How to Control Black Spot Fungus Disease on Roses
By: Julie Day
Black spot (blackspot) is one of the most common diseases of rose bushes; and, if left unchecked, it can cause quite a bit of damage to your rose garden. Caused by the fungus Diplocarpon rosae, black spot begins just as its name suggests, with black spots showing up on the surface of the leaves.
As the spots grow larger, they become ringed with yellow, eventually causing the whole leaf to turn yellow and fall off. The stems may also have black or purplish spots. If not treated, black spot will leave your rose garden not only naked, but significantly weakened and unprepared for the next winter.
Like most fungal diseases, black spot thrives in wet, humid weather, usually when temperatures are in the mid-70s F. The spots produce spores which spread to other leaves and plants. The spores can survive the winter in fallen leaves and infected canes.
While black spot may seem relentless in your rose garden, it’s actually fairly easy to manage with the right treatment and prevention strategies. Here’s how to control black spot in your garden.
How to Treat Black Spot
If you’ve noticed telltale black spots on your roses, you should:
Prune away the infected leaves and infected stems. Also, rake up all the fallen leaves under the plant. Throw the infected debris away, rather than putting it on your compost pile.
Spray RosesOnce the infected leaves and stems have been removed, treat your rose with a fungicide. Spray the entire plant, making sure to get the tops and bottoms of the leaves as well as the stems. You can use a chemical fungicide or any number of organic options such as:
- Lime Sulfur
- Neem Oil
- Potassium or Ammonium Bicarbonate
How to Prevent Black Spot
With the right gardening habits in place, you can keep black spot at bay on your roses by:
- Preventative Treatment: If black spot is a perennial problem in your yard, begin a preventative treatment with an organic, nontoxic, fungicidal spray (such as those on the list above), right before your roses sprout in spring. Continue treatment every couple of weeks to keep the fungus under control.