Fungus on rose leaves can ruin your entire garden if left unchecked.
Caused by the fungus Diplocarpon rosae, black spot is one of the most common diseases of rose bushes.
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 Fahrenheit.
The spots produce spores that spread to other leaves and plants. The spores can survive the winter in fallen leaves and infected canes.
How to Treat Black Spot
While black spot may seem relentless in your rose garden, it’s actually easy to manage with the right treatment and prevention strategies. Here’s how to control black spot in your garden.
If you’ve noticed telltale black spots on your roses, you should:
Prune away the infected rose leaves and stems. Also, rake up all the fallen leaves under the plant. Throw the infected debris away, rather than putting it on your compost pile.
Once 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 these gardening habits in place, you can keep black spot at bay:
- Water in the Morning: This allows the moisture to evaporate from the leaves.
- Avoid Watering Foliage: Especially if the weather is warm and humid, avoid getting the foliage wet when watering.
- Choose Resistant Roses: Some varieties of rose are resistant to black spot.
- Allow Air Circulation: Choose a planting site with plenty of air circulation, and be sure to space your roses 3-4 feet apart. Prune dense bushes to let air circulate between the branches.
- Choose Sunny Location: Roses prefer full sun anyway, and choosing a sunny location for your rose garden will help burn off moisture.
- Keep Garden Clean: Regularly rake and clean up around your roses, and throw away any fallen leaves. This is especially important in fall and winter, to remove spores that might be awaiting the spring warm-up to become active. After the cold-weather cleanup, apply fresh mulch to keep any remaining spores away from your roses.
- Stay Alert: Inspect your roses regularly, and immediately remove and throw away any spotted leaves, both on the plant and on the ground. If the problem isn’t severe, this may be all the treatment you need.
- Preventive Treatment: If black spot is a perennial problem in your yard, begin a preventive 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.