How to Treat and Prevent Black Spot Disease on Roses
It happens to just about everyone who grows roses in a humid environment. You plant your roses and watch as they grow beautifully, blessing you with their magnificent blooms and heavenly aroma.
Then, one day you spot it - a few black spots on your rose's leaves. Soon the spots have spread from leaf to leaf, and from plant to plant. The leaves start yellowing, then dropping, leaving your once glorious rose denuded to the canes except for the blossoms.
But don't despair; there is help available for everyone's roses, whether they prefer the natural organic methods of disease control or they are the type who don't mind gardening with chemicals.
I have a few handy tips that may help prevent and/or minimize blackspot infestations, too.
What is Black Spot Disease?
Black spot, Diplocarpon rosae, is a fungal disease specific to roses . It is caused by a common fungus that thrives in wet, warm, and humid conditions.
The spots are black and round with irregular edges. When a fungal infection is acute, the individual spots may appear as one large mass. The leaf material around the spots will begin to yellow, and the leaf will eventually drop off.
Occasionally, black spot disease will spread to the rose cane itself. The affected area will appear purplish at first, then will turn black.
Black spot is not usually a fatal disease for roses, though a weakened, infected plant will not thrive, and will be less able to combat other environmental stressors.
Though many plants may harbor the fungus, black spot disease only affects rose leaves. The fungal spores will overwinter in the soil and in dropped leaf litter. The fungus can be spread by splashing water or the wind.
To begin its life cycle, the black spot fungus needs just the right environmental conditions. It begins to grow when exposed to wet conditions and temperatures of 70 degrees F. Once the fungus invades the leaf and becomes visible, the leaf is permanently damaged.
Can Black Spot on Roses be Prevented?
While you may never be able to entirely prevent black spot disease, there are a few things you can do to greatly reduce the risk of a black spot fungal infection.
- Since the black spot spores need seven hours of continuous exposure to water to "sprout", it is best to water your roses in the morning. The leaves will have a chance to dry before the spores can begin to grow. Obviously, if it rains during the night or evening, there isn't much you can do to prevent the spores from coming to life.
- Water only the soil. Splashing water on infected leaves will spread the fungus from leaf to leaf, and from plant to plant.
- Do not plant your roses too close together. The crowded plants will prevent air from circulating and drying the leaves, allowing the fungus to grow.
- Do not plant the roses where they will receive too much shade. The shade will prevent leaves from drying quickly after a wetting from watering or rain.
Plant selection tips:
- Many varieties of roses are being bred that are resistant to black spot disease. Note that I say "resistant," not immune. The resistant roses may still get black spot, but will tolerate the disease better.
- Some resistant rose varieties include: Henry Hudson, Jens Munk, Charles Albanel, Plena rugosa, David Thompson, John Cabot, and Reine des Violettes. For a more comprehensive list see Paul Barden's BlogSpot.
Environmental prevention tips:
- Clean up infected, dropped leaves immediately
- Dispose of infected leaves properly. Do not use the leaves in a mulch or compost pile. Have them removed with yard debris, or burn them if burning is allowed in your area.
Chemical Black Spot Disease Treatments
Effective fungicides for black spot control should contain at least one of these anti-fungal ingredients:
Follow the directions closely, as most have to be sprayed onto the plants at a regular interval. Remember, the damaged leaves will not recover, but it is possible to keep the black spot disease from spreading.
Unless you have an active insect problem on your roses, it is not necessary to use a fungicide and insecticide combination.
An organic sulfur based fungicide
Organic Black Spot Disease Control
Remember, once the leaf has been damaged it will not recover. These sprays will kill the fungus and/or keep it from spreading.
Baking soda and vinegar: When vinegar and baking soda are sprayed on a fungus, changes the pH of the fungus thus killing it.
1½ tbs baking soda, 2 tbs vegetable oil,1½ tbs liquid hand soap, 1 tbs vinegar. Add to 1 gallon of water and spray every six to seven days onto rose leaves. The soap helps the other ingredients adhere to the leaves.
Sulfur spray: Sulfur has been effectively used by gardeners to control pests and diseases in the garden for centuries. Sulfur sprays and dusts are available at gardening centers, and are honey bee safe.
Use extra caution when using sulfur and do not inhale the dust or fumes.
Baking soda and oil: The baking soda changes changes the pH of the black spot, effectively killing it.
1 gallon water, 3 tsp baking soda, 1 tsp canola oil. Mix and apply mixture weekly. Be aware that spraying roses on hot sunny days may cause the leaves to "sunburn."
Milk and Water: The milk changes the pH of the fungus and kills it.
Make a mixture of one part milk, two parts water. Spray weekly.
Neem Oil: Neem oil is made from the evergreen Neem tree. The oil is an effective antifungicide and is commercially available in many garden centers.
The Neem oil works by preventing the formation of the black spot fungus spores. Neem oil comes in different strengths, so mix according to the bottle's directions.
Be cautious about spraying Neem oil on leaves which get many hours of full, midday sun as sunburn can occur.
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