Blackspot is a serious disease to roses. Caused by the fungus Diplocarpon rosae, it can cause rapid defoliation and also affects flower buds and canes. It usually strikes young leaves less than two weeks old, and begins as small black spots on the leaves. As the fungus spreads, the leaf area around the black spots turns yellow, then brown, and soon the entire leaf is affected and drops from the cane. Diligent control of the fungus is necessary. Although chemical fungicides are available, safer organic remedies are also effective. Treatments should be repeated throughout the season.
Beginning in the early spring, spray your roses with a wettable sulfur. Sulfur is an old remedy for blackspot, and it works as a very light film on the plant that creates a hostile environment for the fungus. Cover the entire plant when you spray, including the undersides of the leaves. Sulfur products such as Safer Garden Fungicide or Orthoganics Garden Sulfur are readily available in garden centers.
Baking Soda Remedy
Mix a solution of 1 tsp. baking soda per one qt. water. Add a few drops of liquid soap so the solution will stick to the waxy rose leaves. Spray the rose, making sure to cover both sides of the leaves.
Manure Tea Remedy
Make manure tea by placing 1 gallon of completely composted manure in a 5-gallon bucket, and fill it with water. Let this stand in a warm place for three days. Strain off the liquid, and spray your rose, covering the entire plant. Manure tea is also effective against powdery mildew and rust, and it is a nutritious foliar feed for roses as well.
Garden Sanitation Remedy
Good garden sanitation is part of every spray treatment remedy. Infected leaves and canes must be cleaned up and destroyed or they will release spores and continue the fungus cycle throughout the growing season. Infected plant debris will release spores that overwinter in the soil and reinfect the plant the following year. Garden sanitation includes removing mulch early in the spring, keeping dropped foliage and debris raked up and removed from around the rose, removing infected leaves as soon as you notice them, and pruning infected canes down to fungus free wood. Canes that harbor the fungus have small purplish spots or splotches on the affected areas.