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Black Spot on Roses - What to Do and How to Prevent It.

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Black Spot on Roses
What to do about black spot on roses.

This is becoming a severe infection. The black spots are starting to merge and will soon cover the leaves and cause them to fall.

Photo: © Marie Iannotti

What is Black Spot?

Black spot is a fungal disease (Diplocarpon rosae), that affects roses. Ideal conditions include hot, humid or rainy summers and hot days with cool, damp nights.

Symptoms:

Somewhat circular black spots on leaves. They usually occur on the upper sides of leaves, but can also develop on the undersides. The outer margins of the black circles are ragged or feathery and they are usually surrounded by a ring of yellow. Spots begin on the lower foliage and move upward. They can appear as early as when the leaves first unfurl. These spots can enlarge and merge. Severally affected leaves often fall off the plants and left unchecked, the entire plant can defoliate.

The fungus can also infect young canes, causing dark purple or black blisters, and even the flowers may show some red spotting. Infected plants will set less flower buds and without leaves, the plant becomes stressed and is susceptible to even more problems.

Cultural Controls:

  • Growing Conditions - Make the growing conditions less hospitable for black spot. Black spot is easier to prevent, than to cure. The spores can remain in the soil and overwinter on leaves and stems, waiting for favorable conditions. They make contact with the rose by splashing up in drops of water.

    Give the plant what it wants. This is true for avoiding any plant disease. A healthy, vigorous plant is less susceptible to problems. Roses prefer a sunny location with well-draining soil and regular weekly watering.

  • Air Flow - Provide good air circulation around and through plant. Don't plant your roses too close to other plants. Prune, if the plant gets too dense and air can not get through.

  • Proper Watering - Avoid getting the leaves wet, while watering. There's not much you can do about rain, but at least limit the time the leaves remain wet.

  • Plant resistant cultivars - Roses are often labeled for resistance, from highly resistant on down. While you are looking for black spot resistance, you might as well look for a rose that is also resistant to rust and powdery mildew. Rugosas, the newer shrub and ground cover roses and many of the Canadian Explorer Series, like John Cabot and William Baffin, show good resistance.

  • Sanitation - Remove any infected leaves and always do a thorough clean up each fall. Remove and dispose of any remaining leaves, when you do your dormant pruning in late winter/early spring. Spores can remain on leaves and stems throughout and will reinfect whenever conditions are favorable. Within 10 days of the first symptoms, the disease has already started spreading. Spores can spread by water and wind.

    Prune out any canes showing signs of infection. Prune 6 – 8 inches below the infection and only prune in dry weather. Disinfect your pruners with a 10% bleach solution or alcohol, between cuts.

  • Mulch - Apply a thick layer of mulch around the plants. Mulch will prevent water from splashing up on the plant and spreading spores.

Sprays for Treatment and Prevention

It is easier to prevent black spot than to cure it, but it is time consuming. If black spot is a reoccurring problem in your area, you may need to spray your plants weekly, starting in early spring.
  • Baking soda spray - Dissolve 1 teaspoon baking soda in 1 quart warm water. Add up to 1 teaspoon liquid soap. Spray leaves thoroughly. This mixture works better as a preventive, than as a cure. Also offers some protection from powdery mildew.

  • Bordeaux Mix - This is a fungicide that contains copper sulfate and hydrated lime. It can be used as a powder or mixed with water and sprayed. Bordeaux mix also repels some insect pests, but it can burn plant leaves. It is generally used as a preventative in the spring, before plants leaf out.

  • Insecticidal soaps with added fungicide - These are basically some type of organic fungicide, often sulfur, added to regular insecticidal soap. The soap helps the fungicide adhere to the plant leaves and spread to coat them.

  • Neem Oil - Neem is an organic fungicide and pesticide, derived from the seeds of the neem tree. It gets inside the plant's system, so you don't need to worry about coating everything or reapplying after a rain. However it can burn plant leaves in hot sun. You should not apply need oil within 2 weeks of using a product containing sulfur.

  • Sulfur - Sulfur both prevents and gets rid of fungus diseases. It is also used to control several insect pests. However it can be mildly toxic to humans and other animals. You should wear protective clothing when you spray it. It can also corrode metal, so use a plastic sprayer . And it can burn plants leaves in hot weather.

    Sulfur comes as a finely ground powder. If you prefer to spray it on, look for one labeled as wettable, so that it will mix with water.

Sources:

"The Organic Gardener's Handbook of Natural Pest and Disease Control", by Fern Marshall Bradley, Barbara W. Ellis, Deborah L. Martin (Rodale, 2010)

University of Illinois Extension Controlling Black Spot Disease of Roses

University of Wisconsin Extension Black Spot

American Rose Society What is Black Spot?

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