fungicide such as triforine, chlorothalonil or thiophanate-methyl. Follow the package directions very carefully, and wear a breathing device, protective clothing and goggles.

Tips and Warnings

  • Fungicides like triforine, chlorothalonil and thiophanate-methyl are very effective for treating fungal infestations, but they are also very powerful chemicals that can cause serious health and environmental problems, like respiratory ailments and ground water contamination.

Things You'll Need

  • Baking soda
  • Summer oil
  • Mild dish soap
  • Cooking oil
  • Calcium sulphide or calcium polysulphide (lime sulphur)
  • Spray bottle
  • Triforine, chlorothalonil or thiophanate-methyl

References

  • Black Spot: Sunset Magazine
  • Getting to Know Low-Impact Pesticides: Calcium Sulphide or Calcium Polysulphide (Lime Sulphur); The Green Pages of the Montreal Botanical Garden
  • Synthetic Fungicides: Sunset Magazine

Who Can Help

  • Gentle Pest Controls: Sunset Magazine
Keywords: fungal diseases of roses, black spot fungus, treating black spot

About this Author

Cheyenne Cartwright has worked in publishing for more than 25 years. She has served as an editor for several large nonprofit institutions, and her writing has appeared in a variety of publications, including "Professional Bull Rider Magazine." She holds a Bachelor of Arts in English from Oklahoma Christian University and a Master of Arts in English from the University of Tulsa.