The fungus Diplocarpon rosae, present in all parts of the world, uses rose bushes as its host to germinate and multiply. The spores land on the foliage, causing black spots on the upper surface of leaves. As the infection progresses, the tissue surrounding the dark areas becomes yellow and the foliage dies. Rose canes are also susceptible to black spot disease, which weakens the plant and stops flower production. Meeting the shrub's cultivation requirements strengthens it against the fungus. If the pathogen finds a way of infiltrating the rose's defences, take a few steps to rescue the plant.
Prune infected leaves and canes, discarding them in the trash. Rake the area surrounding the rose shrub to remove any infected plant debris that might have fallen on the ground. Do not recycle this material, as composting doesn't kill the fungus.
Blend 2 tsp baking soda, 2 drops seaweed extract, 2 drops liquid dishwashing detergent and 5 quarts water. Stir the mixture and add it to a spray bottle. Spray the solution on all areas of the rosebush with black spot to kill the spores.
Add 1 gallon of well-rotted manure to a 5-gallon bucket. Fill the container with water and stir. Place the bucket in a warm area and let the manure steep for three days. Strain the manure tea with cheesecloth, scooping out enough to fill a spray bottle. Spray the infected plant tissue with this formula to kill the fungal spores.
Use a low-toxicity fungicide on infected rose shrubs. Select a product containing neem oil, sulphur or copper as its active ingredient. Apply it according to the manufacturer's label.
Spray a rose bush suffering from black spot disease with a synthetic fungicide. Select a product containing trifloxystrobin, maneb, triforine or another chemical labelled for use on roses with black spot. Follow the application method, rates and schedule indicated in the manufacturer's instructions for the brand you select.