Black Spot is a common rose disease caused by the fungus, Diplocarpon rosae. Black spot begins as small brown or black spots on leaves, which eventually causes the leaves to yellow and fall off. The disease does not kill the plant, but will weaken the plant over time. The rose plant will produce fewer blooms and is more susceptible to winter kill. The disease is active during cool, moist weather.
The best way to control black spot is prevention. Here are some tips on preventing black spot on your roses:
- Some roses are more prone to disease, such as hybrid tea roses, floribundas, and climbers. Choose disease resistant varieties of roses, like antique roses. Talk to a local garden expert to learn about the best roses to grow in your area.
- As fungi thrive in cool, moist areas, it’s best to plant your roses in a sunny location-one that receives at least 6 hours of sunlight.
- Give roses enough air circulation.
If your roses are affected with black spot, here are some ways to help control the disease:
- Remove and destroy diseased leaves immediately, including ones on the ground. To prevent further cases of black rot, apply a layer of mulch on the soil and prune diseased branches in the spring.
- Keep leaves dry. Fungi need moisture to grow, so keep the foliage as dry as possible. Avoid overhead watering; instead use drip irrigation or water at the base of the plant.
- Improve the air circulation around the plants. Crowded plants are prone to disease. Pruning plants improves air circulation and helps keep the leaves dry.
- Use black spot fungicide. Fungicides don’t cure infected leaves. However, when applied regularly, they can protect new leaves from black spot.
- Diluted neem oil is a natural and effective alternative to fungicide. Apply regularly for the best results.
- Plant Pests and Diseases (growerdirect.com)