A beautiful garden full of fragrant, colorful, healthy roses is the goal of many gardeners. Rose plants are affected by many diseases, but fungal diseases tend to be the most prevalent. Knowledge of prevention and treatment methods can help mitigate the damage they do. Plan in advance to prevent and contain diseases and enjoy the bounty of blooms.
Types of Diseases
The three types of diseases most likely to occur are black spot, mildew and rust. Black spot is a fungus that attacks the plant. It is easy to see, as it leaves black spots on the leaves. The spots become larger as the fungus develops. Leaves turn yellow and will fall off the plant as the disease worsens. Mildew is another fungal disease that is common on rose bushes. It will appear as a white, powdery dust on the leaves of the plant. The leaves will begin to crinkle and mold will appear and then spread in fine filaments to the other parts of the plant. Rust is also a fungus. It appears as small rust-colored spots on the undersides of leaves. Streaky rust-colored lines may also appear on young stalks. After awhile, spots turn black and leaves fall off.
Blackspot is typically a problem during warm, wet weather. It takes seven hours for the fungus to germinate, and if temperatures and conditions last that long, it may take hold. Mildew appears during hot, dry weather with cool, wet nights. Gardens where conditions are damp and shady are susceptible to mildew. Spores need three hours of moisture to germinate. Rust is predominately present west of the Rocky Mountains. It requires cool, wet weather and grows particularly well in foggy, misty conditions. Four hours of these conditions enable rust to germinate and begin to grow on the plant.
To prevent black spot, spray rose plants with a fungicidal soap and sulfur while they are still dormant. Spores cannot germinate underneath the film of sulfur. You will need to re-spray the plants frequently, as the sulfur and soap will wash off in the rain. Another way to prevent black spot, is to avoid watering the leaves of the plant; aim the hose at the roots instead. Place rose bushes at least three feet apart to allow plenty of ventilation. To prevent mildew, keep your roses well watered at the roots. Avoid watering the leaves of the plant. Give rose bushes plenty of room for ventilation when planting. The sulfur and fungicidal soap spray is also effective when applied while roses are dormant. Well-aerated soil is crucial for prevention of rust. Immediately remove any rust-damaged leaves from the ground. Do not work in a wet garden, or you may spread rust spores around even more.
Treatment for all three diseases is the same. First, remove the diseased leaves from the plant and prune any damaged branches. Remove the pruned matter from your garden so that the spores do not have a chance to migrate back to the bushes. Spray bushes every few weeks with fungicidal soap. You can also purchase disease-resistant plants.
RoseMagazine.com offers a good recipe for your own disease-fighting spray: 1 medium onion, chopped 3 cloves garlic, crushed 1/2 jalapeno pepper 1 1/2 tsp baking soda 1 or 2 drops dish-washing liquid Steep garlic, onion and pepper in warm water for one hour. Strain through cheesecloth and retain liquid. Add baking soda. Dilute into spray bottle with 1 part liquid and 4 parts warm water. Mist plants with mixture.