Black spot (Marssonina rosae or Diplocarpon rosae) is a common fungal disease that affects roses. This fungal disease forms black spots all over the foliage of rose bushes causing the leaves to turn yellow and fall off. If left untreated, all of the leaves can fall off and the fungus can weaken the entire rose bush.
Due to the fact that the spores of black spot travel with the help of water, black spot disease thrives during cool moist weather conditions. Mild wet spring days and moist dewy nights are prime conditions for black spot disease to attack and spread. Once the fungus becomes active, it cannot be stopped without fungal controls. Prevention and prompt intervention when first spotted are critical to help keep your roses healthy and beautiful.
Signs of Black Spot
The first signs of black spot will begin to appear on the lower leaves of the rose bush. Circular black spots on the upper sides of the leaves will develop. Sometimes the spots are so big that they merge together. The diseased leaves will turn yellow and eventually fall off of the plant. If left untreated, the fungus moves upward and the entire plant can lose its leaves. Black spot not only affects the leaves of roses but the canes as well. Old and new canes can be infected with black spot disease. Black spot on canes will look like purple or black dots.
Black Spot Prevention
Once your rose shrub has black spot, even after you have treated the plant, the spots on the leaves will remain until all diseased leaves have fallen off. Prevention is key when treating black spot on roses. Planting your rose in an area that gets full sunlight will help to keep the leaves dry. Spacing out rose bushes allows for good air circulation which also helps to dry out the foliage and prevent the disease from spreading. Pruning the diseased canes is also very important. During the growing season, avoid getting the leaves wet when watering. It is a good idea to spray your roses in the spring with a fungicide and continue treating every two weeks so that new leaves are protected from the start.
Black Spot Treatment
Once you see black spots on your rose leaves, you need to address the issue immediately. Begin by pruning away all of the infected leaves and infected stems. Dispose all of the infected material. Once that is done you will need to spray the entire plant- above/below the leaves and stems with a commercial fungicide or a homemade solution. If the black spot returns, you may need to spray your roses weekly.
If you choose to stay away from chemical sprays, there are many DIY methods of treatments. You can make a Bordeaux mix which will also help to repel some insect pests. A baking soda and soap spray can also help with powdery mildew issues. Neem oil is an organic fungicide and pesticide derived from the seeds of the neem plant. Sulfur powder is also an option.
Tips for Preventing Black Spot on Roses
To help your roses and other garden plants stay healthy and disease free, follow these simple tips while maintaining your garden area.
- Provide good air circulation between plants.
- Leaves dry quicker when plants are not overcrowded.
- Water roses early in the morning.
- Sun throughout the day will help to dry leaves. Wet leaves at night foster disease.
- Avoid getting the foliage wet when watering.
- Black spot uses water to spread disease.
- Prune infected canes and leaves. Dispose debris in the trash.
- Sterilize pruning tools before and after use.