Guide: Black Spot on Roses
July is here and so is the balmy weather. While we enjoy the sun by the pool or the beach, our gardens soak up the rays for beautiful blossoms. But….uh-oh, your roses have some odd brown-black spots developing on their leaves. More than likely, your roses have been struck by Black Spot, a fungal disease that primarily affects roses. It’s very common, but unfortunately it’s easier to prevent it than it is to cure it.
Black spot begins on the lower leaves of roses and moves upwards, weakening the plant. It thrives in hot, humid or rainy summers and hot days with cool, damp nights. It can appear as early as when rose leaves first unfurl, which sounds a little ominous. Black spot can overwinter and wait for ideal growing conditions, such as humid, balmy weather.
To prevent this nasty fungal disease, take these steps:
1. Plant your roses in a sunny location with well-draining soil.
Soil that stays too wet too often creates a breeding ground for fungus. You can improve your soil drainage by adding compost or peat moss.
2. Make sure your rose bush has good air flow.
Don’t plant them too close to other plants, and prune them to open up space between the branches.
3. Plant resistant cultivars.
Some especially-resistant roses include the Oso Easy line by Proven Winners, as well as the Knockout series. The yellow rose variety “Julia Child” also has great disease resistance.
4. Water properly.
Roses should be watered thoroughly once a week. Avoid getting the leaves wet while watering.
Remove any infected leaves and perform a thorough cleanup every fall. Spores can remain on the leaves and stems and will reinfect the plant when conditions are favorable. Trim any infected stems about 6 to 8 inches below the infection. Be sure to disinfect your pruners with a 10% bleach solution to prevent spreading to other plants.
Apply a thick layer of mulch around your plants. This will prevent water from splashing up onto the plant and spreading spores.
Should your roses still become infected, there are some topical fungicides you can apply to clear up black spot. Ortho Rose + Flower Disease Control comes in a concentrate and works through the leaves and stems, so it won’t be washed off in the rain. However, it should still be applied during dry weather.
A home remedy uses baking soda, but this method works better as a preventative than a cure. Baking soda will change the pH level on leaf surfaces, making it harder for black spot to infect your plants. Dissolve 1 teaspoon of baking soda into 1 quart of warm water. To allow the mixture to stick to the leaves, add up to 1 teaspoon of dish soap.
Neem Oil also works through the leaves and is an organic fungicide. Don’t apply in the middle of the day; it can burn plant leaves in the hot sun.
Fungicides containing sulfur are also good preventative measures against diseases. It is mildly toxic to humans and animals, so you should wear protective clothing when you spray it. A plastic spray bottle is the best method for applying a sulfur fungicide, as it can corrode metal. It comes as a finely ground powder, so look for a sulfur fungicide that is water-soluble if you prefer to spray it.