How to Treat and Prevent Black Spots on Roses
Marie Iannotti / The Spruce
Black spot is a fungal disease (Diplocarpon rosae) that affects roses. The fungus develops as black spots on the leaves, which eventually causes the leaves to turn yellow and drop off. Besides looking unsightly, it can seriously weaken the rose plant. Black spot thrives during hot, humid, or rainy summers and hot days with cool, damp nights.
What Does Black Spot Do?
Black spot will look like somewhat circular black spots on leaves. It usually occurs 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 leaves and move upward. They can appear as early as when the leaves first unfurl. These spots can enlarge and eventually merge. Affected leaves often fall off the plants, and if left unchecked, the entire plant can defoliate.
The fungus can also infect young canes, causing dark purple or black blisters on the canes, and even the flowers may show some red spotting. Infected plants will set fewer flower buds and without leaves, the plants become stressed and susceptible to more problems.
Environmental Controls for the Problem
Black spot is easier to prevent than to cure. Existing spores can remain in the soil and overwinter on leaves and stems, waiting for favorable conditions. The spores make contact with the rose by splashing onto it in drops of water.
Give the Plant Ideal Growing Conditions
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.
Good Air Flow
Provide good air circulation around and through your rose plants. Do not plant your roses too close to other plants. Prune to open the spaces between canes, if the plant gets too dense and air cannot get through.
Avoid getting the leaves wet while watering. There is not much you can do about rain, but at least limit the time the leaves remain wet.
Remove any infected leaves and always do a thorough cleanup 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 and can 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 to 8 inches below the infection and only prune in dry weather. Disinfect your pruners with a 10 percent bleach solution or alcohol between cuts.
Apply a thick layer of mulch around the plants. Mulch will prevent water from splashing up on the plant and spreading spores.
Topical Sprays for Treatment and Prevention
There are commercial and homemade, DIY-solutions you can use to try to cure black spot. The treatment may seem time-consuming; it is a pesky problem. And, if after you have treated it, the black spots reoccur, you may need to spray your plants weekly starting in early spring.
- Baking soda spray: Dissolve 1 teaspoon baking soda in 1 quart of warm water. Add up to 1 teaspoon of liquid soap. Spray leaves thoroughly. This mixture works better as a preventive than as a cure. It 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: You can use an organic fungicide, which is often sulfur added to regular insecticidal soap. The soap coats the leaves and helps the fungicide adhere to the plant.
- 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 do not need to worry about coating everything or reapplying after rain. However, it can burn plant leaves in the hot sun. You should not apply neem oil within two weeks of using a product containing sulfur.
- Sulfur: Sulfur prevents and gets rid of fungus diseases. It is also used to control several insect pests. Sulfur comes as a finely ground powder. If you prefer to spray it on, look for one that is labeled as wettable so that it will mix with water.
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.
Roses are often labeled for resistance, from highly resistant on down. If you are looking into black spot-resistance, you might as well look for a rose that is also resistant to rust and powdery mildew. Rugosas, a newer shrub and ground cover rose, and many of the Canadian Explorer series roses like “John Cabot” and “William Baffin,” show good resistance.
Black spot is one of the most common rose diseases. It weakens the rose and ruins its appearance. Here's how to treat and prevent it.
How to Cure Black Spots on Plants
Black spots are among the most common symptoms of plant disease, whether the problem is a fungus or a bacterial infection. A problem for the casual gardener is that the spots of various diseases may look strikingly similar, and knowing the best way to cure black spots on plants depends on diagnosing the cause. Always begin by researching the common diseases of the plant species. It’s important to reach a define diagnosis before you attempt to treat the plant. Fortunately, a consistent control program is all that’s needed to deal with most plant diseases.
Examine the plant for other disease symptoms known to be associated with black spots on the species. For example, search roses for other black spot disease symptoms caused by the fungal pathogen Diplocarpon rosae, including diminished health, black spots on upper sides of leaves, yellowed foliage, and leaf drop. Look over canes for the presence of purple-hued areas of dying plant tissue. Look closely at spots for the presence of tiny fruiting bodies.
Examine both deciduous and evergreen trees and shrubs displaying black spots for signs of anthracnose. The University of California Integrated Pest Management Program says symptoms to look for include black, “tar-like” spots, necrotic leaf tissue, cankered branches and curled leaves. Examine trees that lose their leaves early for distorted branches.
Regularly monitor vegetable crops such as tomatoes. Look for blight issues, such as early blight, caused by the fungal pathogen Alternaria solani, which causes black spots in the form of a bull’s eye on lower leaves. Examine the ground for yellowed leaves that have dropped early. Search fruit for dark, sunken areas of plant tissue. Touch spots to see if they are wet, a sign of late blight caused by the fungal pathogen Phytophthora infestans, which may cause large, callused spots.
Examine fruit trees for the presence of a bacterial spot infection caused by the bacterial pathogen Xanthomonas pruni. Look for tiny, black, irregular spots, which group near the ends of leaves and along veins. Spots may appear wet. Search for areas on the leaf where spots have died and fallen from the plant, leaving holes.
Examine ornamental plants, as well as fruits and vegetables, for Botrytis blight, also referred to as gray mold. Search for symptoms caused by the fungal pathogen Botrytis cinerea, including raised black spots on plant tissue, wet spots on flowers and other soft plant tissue, and the tell-tale sign of infection, a gray-hued, soft mold growth on areas of dying plant tissue.
Control black-spot diseases with cultural methods first, as employing toxic chemicals when they’re not needed can cause needless harm to desired plants and the environment, and may contribute to chemical resistance.
Use pruning shears to remove affected plant parts, and destroy the removed material, including fallen debris. This helps to reduce the severity of disease and prevent its spread. Sterilize all equipment, such as pruning tools, between each cut and each plant. Wash your hands regularly, or change gloves, to guard against becoming an agent of disease transfer.
Avoid overhead watering. Instead, irrigate soil directly with methods such as drip irrigation. Overhead irrigation causes standing water on leaf surfaces, which can provide the ideal environment for proliferation of both fungal and bacterial pathogens.
Put a 2- to 3-inch layer of organic mulch such as wood chips under surrounding plants without pressing it against stems or trunks. Mulch assists in weed prevention, conserves moisture and protects plants from accidental injuries that may weaken plants and provide an entry point for disease.
Apply low-toxicity fungicides or bactericides when possible, to avoid damaging the balance of beneficial bugs in the garden and to prevent further injury to the desired plant. Saturate roses with neem oil or a sulfur-based pesticide, for example.
Apply higher-toxicity fungicides and bactericides when the problem is severe or when low-toxicity methods fail. Follow the directions on the labels carefully. Though some diseases respond best to fungicide applications as a preventive measure, fungicides also can decrease the intensity of disease or protect new growth.
Apply a fungicide with an active ingredient, such as chlorothalonil or mancozeb, to roses with black spot disease every one to two weeks. Apply fungicides with chlorothalonil to plants affected with anthracnose at bud break, and again in two weeks in the case of wet weather; apply to blighted vegetables as well, before symptoms occur and during active growth. Apply chlorothalonil to plants infected with gray mold as a preventive measure once every five to seven days in wet weather or every week to 10 days during warm, dry weather, applying only one-third the usual rate to plant blossoms, advises the University of Illinois Extension. Apply a copper-based spray and an antibiotic, alternating between the two, to prevent and control bacterial spot.
How to Cure Black Spots on Plants. Black spots are among the most common symptoms of plant disease, whether the problem is a fungus or a bacterial infection. A problem for the casual gardener is that the spots of various diseases may look strikingly similar, and knowing the best way to cure black spots on plants …