As far as rose diseases go, black spots are definitely among the most serious diseases to affect these plants. As a fungal disease caused by Diplocarpon rosae, black spot is a highly adaptive and resistant fungus that can and will cause your roses to lose all their leaves.
A rose affected by black spot can eventually even lose its vigor and cause a decline in rose population in areas, where the disease is left uncontrolled.
I’m going to cover the symptoms and causes of black spots on roses, as well as the treatment options available. I will also explain what you can do to prevent black spot disease on roses.
Causes of Black Spots on Roses
As I mentioned, a fungus is responsible for the purple-black spots that develop on the leaves of your roses, causing them to eventually fall off.
The problem with this fungus is that it’s genetically diverse and new strains will develop quickly. So, even if some rose varieties have resistance bred into them, new strains will quickly dismantle this resistance, leaving roses defoliated.
Usually, upper leaves are the first ones to exhibit symptoms. As the fungus spreads, the leaves turn yellow, then fall off. Some leaves will fall off even without turning yellow.
Once black spots appear on the leaves, they remain that way even if treatment is applied. If the treatment is successful, the new leaves that emerge will be healthy.
Young rose canes can also be affected by black, scabby lesions. Even flower buds and flowers can show dark reddish spots.
Besides the aesthetic drawbacks of black spot disease on roses, the plant also becomes vulnerable to other problems. With rose plants completely defoliated, they undergo massive stress, their natural defenses falter, making them vulnerable to other plant diseases and pests.
Dealing with Black Spots on Roses
There are two approaches to treating black spots on roses. One is a non-chemical approach that focuses on prevention and creating unfavorable conditions for the disease, the other aims to solve an existing black spot disease with the help of fungicides.
First, I will discuss how you can prevent this disease from taking over your roses in the first place. Then I will discuss fungicidal treatment options for cases when the disease is already present.
– Preventing Black Spots on Roses
The spores of Diplocarpon rosae, the fungus responsible for causing black spot, enjoy warm and humid conditions.
The spores of the Diplocarpon rosae can overwinter on fallen leaves and when environmental conditions are favorable, water droplets can splash them onto healthy leaves, causing reinfection.
Here’s what your roses need to prevent infections or reinfections:
- Plant your roses in a sunny location (full sun is ideal)
- Avoid overhead sprinkles and water only at the root level
- Clean dead, diseased or fallen leaves off the ground
- Apply a thick layer of mulch around the plant to prevent the soil from splashing spores onto the plant
- Prune your roses to remove diseased leaves and to improve air circulation between the canes
Applying topical insecticidal sprays as a preventative such as neem oil, baking soda, sulfur, Bordeaux mix can also help keep spores from taking a hold of your roses.
– Treating Black Spots on Roses
You also need to spray your roses with fungicides such as:
- Tebuconazole with trifloxystrobin
You can also use insecticides that contain the above ingredients, but fungicides are your best bet against clearing up a black spot infestation.
It will usually take multiple applications over a period of 7 to 10 weeks to treat a black spot infestation, but remember that once a leaf is attacked, it will not revert back to its previous state.
Can Vinegar Cure Black Spots on Roses?
White vinegar can be used to prevent black spot disease on roses. Vinegar has antifungal properties and it’s a natural way to keep a black spot infestation from coming back, but it can also be used to treat an already ongoing infestation.
A white vinegar mixture I found helpful is the one that combines 1 tablespoon of white vinegar, 1 tablespoon of baking soda, 1 tablespoon of superfine horticultural oil added to 1 gallon of water.
The mixture can be sprayed on affected leaves or used as a preventative treatment.
Can You Use Hydrogen Peroxide for Black Spots on Roses?
Gardeners also report having successfully eradicated black spot diseases from their roses by using hydrogen peroxide (H2O2) diluted in water (1 tablespoon of H2O2 at 3% concentration added to 1 cup of water) or in combination with other anti-fungicides.
Hydrogen peroxide is widely used as a foliar spray to control fungal issues and pest problems in many other plants, therefore, it’s known among gardeners for its antifungal effects.
Do Rose Black Spots Infest other Plants?
Although named after the roses it affects, the fungus causing rose black spots are not exclusive to roses and they can spread and infect other plants as well.
It usually appears in spring in moist conditions and runs rampant until temperatures rise above 85 °F, creating an unfavorable environment for the fungus.
Any plant with green, fleshy leaves can be affected by rose black spots, so it’s important to control an infection as soon as you notice signs of it.
Black spots on roses are not harmless and they should be addressed as a serious threat to your roses. You should try your best to remove diseased leaves and sections by pruning your rose.
You should also practice preventative care and create conditions unfavorable for the spread and viability of fungus spores such as clearing away fallen leaves, mulching your roses, planting them in a sunny location, and practicing good watering habits.
An active infection should be treated with natural or commercial fungicides until the problem goes away, but periodically applying preventative treatment is even better.
Rose black spots will easily defoliate roses, weaken the plant’s natural defenses and open the way to other diseases and problems.